Merge pull request #5408 from mono/fast-build-check-for-test

Fast build check for test

8 files changed, 806 insertions, 613 deletions

diff --git a/main/src/addins/MonoDevelop.UnitTesting/Services/UnitTestService.cs b/main/src/addins/MonoDevelop.UnitTesting/Services/UnitTestService.cs
index 865e531c30..8ebf9a13c3 100644
--- a/main/src/addins/MonoDevelop.UnitTesting/Services/UnitTestService.cs
+++ b/main/src/addins/MonoDevelop.UnitTesting/Services/UnitTestService.cs
@@ -105,7 +105,7 @@ namespace MonoDevelop.UnitTesting
 
 		public static AsyncOperation RunTest (UnitTest test, MonoDevelop.Projects.ExecutionContext context)
 		{
-			var result = RunTest (test, context, IdeApp.Preferences.BuildBeforeRunningTests);
+			var result = RunTest (test, context, true);
 			result.Task.ContinueWith (t => OnTestSessionCompleted (), TaskScheduler.FromCurrentSynchronizationContext ());
 			return result;
 		}
@@ -126,32 +126,26 @@ namespace MonoDevelop.UnitTesting
 			if (buildOwnerObject) {
 				var build_targets = new HashSet<IBuildTarget> ();
 				foreach (var t in tests) {
-					IBuildTarget bt = t.OwnerObject as IBuildTarget;
-					if (bt != null)
+					if (t.OwnerObject is IBuildTarget bt)
 						build_targets.Add (bt);
 				}
-				if (build_targets.Count > 0) {
-					if (!IdeApp.ProjectOperations.CurrentRunOperation.IsCompleted) {
-						MonoDevelop.Ide.Commands.StopHandler.StopBuildOperations ();
-						await IdeApp.ProjectOperations.CurrentRunOperation.Task;
-					}
 
-					foreach (var bt in build_targets) {
-						var res = await IdeApp.ProjectOperations.Build (bt, cs.Token).Task;
-						if (res.HasErrors)
-							return;
-					}
+				var res = await IdeApp.ProjectOperations.CheckAndBuildForExecute (
+					build_targets, IdeApp.Workspace.ActiveConfiguration, IdeApp.Preferences.BuildBeforeRunningTests,
+					false, null, cs.Token);
+
+				if (!res)
+					return;
 
-					var test_names = new HashSet<string> (tests.Select ((v) => v.FullName));
+				var test_names = new HashSet<string> (tests.Select ((v) => v.FullName));
 
-					await RefreshTests (cs.Token);
+				await RefreshTests (cs.Token);
 
-					tests = test_names.Select ((fullName) => SearchTest (fullName)).Where ((t) => t != null).ToList ();
+				tests = test_names.Select ((fullName) => SearchTest (fullName)).Where ((t) => t != null).ToList ();
 
-					if (tests.Any ())
-						await RunTests (tests, context, false, checkCurrentRunOperation, cs);
-					return;
-				}
+				if (tests.Any ())
+					await RunTests (tests, context, false, checkCurrentRunOperation, cs);
+				return;
 			}
 			
 			if (checkCurrentRunOperation && !IdeApp.ProjectOperations.ConfirmExecutionOperation ())
@@ -176,7 +170,7 @@ namespace MonoDevelop.UnitTesting
 
 		public static AsyncOperation RunTests (IEnumerable<UnitTest> tests, MonoDevelop.Projects.ExecutionContext context)
 		{
-			var result = RunTests (tests, context, IdeApp.Preferences.BuildBeforeRunningTests);
+			var result = RunTests (tests, context);
 			result.Task.ContinueWith (t => OnTestSessionCompleted (), TaskScheduler.FromCurrentSynchronizationContext ());
 			return result;
 		}
diff --git a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects/Solution.cs b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects/Solution.cs
index 6fcc011e57..1086aac0c2 100644
--- a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects/Solution.cs
+++ b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects/Solution.cs
@@ -317,10 +317,9 @@ namespace MonoDevelop.Projects
 		void CollectItemProperties (PropertyBag props, SolutionFolderItem item, string path)
 		{
 			if (!item.UserProperties.IsEmpty && item.ParentFolder != null)
-				props.SetValue (path, item.UserProperties);
-			
-			SolutionFolder sf = item as SolutionFolder;
-			if (sf != null) {
+				props.SetValue (path, item.UserProperties);

+

+			if (item is SolutionFolder sf) {
 				foreach (SolutionFolderItem ci in sf.Items)
 					CollectItemProperties (props, ci, path + "." + ci.Name);
 			}
@@ -328,10 +327,9 @@ namespace MonoDevelop.Projects
 		
 		void CleanItemProperties (PropertyBag props, SolutionFolderItem item, string path)
 		{
-			props.RemoveValue (path);
-			
-			SolutionFolder sf = item as SolutionFolder;
-			if (sf != null) {
+			props.RemoveValue (path);

+

+			if (item is SolutionFolder sf) {
 				foreach (SolutionFolderItem ci in sf.Items)
 					CleanItemProperties (props, ci, path + "." + ci.Name);
 			}
@@ -343,10 +341,9 @@ namespace MonoDevelop.Projects
 			if (info != null) {
 				item.LoadUserProperties (info);
 				props.RemoveValue (path);
-			}
-			
-			SolutionFolder sf = item as SolutionFolder;
-			if (sf != null) {
+			}

+

+			if (item is SolutionFolder sf) {
 				foreach (SolutionFolderItem ci in sf.Items)
 					LoadItemProperties (props, ci, path + "." + ci.Name);
 			}
@@ -373,7 +370,7 @@ namespace MonoDevelop.Projects
 
 		public SolutionConfiguration AddConfiguration (string id, bool createConfigForItems)
 		{
-			SolutionConfiguration conf = new SolutionConfiguration (id);
+			var conf = new SolutionConfiguration (id);
 			foreach (SolutionItem item in Items.Where (it => it.SupportsBuild())) {
 				if (createConfigForItems && item.GetConfiguration (new ItemConfigurationSelector (id)) == null) {
 					SolutionItemConfiguration newc = item.CreateConfiguration (id);
@@ -389,7 +386,7 @@ namespace MonoDevelop.Projects
 		
 		public override ReadOnlyCollection<string> GetConfigurations ()
 		{
-			List<string> configs = new List<string> ();
+			var configs = new List<string> ();
 			foreach (SolutionConfiguration conf in Configurations)
 				configs.Add (conf.Id);
 			return configs.AsReadOnly ();
@@ -445,12 +442,13 @@ namespace MonoDevelop.Projects
 
 		public ReadOnlyCollection<T> GetAllSolutionItemsWithTopologicalSort<T> (ConfigurationSelector configuration) where T: SolutionItem
 		{
-			return RootFolder.GetAllItemsWithTopologicalSort<T> (configuration);
+			var list = new List<T> (GetAllItems<T> ());
+			return SolutionItem.TopologicalSort (list, configuration);
 		}
 		
 		public ReadOnlyCollection<Project> GetAllProjectsWithTopologicalSort (ConfigurationSelector configuration)
 		{
-			return RootFolder.GetAllProjectsWithTopologicalSort (configuration);
+			return GetAllSolutionItemsWithTopologicalSort<Project> (configuration);
 		}
 
 		public override IEnumerable<Project> GetProjectsContainingFile (FilePath fileName)
@@ -857,16 +855,14 @@ namespace MonoDevelop.Projects
 
 		/*protected virtual*/ bool OnGetCanExecute(ExecutionContext context, ConfigurationSelector configuration, SolutionRunConfiguration runConfiguration)
 		{
-			var ssc = runConfiguration as SingleItemSolutionRunConfiguration;
-			if (ssc != null)
+			if (runConfiguration is SingleItemSolutionRunConfiguration ssc)
 				return ssc.Item.CanExecute (context, configuration, ssc.RunConfiguration);
 
-			var msc = runConfiguration as MultiItemSolutionRunConfiguration;
-			if (msc != null) {
+			if (runConfiguration is MultiItemSolutionRunConfiguration msc) {
 				var multiProject = context.ExecutionTarget as MultiProjectExecutionTarget;
 				foreach (StartupItem it in msc.Items) {
-					var localContext = context;
-					//Set project specific execution target to context if exists
+					var localContext = context;

+					//Set project specific execution target to context if exists

 					if (multiProject?.GetTarget (it.SolutionItem) != null)
 						localContext = new ExecutionContext (context.ExecutionHandler, context.ExternalConsoleFactory, multiProject?.GetTarget (it.SolutionItem));
 					if (it.SolutionItem.CanExecute (localContext, configuration, it.RunConfiguration))
@@ -879,26 +875,24 @@ namespace MonoDevelop.Projects
 		
 		/*protected virtual*/ async Task OnExecute (ProgressMonitor monitor, ExecutionContext context, ConfigurationSelector configuration, SolutionRunConfiguration runConfiguration)
 		{
-			var ssc = runConfiguration as SingleItemSolutionRunConfiguration;
-			if (ssc != null) {
+			if (runConfiguration is SingleItemSolutionRunConfiguration ssc) {
 				await ssc.Item.Execute (monitor, context, configuration, ssc.RunConfiguration);
 				return;
 			}
-			var msc = runConfiguration as MultiItemSolutionRunConfiguration;
-			if (msc != null) {
+			if (runConfiguration is MultiItemSolutionRunConfiguration msc) {
 				var tasks = new List<Task> ();
 				var monitors = new List<AggregatedProgressMonitor> ();
 				monitor.BeginTask ("Executing projects", 1);
 
 				var multiProject = context.ExecutionTarget as MultiProjectExecutionTarget;
 				foreach (StartupItem it in msc.Items) {
-					var localContext = context;
-					//Set project specific execution target to context if exists
+					var localContext = context;

+					//Set project specific execution target to context if exists

 					if (multiProject?.GetTarget (it.SolutionItem) != null)
 						localContext = new ExecutionContext (context.ExecutionHandler, context.ConsoleFactory, multiProject?.GetTarget (it.SolutionItem));
 					if (!it.SolutionItem.CanExecute (localContext, configuration, it.RunConfiguration))
 						continue;
-					AggregatedProgressMonitor mon = new AggregatedProgressMonitor ();
+					var mon = new AggregatedProgressMonitor ();
 					mon.AddFollowerMonitor (monitor, MonitorAction.ReportError | MonitorAction.ReportWarning | MonitorAction.FollowerCancel);
 					monitors.Add (mon);
 					tasks.Add (it.SolutionItem.Execute (mon, localContext, configuration, it.RunConfiguration));
@@ -922,17 +916,213 @@ namespace MonoDevelop.Projects
 		}
 
 		/*protected virtual*/ void OnStartupItemChanged(EventArgs e)
-		{
-			if (StartupItemChanged != null)
-				StartupItemChanged (this, e);
+		{

+			StartupItemChanged?.Invoke (this, e);
 		}
 
 		void OnStartupConfigurationChanged (EventArgs e)
+		{

+			StartupConfigurationChanged?.Invoke (this, e);
+		}

+

+		/// <summary>

+		/// Builds a set of SolutionItems from this solution and their dependencies. They will be built in parallel, and common dependencies will be deduplicated.

+		/// </summary>

+		public async Task<BuildResult> CleanItems (ProgressMonitor monitor, ConfigurationSelector configuration, IEnumerable<SolutionItem> items, OperationContext operationContext = null, string beginTaskMessage = null)
+		{
+			SolutionConfiguration slnConf = GetConfiguration (configuration);
+			if (slnConf == null)
+				return new BuildResult ();
+
+			ReadOnlyCollection<SolutionItem> sortedItems;
+			try {
+				sortedItems = GetItemsAndDependenciesSortedForBuild (items, configuration);
+			} catch (CyclicDependencyException) {
+				monitor.ReportError (GettextCatalog.GetString ("Cyclic dependencies are not supported."), null);
+				return new BuildResult ("", 1, 1);
+			}
+
+			if (operationContext == null)
+				operationContext = new OperationContext ();
+
+			monitor.BeginTask (
+				beginTaskMessage ?? GettextCatalog.GetString ("Cleaning {0} items in solution {1} ({2})", sortedItems.Count, Name, configuration.ToString ()),
+				sortedItems.Count
+			);
+
+			bool operationStarted = false;
+			BuildResult result = null;
+
+			try {
+				operationStarted = await BeginBuildOperation (monitor, configuration, operationContext);
+
+				return result = await RunParallelBuildOperation (monitor, configuration, sortedItems, (ProgressMonitor m, SolutionItem item) => {
+					return item.Clean (m, configuration, operationContext);
+				}, false);
+			} finally {
+				if (operationStarted)
+					await EndBuildOperation (monitor, configuration, operationContext, result);
+				monitor.EndTask ();
+			}
+		}

+

+		/// <summary>

+		/// Builds a set of SolutionItems from this solution and their dependencies. They will be built in parallel, and common dependencies will be deduplicated.

+		/// </summary>

+		public async Task<BuildResult> BuildItems (ProgressMonitor monitor, ConfigurationSelector configuration, IEnumerable<SolutionItem> items, OperationContext operationContext = null, string beginTaskMessage = null)
 		{
-			if (StartupConfigurationChanged != null)
-				StartupConfigurationChanged (this, e);
+			SolutionConfiguration slnConf = GetConfiguration (configuration);
+			if (slnConf == null)
+				return new BuildResult ();
+
+			ReadOnlyCollection<SolutionItem> sortedItems;

+

+			try {
+				sortedItems = GetItemsAndDependenciesSortedForBuild (items, configuration);
+			} catch (CyclicDependencyException) {
+				monitor.ReportError (GettextCatalog.GetString ("Cyclic dependencies are not supported."), null);
+				return new BuildResult ("", 1, 1);
+			}
+
+			if (operationContext == null)
+				operationContext = new OperationContext ();
+
+			bool operationStarted = false;
+			BuildResult result = null;
+
+			try {
+
+				if (Runtime.Preferences.SkipBuildingUnmodifiedProjects)
+					sortedItems = sortedItems.Where (si => {
+						if (si is Project p)
+							return p.FastCheckNeedsBuild (configuration);
+						return true;//Don't filter things that don't have FastCheckNeedsBuild

+					}).ToList ().AsReadOnly ();
+
+				monitor.BeginTask (
+					beginTaskMessage ?? GettextCatalog.GetString ("Building {0} items in solution {1} ({2})", sortedItems.Count, Name, configuration.ToString ()),
+					sortedItems.Count
+				);
+
+				operationStarted = await BeginBuildOperation (monitor, configuration, operationContext);
+
+				return result = await RunParallelBuildOperation (monitor, configuration, sortedItems, (ProgressMonitor m, SolutionItem item) => {
+					return item.Build (m, configuration, false, operationContext);
+				}, false);
+
+			} finally {
+				if (operationStarted)
+					await EndBuildOperation (monitor, configuration, operationContext, result);
+				monitor.EndTask ();
+			}

+		}

+

+		static async Task<BuildResult> RunParallelBuildOperation (ProgressMonitor monitor, ConfigurationSelector configuration, IEnumerable<SolutionItem> sortedItems, Func<ProgressMonitor, SolutionItem, Task<BuildResult>> buildAction, bool ignoreFailed)
+		{
+			var toBuild = new List<SolutionItem> (sortedItems);
+			var cres = new BuildResult { BuildCount = 0 };

+

+			// Limit the number of concurrent builders to processors / 2

+

+			var slotScheduler = new TaskSlotScheduler (Environment.ProcessorCount / 2);

+

+			// Create a dictionary with the status objects of all items

+

+			var buildStatus = new Dictionary<SolutionItem, BuildStatus> ();
+			foreach (var it in toBuild)
+				buildStatus.Add (it, new BuildStatus ());

+

+			// Start the build tasks for all itemsw

+

+			foreach (var itemToBuild in toBuild) {
+				if (monitor.CancellationToken.IsCancellationRequested)
+					break;
+
+				var item = itemToBuild;
+
+				var myStatus = buildStatus[item];
+
+				var myMonitor = monitor.BeginAsyncStep (1);

+

+				// Get a list of the status objects for all items on which this one depends

+

+				var refStatus = item.GetReferencedItems (configuration).Select (it => {
+					buildStatus.TryGetValue (it, out var bs);
+					return bs;
+				}).Where (t => t != null).ToArray ();

+

+				// Build the item when all its dependencies have been built

+

+				var refTasks = refStatus.Select (bs => bs.Task);
+
+				myStatus.Task = Task.WhenAll (refTasks).ContinueWith (async t => {
+					if (!ignoreFailed && (refStatus.Any (bs => bs.Failed) || t.IsFaulted)) {
+						myStatus.Failed = true;
+					} else {
+						using (await slotScheduler.GetTaskSlot ())
+							myStatus.Result = await buildAction (myMonitor, item);
+						myStatus.Failed = myStatus.Result != null && myStatus.Result.ErrorCount > 0;
+					}
+					myMonitor.Dispose ();
+				}, Runtime.MainTaskScheduler).Unwrap ();
+
+				if (!Runtime.Preferences.ParallelBuild.Value)
+					await myStatus.Task;
+			}

+

+			// Wait for all tasks to end

+

+			await Task.WhenAll (buildStatus.Values.Select (bs => bs.Task));

+

+			// Generate the errors in the order they were supposed to build

+

+			foreach (var it in toBuild) {
+				if (buildStatus.TryGetValue (it, out var bs) && bs.Result != null)
+					cres.Append (bs.Result);
+			}
+
+			return cres;
 		}
 
+		class BuildStatus
+		{
+			public bool Failed;
+			public Task Task;
+			public BuildResult Result;
+		}
+
+		/// <summary>

+		/// Given a set of SolutionItems from this solution, collects them and their buildable dependencies, and toplogically sorts them in preparation for a build.

+		/// </summary>
+		ReadOnlyCollection<SolutionItem> GetItemsAndDependenciesSortedForBuild (IEnumerable<SolutionItem> items, ConfigurationSelector configuration)

+		{

+			var slnConf = GetConfiguration (configuration);

+			var collected = new HashSet<SolutionItem> ();

+

+			foreach (var item in items) {

+				if (item.ParentSolution != this) {

+					throw new ArgumentException ("All items must be in this solution", nameof(items));

+				}

+				if (slnConf.BuildEnabledForItem (item) && collected.Add (item)) {

+					CollectBuildableDependencies (collected, item, configuration, slnConf);

+				}

+			}

+

+			return SolutionItem.TopologicalSort (collected, configuration);

+		}

+

+		/// <summary>

+		/// Recursively collects buildable dependencies.

+		/// </summary>
+		internal static void CollectBuildableDependencies (HashSet<SolutionItem> collected, SolutionItem item, ConfigurationSelector configuration, SolutionConfiguration conf)
+		{
+			foreach (var it in item.GetReferencedItems (configuration)) {

+				if (collected.Contains (it) || !conf.BuildEnabledForItem (it))

+					continue;

+				collected.Add (it);

+				CollectBuildableDependencies (collected, it, configuration, conf);

+			}
+		}
 
 		[ThreadSafe]
 		public MSBuildFileFormat FileFormat {
@@ -988,37 +1178,33 @@ namespace MonoDevelop.Projects
 			
 			solutionItems = null;
 
-			SolutionFolder sf = args.SolutionItem as SolutionFolder;
-			if (sf != null) {
+			if (args.SolutionItem is SolutionFolder sf) {
 				foreach (SolutionFolderItem eitem in sf.GetAllItems<SolutionFolderItem> ())
 					SetupNewItem (eitem, null);
-			}
-			else {
+			} else {
 				SetupNewItem (args.SolutionItem, args.ReplacedItem);
 			}
 
-			OnRootDirectoriesChanged ();
-			
-			if (SolutionItemAdded != null)
-				SolutionItemAdded (this, args);
+			OnRootDirectoriesChanged ();

+

+			SolutionItemAdded?.Invoke (this, args);
 		}
 		
 		void SetupNewItem (SolutionFolderItem item, SolutionFolderItem replacedItem)
 		{
-			SolutionItem eitem = item as SolutionItem;
-			if (eitem != null) {
+			if (item is SolutionItem eitem) {
 				eitem.ConvertToFormat (FileFormat);
 				eitem.NeedsReload = false;
 				if (eitem.SupportsConfigurations () || replacedItem != null) {
-					if (replacedItem == null) {
-						// Register the new entry in every solution configuration
+					if (replacedItem == null) {

+						// Register the new entry in every solution configuration

 						foreach (SolutionConfiguration conf in Configurations)
-							conf.AddItem (eitem);
-						// If there is no startup project or it is an invalid one, use the new project as startup if possible
+							conf.AddItem (eitem);

+						// If there is no startup project or it is an invalid one, use the new project as startup if possible

 						if (!Loading && (StartupItem == null || !StartupItem.SupportsExecute ()) && eitem.SupportsExecute ())
 							StartupItem = eitem;
-					} else {
-						// Reuse the configuration information of the replaced item
+					} else {

+						// Reuse the configuration information of the replaced item

 						foreach (SolutionConfiguration conf in Configurations)
 							conf.ReplaceItem ((SolutionItem)replacedItem, eitem);
 
@@ -1034,23 +1220,19 @@ namespace MonoDevelop.Projects
 		
 		internal /*protected virtual*/ void OnSolutionItemRemoved (SolutionItemChangeEventArgs args)
 		{
-			solutionItems = null;
-			
-			SolutionFolder sf = args.SolutionItem as SolutionFolder;
-			if (sf != null) {
+			solutionItems = null;

+

+			if (args.SolutionItem is SolutionFolder sf) {
 				foreach (SolutionItem eitem in sf.GetAllItems<SolutionItem> ())
 					DetachItem (eitem, args.Reloading);
-			}
-			else {
-				SolutionItem item = args.SolutionItem as SolutionItem;
-				if (item != null)
+			} else {
+				if (args.SolutionItem is SolutionItem item)
 					DetachItem (item, args.Reloading);
 			}
 
-			OnRootDirectoriesChanged ();
-			
-			if (SolutionItemRemoved != null)
-				SolutionItemRemoved (this, args);
+			OnRootDirectoriesChanged ();

+

+			SolutionItemRemoved?.Invoke (this, args);
 		}
 		
 		void DetachItem (SolutionItem item, bool reloading)
@@ -1082,7 +1264,7 @@ namespace MonoDevelop.Projects
 				if (project == projectToRemove)
 					continue;
 				
-				List<ProjectReference> toDelete = new List<ProjectReference> ();
+				var toDelete = new List<ProjectReference> ();
 				
 				foreach (ProjectReference pref in project.References) {
 					if (pref.ReferenceType == ReferenceType.Project && pref.Reference == projectToRemove.Name)
@@ -1100,7 +1282,7 @@ namespace MonoDevelop.Projects
 		internal void ReadSolution (ProgressMonitor monitor)
 		{
 			var sln = new SlnFile ();
-			sln.Read (this.FileName);
+			sln.Read (FileName);
 
 			using (currentLoadContext = new SolutionLoadContext (this))
 				SolutionExtension.OnReadSolution (monitor, sln);
@@ -1176,63 +1358,53 @@ namespace MonoDevelop.Projects
 		}
 		
 		internal /*protected virtual*/ void OnFileAddedToProject (ProjectFileEventArgs args)
-		{
-			if (FileAddedToProject != null)
-				FileAddedToProject (this, args);
+		{

+			FileAddedToProject?.Invoke (this, args);
 		}
 		
 		internal /*protected virtual*/ void OnFileRemovedFromProject (ProjectFileEventArgs args)
-		{
-			if (FileRemovedFromProject != null)
-				FileRemovedFromProject (this, args);
+		{

+			FileRemovedFromProject?.Invoke (this, args);
 		}
 		
 		internal /*protected virtual*/ void OnFileChangedInProject (ProjectFileEventArgs args)
-		{
-			if (FileChangedInProject != null)
-				FileChangedInProject (this, args);
+		{

+			FileChangedInProject?.Invoke (this, args);
 		}
 		
 		internal /*protected virtual*/ void OnFilePropertyChangedInProject (ProjectFileEventArgs args)
-		{
-			if (FilePropertyChangedInProject != null)
-				FilePropertyChangedInProject (this, args);
+		{

+			FilePropertyChangedInProject?.Invoke (this, args);
 		}
 		
 		internal /*protected virtual*/ void OnFileRenamedInProject (ProjectFileRenamedEventArgs args)
-		{
-			if (FileRenamedInProject != null)
-				FileRenamedInProject (this, args);
+		{

+			FileRenamedInProject?.Invoke (this, args);
 		}
 		
 		internal /*protected virtual*/ void OnReferenceAddedToProject (ProjectReferenceEventArgs args)
-		{
-			if (ReferenceAddedToProject != null)
-				ReferenceAddedToProject (this, args);
+		{

+			ReferenceAddedToProject?.Invoke (this, args);
 		}
 		
 		internal /*protected virtual*/ void OnReferenceRemovedFromProject (ProjectReferenceEventArgs args)
-		{
-			if (ReferenceRemovedFromProject != null)
-				ReferenceRemovedFromProject (this, args);
+		{

+			ReferenceRemovedFromProject?.Invoke (this, args);
 		}
 		
 		internal /*protected virtual*/ void OnEntryModified (SolutionItemModifiedEventArgs args)
-		{
-			if (EntryModified != null)
-				EntryModified (this, args);
+		{

+			EntryModified?.Invoke (this, args);
 		}
 		
 		internal /*protected virtual*/ void OnEntrySaved (SolutionItemSavedEventArgs args)
-		{
-			if (EntrySaved != null)
-				EntrySaved (this, args);
+		{

+			EntrySaved?.Invoke (this, args);
 		}
 		
 		internal /*protected virtual*/ void OnItemReloadRequired (SolutionItemEventArgs args)
-		{
-			if (ItemReloadRequired != null)
-				ItemReloadRequired (this, args);
+		{

+			ItemReloadRequired?.Invoke (this, args);
 		}
 		
 #endregion
@@ -1399,9 +1571,8 @@ namespace MonoDevelop.Projects
 		public Solution Solution { get; private set; }
 
 		void IDisposable.Dispose ()
-		{
-			if (LoadCompleted != null)
-				LoadCompleted (this, EventArgs.Empty);
+		{

+			LoadCompleted?.Invoke (this, EventArgs.Empty);
 		}
 	}
 }
diff --git a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects/SolutionFolder.cs b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects/SolutionFolder.cs
index 6b94381310..70c29a0808 100644
--- a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects/SolutionFolder.cs
+++ b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects/SolutionFolder.cs
@@ -140,15 +140,15 @@ namespace MonoDevelop.Projects
 
 			foreach (SolutionFolderItem it in Items) {

 				FilePath subdir;
-				if (it is SolutionFolder) {
-					SolutionFolder sf = (SolutionFolder) it;
+				if (it is SolutionFolder sf) {
 					if (sf.HasCustomBaseDirectory)
 						subdir = sf.BaseDirectory;
 					else
 						subdir = sf.GetCommonPathRoot ();
-				} else
-					subdir = it.BaseDirectory;
-				
+				} else {

+					subdir = it.BaseDirectory;

+				}
+

 				if (subdir.IsNullOrEmpty)

 					return FilePath.Null;
 				
@@ -211,17 +211,15 @@ namespace MonoDevelop.Projects
 			if (Items.IndexOf (sitem) == -1)
 				throw new InvalidOperationException ("Solution item '" + sitem.Name + "' does not belong to folder '" + Name + "'");
 
-			SolutionItem item = sitem as SolutionItem;
-			if (item != null) {
-				// Load the new item
-				
+			if (sitem is SolutionItem item) {

+				// Load the new item

+

 				SolutionItem newItem;
 				try {
 					if (ParentSolution.IsSolutionItemEnabled (item.FileName)) {
 						using (var ctx = new SolutionLoadContext (ParentSolution))
 							newItem = await Services.ProjectService.ReadSolutionItem (monitor, item.FileName, null, ctx: ctx, itemGuid: item.ItemId);
-					}
-					else {
+					} else {
 						UnknownSolutionItem e = new UnloadedSolutionItem () {
 							FileName = item.FileName
 						};
@@ -230,37 +228,37 @@ namespace MonoDevelop.Projects
 						newItem = e;
 					}
 				} catch (Exception ex) {
-					UnknownSolutionItem e = new UnknownSolutionItem ();
-					e.LoadError = ex.Message;
-					e.FileName = item.FileName;
-					newItem = e;
+					newItem = new UnknownSolutionItem {

+						LoadError = ex.Message,

+						FileName = item.FileName

+					};
 				}
 
-				if (!Items.Contains (item)) {
-					// The old item is gone, which probably means it has already been reloaded (BXC20615), or maybe removed.
-					// In this case, there isn't anything else we can do
-					newItem.Dispose ();
-
-					// Find the replacement if it exists
+				if (!Items.Contains (item)) {

+					// The old item is gone, which probably means it has already been reloaded (BXC20615), or maybe removed.

+					// In this case, there isn't anything else we can do

+					newItem.Dispose ();

+

+					// Find the replacement if it exists

 					return Items.OfType<SolutionItem> ().FirstOrDefault (it => it.FileName == item.FileName);
-				}
-
-				// Replace in the file list
+				}

+

+				// Replace in the file list

 				Items.Replace (item, newItem);
 
 				item.ParentFolder = null;
 				DisconnectChildEntryEvents (item);
-				ConnectChildEntryEvents (newItem);
-	
+				ConnectChildEntryEvents (newItem);

+

 				NotifyModified ("Items");
-				OnItemRemoved (new SolutionItemChangeEventArgs (item, ParentSolution, true) { ReplacedItem = item } , true);
-				OnItemAdded (new SolutionItemChangeEventArgs (newItem, ParentSolution, true) { ReplacedItem = item }, true);
-				
+				OnItemRemoved (new SolutionItemChangeEventArgs (item, ParentSolution, true) { ReplacedItem = item }, true);
+				OnItemAdded (new SolutionItemChangeEventArgs (newItem, ParentSolution, true) { ReplacedItem = item }, true);

+

 				item.Dispose ();
 				return newItem;
-			}
-			else
-				return sitem;
+			}

+

+			return sitem;
 		}
 		
 		internal void NotifyItemAdded (SolutionFolderItem item, bool newToSolution)
@@ -274,7 +272,7 @@ namespace MonoDevelop.Projects
 		void ConnectChildEntryEvents (SolutionFolderItem item)
 		{
 			if (item is Project) {
-				Project project = item as Project;
+				var project = item as Project;
 				project.FileRemovedFromProject += NotifyFileRemovedFromProject;
 				project.FileAddedToProject += NotifyFileAddedToProject;
 				project.FileChangedInProject += NotifyFileChangedInProject;
@@ -287,7 +285,7 @@ namespace MonoDevelop.Projects
 			}
 			
 			if (item is SolutionFolder) {
-				SolutionFolder folder = item as SolutionFolder;
+				var folder = item as SolutionFolder;
 				folder.FileRemovedFromProject += NotifyFileRemovedFromProject;
 				folder.FileAddedToProject += NotifyFileAddedToProject;
 				folder.FileChangedInProject += NotifyFileChangedInProject;
@@ -327,11 +325,10 @@ namespace MonoDevelop.Projects
 		
 		public void AddItem (SolutionFolderItem item, bool createSolutionConfigurations)
 		{
-			Items.Add (item);
-			
-			SolutionItem eitem = item as SolutionItem;
-			if (eitem != null && createSolutionConfigurations && eitem.SupportsBuild ()) {
-				// Create new solution configurations for item configurations
+			Items.Add (item);

+

+			if (item is SolutionItem eitem && createSolutionConfigurations && eitem.SupportsBuild ()) {

+				// Create new solution configurations for item configurations

 				foreach (ItemConfiguration iconf in eitem.Configurations) {
 					bool found = false;
 					foreach (SolutionConfiguration conf in ParentSolution.Configurations) {
@@ -341,8 +338,8 @@ namespace MonoDevelop.Projects
 						}
 					}
 					if (!found) {
-						SolutionConfiguration sconf = new SolutionConfiguration (iconf.Id);
-						// Add all items to the new configuration
+						var sconf = new SolutionConfiguration (iconf.Id);

+						// Add all items to the new configuration

 						foreach (var it in ParentSolution.GetAllItems<SolutionItem> ())
 							sconf.AddItem (it);
 						ParentSolution.Configurations.Add (sconf);
@@ -361,7 +358,7 @@ namespace MonoDevelop.Projects
 		void DisconnectChildEntryEvents (SolutionFolderItem entry)
 		{
 			if (entry is Project) {
-				Project pce = entry as Project;
+				var pce = entry as Project;
 				pce.FileRemovedFromProject -= NotifyFileRemovedFromProject;
 				pce.FileAddedToProject -= NotifyFileAddedToProject;
 				pce.FileChangedInProject -= NotifyFileChangedInProject;
@@ -374,7 +371,7 @@ namespace MonoDevelop.Projects
 			}
 			
 			if (entry is SolutionFolder) {
-				SolutionFolder cce = entry as SolutionFolder;
+				var cce = entry as SolutionFolder;
 				cce.FileRemovedFromProject -= NotifyFileRemovedFromProject;
 				cce.FileAddedToProject -= NotifyFileAddedToProject;
 				cce.FileChangedInProject -= NotifyFileChangedInProject;
@@ -421,27 +418,29 @@ namespace MonoDevelop.Projects
 			return GetAllItems<SolutionFolderItem> ();
 		}
 		
+		[Obsolete("This method will be removed in future releases")]
 		public ReadOnlyCollection<T> GetAllItemsWithTopologicalSort<T> (ConfigurationSelector configuration) where T: SolutionItem
 		{
-			List<T> list = new List<T> ();
+			var list = new List<T> ();
 			GetAllItems<T> (list, this);
 			return SolutionItem.TopologicalSort<T> (list, configuration);
 		}
 		
 		public ReadOnlyCollection<Project> GetAllProjects ()
 		{
-			List<Project> list = new List<Project> ();
-			GetAllItems<Project> (list, this);
+			var list = new List<Project> ();
+			GetAllItems (list, this);
 			return list.AsReadOnly ();
 		}
 		
 		// The projects are returned in the order
 		// they should be compiled, acording to their references.
+		[Obsolete("This method will be removed in future releases")]
 		public ReadOnlyCollection<Project> GetAllProjectsWithTopologicalSort (ConfigurationSelector configuration)
 		{
-			List<Project> list = new List<Project> ();
-			GetAllItems<Project> (list, this);
-			return SolutionItem.TopologicalSort<Project> (list, configuration);
+			var list = new List<Project> ();
+			GetAllItems (list, this);
+			return SolutionItem.TopologicalSort (list, configuration);
 		}
 		
 		void GetAllItems<T> (List<T> list, SolutionFolderItem item) where T: SolutionFolderItem
@@ -450,54 +449,49 @@ namespace MonoDevelop.Projects
 				list.Add ((T)item);
 			}
 		
-			if (item is SolutionFolder) {
-				foreach (SolutionFolderItem ce in ((SolutionFolder)item).Items)
-					GetAllItems<T> (list, ce);
+			if (item is SolutionFolder sf) {
+				foreach (SolutionFolderItem ce in (sf).Items)
+					GetAllItems (list, ce);
 			}
 		}
 		
+		[Obsolete("This method will be removed in future releases")]
 		public ReadOnlyCollection<SolutionItem> GetAllBuildableEntries (ConfigurationSelector configuration, bool topologicalSort, bool includeExternalReferences)
 		{
-			var list = new List<SolutionItem> ();
-			GetAllBuildableEntries (list, configuration, includeExternalReferences);
+			var list = new List<SolutionItem> ();

+			if (ParentSolution != null)
+				return list.AsReadOnly ();
+
+			SolutionConfiguration conf = ParentSolution.GetConfiguration (configuration);
+			if (conf == null)
+				return list.AsReadOnly ();
+
+			var collected = new HashSet<SolutionItem> ();
+			CollectBuildableEntries (collected, configuration, conf, includeExternalReferences);
+			list.AddRange (collected);
+
 			if (topologicalSort)
-				return SolutionItem.TopologicalSort<SolutionItem> (list, configuration);
+				return SolutionItem.TopologicalSort (list, configuration);
 			else
 				return list.AsReadOnly ();
-		}
-		
+		}

+

+		[Obsolete("This method will be removed in future releases")]
 		public ReadOnlyCollection<SolutionItem> GetAllBuildableEntries (ConfigurationSelector configuration)
 		{
 			return GetAllBuildableEntries (configuration, false, false);
 		}
-		
-		void GetAllBuildableEntries (List<SolutionItem> list, ConfigurationSelector configuration, bool includeExternalReferences)
-		{
-			if (ParentSolution == null)
-				return;
-			SolutionConfiguration conf = ParentSolution.GetConfiguration (configuration);
-			if (conf == null)
-				return;
 
+		void CollectBuildableEntries (HashSet<SolutionItem> collected, ConfigurationSelector configuration, SolutionConfiguration slnConf, bool includeDependencies)
+		{
 			foreach (SolutionFolderItem item in Items) {
-				if (item is SolutionFolder)
-					((SolutionFolder)item).GetAllBuildableEntries (list, configuration, includeExternalReferences);
-				else if ((item is SolutionItem) && conf.BuildEnabledForItem ((SolutionItem) item) && ((SolutionItem)item).SupportsBuild ())
-					GetAllBuildableReferences (list, (SolutionItem)item, configuration, conf, includeExternalReferences, false);
-			}
-		}
-
-		void GetAllBuildableReferences (List<SolutionItem> list, SolutionItem item, ConfigurationSelector configuration, SolutionConfiguration conf, bool includeExternalReferences, bool isDirectReference)
-		{
-			if (list.Contains (item) || !conf.BuildEnabledForItem (item))
-				return;
-			// Skip unsupported projects which are not directly referenced by other (supported) projects
-			if (!isDirectReference && item.IsUnsupportedProject)
-				return;
-			list.Add (item);
-			if (includeExternalReferences) {
-				foreach (var it in item.GetReferencedItems (configuration))
-					GetAllBuildableReferences (list, it, configuration, conf, includeExternalReferences, true);
+				if (item is SolutionFolder sf)
+					sf.CollectBuildableEntries (collected, configuration, slnConf, includeDependencies);
+				else if (item is SolutionItem si && slnConf.BuildEnabledForItem (si) && si.SupportsBuild () && collected.Add (si)) {

+					if (includeDependencies) {

+						Solution.CollectBuildableDependencies (collected, si, configuration, slnConf);

+					}

+				}
 			}
 		}
 
@@ -542,15 +536,14 @@ namespace MonoDevelop.Projects
 		{
 			string path = Path.GetFullPath (fileName);
 			foreach (SolutionFolderItem it in Items) {
-				if (it is SolutionFolder) {
-					SolutionItem r = ((SolutionFolder)it).FindSolutionItem (fileName);
+				if (it is SolutionFolder sf) {
+					SolutionItem r = sf.FindSolutionItem (fileName);
 					if (r != null)
 						return r;
 				}
-				else if (it is SolutionItem) {
-					SolutionItem se = (SolutionItem) it;
+				else if (it is SolutionItem se) {
 					if (!string.IsNullOrEmpty (se.FileName) && path == Path.GetFullPath (se.FileName))
-						return (SolutionItem) it;
+						return (SolutionItem)it;
 				}
 			}
 			return null;
@@ -577,161 +570,39 @@ namespace MonoDevelop.Projects
 			return true;
 		}
 
-		public async Task<BuildResult> Clean (ProgressMonitor monitor, ConfigurationSelector configuration, OperationContext operationContext = null)
+		public Task<BuildResult> Clean (ProgressMonitor monitor, ConfigurationSelector configuration, OperationContext operationContext = null)
 		{
-			if (ParentSolution == null)
-				return new BuildResult();
-			SolutionConfiguration conf = ParentSolution.GetConfiguration (configuration);
-			if (conf == null)
-				return new BuildResult();
-
-			if (operationContext == null)
-				operationContext = new OperationContext ();
-
-			ReadOnlyCollection<SolutionItem> allProjects;
-			try {
-				allProjects = GetAllBuildableEntries (configuration, true, true);
-			} catch (CyclicDependencyException) {
-				monitor.ReportError (GettextCatalog.GetString ("Cyclic dependencies are not supported."), null);
-				return new BuildResult ("", 1, 1);
-			}
-
-			monitor.BeginTask (GettextCatalog.GetString ("Cleaning Solution: {0} ({1})", Name, configuration.ToString ()), allProjects.Count);
-
-			bool operationStarted = false;
-			BuildResult result = null;
-
-			try {
-				operationStarted = ParentSolution != null && await ParentSolution.BeginBuildOperation (monitor, configuration, operationContext);
-
-				return result = await RunParallelBuildOperation (monitor, configuration, allProjects, (ProgressMonitor m, SolutionItem item) => {
-					return item.Clean (m, configuration, operationContext);
-				}, false);
-			}
-			finally {
-				if (operationStarted)
-					await ParentSolution.EndBuildOperation (monitor, configuration, operationContext, result);
-				monitor.EndTask ();
-			}
-		}
+			var slnConf = ParentSolution?.GetConfiguration (configuration);
+			if (slnConf == null)
+				return Task.FromResult (new BuildResult ());

+

+			//don't collect dependencies, CleanItems will do it

+			var collected = new HashSet<SolutionItem> ();
+			CollectBuildableEntries (collected, configuration, slnConf, false);
 
-		class BuildStatus
-		{
-			public bool Failed;
-			public Task Task;
-			public BuildResult Result;
+			return ParentSolution.CleanItems (
+				monitor, configuration, collected, operationContext,
+				IsRoot ? GettextCatalog.GetString ("Cleaning solution {0} ({1})", Name, configuration.ToString ()) : null
+			);
 		}
 
-		public async Task<BuildResult> Build (ProgressMonitor monitor, ConfigurationSelector configuration, bool buildReferencedTargets = false, OperationContext operationContext = null)
+		public Task<BuildResult> Build (ProgressMonitor monitor, ConfigurationSelector configuration, bool buildReferencedTargets = false, OperationContext operationContext = null)
 		{
-			ReadOnlyCollection<SolutionItem> allProjects;
-				
-			try {
-				allProjects = GetAllBuildableEntries (configuration, true, true);
-			} catch (CyclicDependencyException) {
-				monitor.ReportError (GettextCatalog.GetString ("Cyclic dependencies are not supported."), null);
-				return new BuildResult ("", 1, 1);
-			}
-
-			if (operationContext == null)
-				operationContext = new OperationContext ();
-
-			bool operationStarted = false;
-			BuildResult result = null;
-
-			try {
-
-				if (Runtime.Preferences.SkipBuildingUnmodifiedProjects)
-					allProjects = allProjects.Where (si => {
-						if (si is Project p)
-							return p.FastCheckNeedsBuild (configuration);
-						return true;//Don't filter things that don't have FastCheckNeedsBuild
-					}).ToList ().AsReadOnly ();
-				monitor.BeginTask (GettextCatalog.GetString ("Building Solution: {0} ({1})", Name, configuration.ToString ()), allProjects.Count);
+			var slnConf = ParentSolution?.GetConfiguration (configuration);
+			if (slnConf == null)
+				return Task.FromResult (new BuildResult ());
 
-				operationStarted = ParentSolution != null && await ParentSolution.BeginBuildOperation (monitor, configuration, operationContext);
+			//don't collect dependencies, BuildItems will do it
+			var collected = new HashSet<SolutionItem> ();
+			CollectBuildableEntries (collected, configuration, slnConf, false);
 
-				return result = await RunParallelBuildOperation (monitor, configuration, allProjects, (ProgressMonitor m, SolutionItem item) => {
-					return item.Build (m, configuration, false, operationContext);
-				}, false);
 
-			} finally {
-				if (operationStarted)
-					await ParentSolution.EndBuildOperation (monitor, configuration, operationContext, result);
-				monitor.EndTask ();
-			}
+			return ParentSolution.BuildItems (
+				monitor, configuration, collected, operationContext,
+				IsRoot ? GettextCatalog.GetString ("Building solution {0} ({1})", Name, configuration.ToString ()) : null
+			);
         }
 
-		internal static async Task<BuildResult> RunParallelBuildOperation (ProgressMonitor monitor, ConfigurationSelector configuration, IEnumerable<SolutionItem> sortedItems, Func<ProgressMonitor,SolutionItem,Task<BuildResult>> buildAction, bool ignoreFailed)
-		{
-			List<SolutionItem> toBuild = new List<SolutionItem> (sortedItems);
-			BuildResult cres = new BuildResult ();
-			cres.BuildCount = 0;
-
-			// Limit the number of concurrent builders to processors / 2
-
-			var slotScheduler = new TaskSlotScheduler (Environment.ProcessorCount / 2);
-
-			// Create a dictionary with the status objects of all items
-
-			var buildStatus = new Dictionary<SolutionItem, BuildStatus> ();
-			foreach (var it in toBuild)
-				buildStatus.Add (it, new BuildStatus ());
-
-			// Start the build tasks for all itemsw
-
-			foreach (var itemToBuild in toBuild) {
-				if (monitor.CancellationToken.IsCancellationRequested)
-					break;
-
-				var item = itemToBuild;
-
-				var myStatus = buildStatus [item];
-
-				var myMonitor = monitor.BeginAsyncStep (1);
-
-				// Get a list of the status objects for all items on which this one depends
-
-				var refStatus = item.GetReferencedItems (configuration).Select (it => {
-					BuildStatus bs;
-					buildStatus.TryGetValue (it, out bs);
-					return bs;
-				}).Where (t => t != null).ToArray ();
-
-				// Build the item when all its dependencies have been built
-
-				var refTasks = refStatus.Select (bs => bs.Task);
-
-				myStatus.Task = Task.WhenAll (refTasks).ContinueWith (async t => {
-					if (!ignoreFailed && (refStatus.Any (bs => bs.Failed) || t.IsFaulted)) {
-						myStatus.Failed = true;
-					} else {
-						using (await slotScheduler.GetTaskSlot ())
-							myStatus.Result = await buildAction (myMonitor, item);
-						myStatus.Failed = myStatus.Result != null && myStatus.Result.ErrorCount > 0;
-					}
-					myMonitor.Dispose ();
-				}, Runtime.MainTaskScheduler).Unwrap ();
-
-				if (!Runtime.Preferences.ParallelBuild.Value)
-					await myStatus.Task;
-			}
-
-			// Wait for all tasks to end
-
-			await Task.WhenAll (buildStatus.Values.Select (bs => bs.Task));
-
-			// Generate the errors in the order they were supposed to build
-
-			foreach (var it in toBuild) {
-				BuildStatus bs;
-				if (buildStatus.TryGetValue (it, out bs) && bs.Result != null)
-					cres.Append (bs.Result);
-			}
-
-			return cres;
-		}
-
 		[Obsolete("This method will be removed in future releases")]
 		public bool NeedsBuilding (ConfigurationSelector configuration)
 		{
@@ -775,7 +646,7 @@ namespace MonoDevelop.Projects
 				sf.Files.Remove (fileName);
 			}
 			foreach (Project projectEntry in GetAllProjects()) {
-				List<ProjectFile> toDelete = new List<ProjectFile> ();
+				var toDelete = new List<ProjectFile> ();
 				foreach (ProjectFile fInfo in projectEntry.Files) {
 					if (fInfo.Name == fileName)
 						toDelete.Add (fInfo);
@@ -881,15 +752,13 @@ namespace MonoDevelop.Projects
 			if (ParentFolder != null)
 				ParentFolder.NotifyItemAddedToFolder (sender, e, newToSolution);
 			else if (ParentSolution != null && newToSolution)
-				ParentSolution.OnSolutionItemAdded (e);
-			if (DescendantItemAdded != null)
-				DescendantItemAdded (sender, e);
+				ParentSolution.OnSolutionItemAdded (e);

+			DescendantItemAdded?.Invoke (sender, e);
 		}
 		
 		internal void NotifyItemRemovedFromFolder (object sender, SolutionItemChangeEventArgs e, bool removedFromSolution)
-		{
-			if (DescendantItemRemoved != null)
-				DescendantItemRemoved (sender, e);
+		{

+			DescendantItemRemoved?.Invoke (sender, e);
 			if (ParentFolder != null)
 				ParentFolder.NotifyItemRemovedFromFolder (sender, e, removedFromSolution);
 			else if (ParentSolution != null && removedFromSolution)
@@ -919,9 +788,8 @@ namespace MonoDevelop.Projects
 		}
 		
 		void OnItemAdded (SolutionItemChangeEventArgs e)
-		{
-			if (ItemAdded != null)
-				ItemAdded (this, e);
+		{

+			ItemAdded?.Invoke (this, e);
 		}
 		
 		void OnItemRemoved (SolutionItemChangeEventArgs e, bool removedFromSolution)
@@ -931,108 +799,88 @@ namespace MonoDevelop.Projects
 		}
 		
 		void OnItemRemoved (SolutionItemChangeEventArgs e)
-		{
-			if (ItemRemoved != null)
-				ItemRemoved (this, e);
+		{

+			ItemRemoved?.Invoke (this, e);
 		}
 		
 		void OnFileRemovedFromProject (ProjectFileEventArgs e)
 		{
 			if (ParentFolder == null && ParentSolution != null)
-				ParentSolution.OnFileRemovedFromProject (e);
-			if (FileRemovedFromProject != null) {
-				FileRemovedFromProject (this, e);
-			}
+				ParentSolution.OnFileRemovedFromProject (e);

+			FileRemovedFromProject?.Invoke (this, e);
 		}
 
 		void OnFileChangedInProject (ProjectFileEventArgs e)
 		{
 			if (ParentFolder == null && ParentSolution != null)
-				ParentSolution.OnFileChangedInProject (e);
-			if (FileChangedInProject != null) {
-				FileChangedInProject (this, e);
-			}
+				ParentSolution.OnFileChangedInProject (e);

+			FileChangedInProject?.Invoke (this, e);
 		}
 		
 		void OnFilePropertyChangedInProject (ProjectFileEventArgs e)
 		{
 			if (ParentFolder == null && ParentSolution != null)
-				ParentSolution.OnFilePropertyChangedInProject (e);
-			if (FilePropertyChangedInProject != null) {
-				FilePropertyChangedInProject (this, e);
-			}
+				ParentSolution.OnFilePropertyChangedInProject (e);

+			FilePropertyChangedInProject?.Invoke (this, e);
 		}
 		
 		void OnFileAddedToProject (ProjectFileEventArgs e)
 		{
 			if (ParentFolder == null && ParentSolution != null)
-				ParentSolution.OnFileAddedToProject (e);
-			if (FileAddedToProject != null) {
-				FileAddedToProject (this, e);
-			}
+				ParentSolution.OnFileAddedToProject (e);

+			FileAddedToProject?.Invoke (this, e);
 		}
 		
 		void OnFileRenamedInProject (ProjectFileRenamedEventArgs e)
 		{
 			if (ParentFolder == null && ParentSolution != null)
-				ParentSolution.OnFileRenamedInProject (e);
-			if (FileRenamedInProject != null) {
-				FileRenamedInProject (this, e);
-			}
+				ParentSolution.OnFileRenamedInProject (e);

+			FileRenamedInProject?.Invoke (this, e);
 		}
 		
 		void OnReferenceRemovedFromProject (ProjectReferenceEventArgs e)
 		{
 			if (ParentFolder == null && ParentSolution != null)
-				ParentSolution.OnReferenceRemovedFromProject (e);
-			if (ReferenceRemovedFromProject != null) {
-				ReferenceRemovedFromProject (this, e);
-			}
+				ParentSolution.OnReferenceRemovedFromProject (e);

+			ReferenceRemovedFromProject?.Invoke (this, e);
 		}
 		
 		void OnReferenceAddedToProject (ProjectReferenceEventArgs e)
 		{
 			if (ParentFolder == null && ParentSolution != null)
-				ParentSolution.OnReferenceAddedToProject (e);
-			if (ReferenceAddedToProject != null) {
-				ReferenceAddedToProject (this, e);
-			}
+				ParentSolution.OnReferenceAddedToProject (e);

+			ReferenceAddedToProject?.Invoke (this, e);
 		}
 
 		void OnItemModified (SolutionItemModifiedEventArgs e)
 		{
 			if (ParentFolder == null && ParentSolution != null)
-				ParentSolution.OnEntryModified (e);
-			if (ItemModified != null)
-				ItemModified (this, e);
+				ParentSolution.OnEntryModified (e);

+			ItemModified?.Invoke (this, e);
 		}
 		
 		void OnItemSaved (SolutionItemSavedEventArgs e)
 		{
 			if (ParentFolder == null && ParentSolution != null)
-				ParentSolution.OnEntrySaved (e);
-			if (ItemSaved != null)
-				ItemSaved (this, e);
+				ParentSolution.OnEntrySaved (e);

+			ItemSaved?.Invoke (this, e);
 		}
 		
 		void OnSolutionItemFileAdded (SolutionItemFileEventArgs args)
-		{
-			if (SolutionItemFileAdded != null)
-				SolutionItemFileAdded (this, args);
+		{

+			SolutionItemFileAdded?.Invoke (this, args);
 		}
 		
 		void OnSolutionItemFileRemoved (SolutionItemFileEventArgs args)
-		{
-			if (SolutionItemFileRemoved != null)
-				SolutionItemFileRemoved (this, args);
+		{

+			SolutionItemFileRemoved?.Invoke (this, args);
 		}
 		
 		void OnItemReloadRequired (SolutionItemEventArgs e)
 		{
 			if (ParentFolder == null && ParentSolution != null)
-				ParentSolution.OnItemReloadRequired (e);
-			if (ItemReloadRequired != null)
-				ItemReloadRequired (this, e);
+				ParentSolution.OnItemReloadRequired (e);

+			ItemReloadRequired?.Invoke (this, e);
 		}
 		
 		public event SolutionItemChangeEventHandler ItemAdded;
@@ -1105,7 +953,7 @@ namespace MonoDevelop.Projects
 		
 		protected override void ClearItems ()
 		{
-			FilePath[] files = new FilePath [Count];
+			var files = new FilePath [Count];
 			CopyTo (files, 0);
 			base.ClearItems();
 			parent.NotifyFilesRemoved (files);
@@ -1134,18 +982,14 @@ namespace MonoDevelop.Projects
 	}
 	
 	public class SolutionItemFileEventArgs: EventArgs
-	{
-		FilePath file;
-		
+	{

 		public SolutionItemFileEventArgs (FilePath file)
 		{
-			this.file = file;
-		}
-
-		public FilePath File {
-			get { return this.file; }
-		}
-	}
+			File = file;
+		}

+

+        public FilePath File { get; }

+    }
 
 	/// <summary>
 	/// Keeps track of slots available for executing an operation
diff --git a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects/SolutionItem.cs b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects/SolutionItem.cs
index e24972f81f..b88e6a0fad 100644
--- a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects/SolutionItem.cs
+++ b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects/SolutionItem.cs
@@ -618,7 +618,7 @@ namespace MonoDevelop.Projects
 				try {
 					SolutionItemConfiguration iconf = GetConfiguration (solutionConfiguration);
 					string confName = iconf != null ? iconf.Id : solutionConfiguration.ToString ();
-					monitor.BeginTask (GettextCatalog.GetString ("Building: {0} ({1})", Name, confName), 1);
+					monitor.BeginTask (GettextCatalog.GetString ("Building {0} ({1})", Name, confName), 1);
 
 					operationStarted = ParentSolution != null && await ParentSolution.BeginBuildOperation (monitor, solutionConfiguration, operationContext);
 
@@ -635,35 +635,19 @@ namespace MonoDevelop.Projects
 				return result;
 			}
 
+			if (ParentSolution == null) {

+				throw new InvalidOperationException ("Cannot build SolutionItem without parent solution");

+			}
+
 			ITimeTracker tt = Counters.BuildProjectAndReferencesTimer.BeginTiming ("Building " + Name, CreateProjectEventMetadata (solutionConfiguration));
 			try {
-				operationStarted = ParentSolution != null && await ParentSolution.BeginBuildOperation (monitor, solutionConfiguration, operationContext);
-
-				// Get a list of all items that need to be built (including this),
-				// and build them in the correct order
-
-				var referenced = new List<SolutionItem> ();
-				var visited = new Set<SolutionItem> ();
-				GetBuildableReferencedItems (visited, referenced, this, solutionConfiguration);
-				if (Runtime.Preferences.SkipBuildingUnmodifiedProjects)
-					referenced.RemoveAll (si => {
-						if (si is Project p)
-							return !p.FastCheckNeedsBuild (solutionConfiguration);
-						return false;//Don't filter things that don't have FastCheckNeedsBuild
-					});
-				var sortedReferenced = TopologicalSort (referenced, solutionConfiguration);
-
-				SolutionItemConfiguration iconf = GetConfiguration (solutionConfiguration);
-				string confName = iconf != null ? iconf.Id : solutionConfiguration.ToString ();
-				monitor.BeginTask (GettextCatalog.GetString ("Building: {0} ({1})", Name, confName), sortedReferenced.Count);
-
-				result = await SolutionFolder.RunParallelBuildOperation (monitor, solutionConfiguration, sortedReferenced, (ProgressMonitor m, SolutionItem item) => {
-					return item.Build (m, solutionConfiguration, false, operationContext);
-				}, false);
+				operationStarted = await ParentSolution.BeginBuildOperation (monitor, solutionConfiguration, operationContext);
+				result = await ParentSolution.BuildItems (
+					monitor, solutionConfiguration, new[] { this }, operationContext,
+					GettextCatalog.GetString ("Building {0} ({1})", Name, solutionConfiguration.ToString ())
+				);
 			} finally {
-				monitor.EndTask ();
 				tt.End ();
-
 				if (operationStarted)
 					await ParentSolution.EndBuildOperation (monitor, solutionConfiguration, operationContext, result);
 			}
@@ -763,7 +747,7 @@ namespace MonoDevelop.Projects
 				try {
 					SolutionItemConfiguration iconf = GetConfiguration (configuration);
 					string confName = iconf != null ? iconf.Id : configuration.ToString ();
-					monitor.BeginTask (GettextCatalog.GetString ("Cleaning: {0} ({1})", Name, confName), 1);
+					monitor.BeginTask (GettextCatalog.GetString ("Cleaning {0} ({1})", Name, confName), 1);
 
 					SolutionItemConfiguration conf = GetConfiguration (configuration);
 					if (conf != null) {
diff --git a/main/src/core/MonoDevelop.Ide/MonoDevelop.Ide.csproj b/main/src/core/MonoDevelop.Ide/MonoDevelop.Ide.csproj
index cd13822a7c..5d77650939 100644
--- a/main/src/core/MonoDevelop.Ide/MonoDevelop.Ide.csproj
+++ b/main/src/core/MonoDevelop.Ide/MonoDevelop.Ide.csproj
@@ -3138,6 +3138,7 @@
     <Compile Include="MonoDevelop.Ide.Gui\ProgressMonitors.cs" />
     <Compile Include="MonoDevelop.Ide.TypeSystem\IMonoDevelopHostDocument.cs" />
     <Compile Include="MonoDevelop.Ide.TypeSystem\MonoDevelopPersistentStorageLocationService.cs" />
+    <Compile Include="MonoDevelop.Ide\ProjectOperations.SolutionItemBuildBatch.cs" />
     <Compile Include="MonoDevelop.Ide\RoslynLogger.cs" />
     <Compile Include="MonoDevelop.Ide\Services.cs" />
     <Compile Include="MonoDevelop.Ide.Gui.OptionPanels\TasksOptionsPanel.cs" />
diff --git a/main/src/core/MonoDevelop.Ide/MonoDevelop.Ide/ProjectOperations.SolutionItemBuildBatch.cs b/main/src/core/MonoDevelop.Ide/MonoDevelop.Ide/ProjectOperations.SolutionItemBuildBatch.cs
new file mode 100644
index 0000000000..d38d774908
--- /dev/null
+++ b/main/src/core/MonoDevelop.Ide/MonoDevelop.Ide/ProjectOperations.SolutionItemBuildBatch.cs
@@ -0,0 +1,133 @@
+//

+// Copyright (C) Microsoft Corp.

+//

+// Permission is hereby granted, free of charge, to any person obtaining

+// a copy of this software and associated documentation files (the

+// "Software"), to deal in the Software without restriction, including

+// without limitation the rights to use, copy, modify, merge, publish,

+// distribute, sublicense, and/or sell copies of the Software, and to

+// permit persons to whom the Software is furnished to do so, subject to

+// the following conditions:

+// 

+// The above copyright notice and this permission notice shall be

+// included in all copies or substantial portions of the Software.

+// 

+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,

+// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF

+// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND

+// NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE

+// LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION

+// OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION

+// WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

+//

+

+using System;

+using System.Collections.Generic;

+using System.Linq;

+using System.Threading.Tasks;

+using MonoDevelop.Core;

+using MonoDevelop.Projects;

+

+namespace MonoDevelop.Ide
+{

+	/// <summary>
+	/// This is the basic interface to the workspace.
+	/// </summary>
+	partial class ProjectOperations

+	{

+		/// <summary>

+		/// Represents a group of solution items being built together.

+		/// </summary>

+		class SolutionItemBuildBatch : IBuildTarget

+		{

+			string name;

+			Solution sln;

+

+			/// <summary>

+			/// Simplifies a group of build targets

+			/// </summary>

+			public static IBuildTarget Create (IEnumerable<IBuildTarget> targets)

+			{

+				Solution sln = null;

+				var buildTargets = new HashSet<SolutionItem> ();

+

+				foreach (var target in targets) {

+					if (target is SolutionItem si) {

+						var parent = si.ParentSolution;

+						if (parent == null) {

+							throw new InvalidOperationException ("Items must be part of a solution");

+						}

+						if (sln != null && sln != parent) {

+							throw new InvalidOperationException ("All items must be in the same solution");

+						} else {

+							sln = parent;

+						}

+						buildTargets.Add (si);

+						continue;

+					}

+					if (target is Solution s) {

+						if (sln != null && sln != s) {

+							throw new InvalidOperationException ("All items must be in the same solution");

+						}

+						return sln;

+					}

+				}

+				if (buildTargets.Count == 1) {

+					return buildTargets.First ();

+				}

+				return new SolutionItemBuildBatch (sln, buildTargets);

+			}

+

+			public ICollection<SolutionItem> Items { get; }

+

+			SolutionItemBuildBatch (Solution sln, ICollection<SolutionItem> items)

+			{

+				this.sln = sln;

+				Items = items;

+			}

+

+			public string Name => name ?? (name = string.Join (";", Items.Select (s => s.Name)));

+

+			public bool CanBuild (ConfigurationSelector configuration)

+			{

+				return Items.All (item => ((IBuildTarget)item).CanBuild (configuration));

+			}

+

+			public Task<BuildResult> Build (ProgressMonitor monitor, ConfigurationSelector configuration, bool buildReferencedTargets = false, OperationContext operationContext = null)

+			{

+				return sln.BuildItems (monitor, configuration, Items);

+			}

+

+			public Task<BuildResult> Clean (ProgressMonitor monitor, ConfigurationSelector configuration, OperationContext operationContext = null)

+			{

+				return sln.CleanItems (monitor, configuration, Items);

+			}

+

+			public bool CanExecute (ExecutionContext context, ConfigurationSelector configuration)

+			{

+				throw new NotSupportedException ("Execution not supported for build groups");

+			}

+

+			public Task Execute (ProgressMonitor monitor, ExecutionContext context, ConfigurationSelector configuration)

+			{

+				throw new NotSupportedException ("Execution not supported for build groups");

+			}

+

+			public IEnumerable<IBuildTarget> GetExecutionDependencies ()

+			{

+				throw new NotSupportedException ("Execution not supported for build groups");

+			}

+

+			public Task PrepareExecution (ProgressMonitor monitor, ExecutionContext context, ConfigurationSelector configuration)

+			{

+				throw new NotSupportedException ("Execution not supported for build groups");

+			}

+

+			[Obsolete]

+			public bool NeedsBuilding (ConfigurationSelector configuration)

+			{

+				return Items.Any (item => ((IBuildTarget)item).NeedsBuilding (configuration));

+			}

+		}

+	}

+}
\ No newline at end of file
diff --git a/main/src/core/MonoDevelop.Ide/MonoDevelop.Ide/ProjectOperations.cs b/main/src/core/MonoDevelop.Ide/MonoDevelop.Ide/ProjectOperations.cs
index b4d4f2e968..e9571d232a 100644
--- a/main/src/core/MonoDevelop.Ide/MonoDevelop.Ide/ProjectOperations.cs
+++ b/main/src/core/MonoDevelop.Ide/MonoDevelop.Ide/ProjectOperations.cs
@@ -1,69 +1,60 @@
-//
-// ProjectOperations.cs
-//
-// Author:
-//   Lluis Sanchez Gual
-//
-// Copyright (C) 2005 Novell, Inc (http://www.novell.com)
-//
-// Permission is hereby granted, free of charge, to any person obtaining
-// a copy of this software and associated documentation files (the
-// "Software"), to deal in the Software without restriction, including
-// without limitation the rights to use, copy, modify, merge, publish,
-// distribute, sublicense, and/or sell copies of the Software, and to
-// permit persons to whom the Software is furnished to do so, subject to
-// the following conditions:
-// 
-// The above copyright notice and this permission notice shall be
-// included in all copies or substantial portions of the Software.
-// 
-// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
-// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
-// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
-// NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
-// LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
-// OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
-// WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
-//
-
-
+//

+// ProjectOperations.cs

+//

+// Author:

+//   Lluis Sanchez Gual

+//

+// Copyright (C) 2005 Novell, Inc (http://www.novell.com)

+//

+// Permission is hereby granted, free of charge, to any person obtaining

+// a copy of this software and associated documentation files (the

+// "Software"), to deal in the Software without restriction, including

+// without limitation the rights to use, copy, modify, merge, publish,

+// distribute, sublicense, and/or sell copies of the Software, and to

+// permit persons to whom the Software is furnished to do so, subject to

+// the following conditions:

+// 

+// The above copyright notice and this permission notice shall be

+// included in all copies or substantial portions of the Software.

+// 

+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,

+// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF

+// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND

+// NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE

+// LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION

+// OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION

+// WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

+//

+

 using System;
-using System.Linq;
-using System.Collections;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.IO;
-
-using MonoDevelop.Projects;
-using MonoDevelop.Projects.Text;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
 using MonoDevelop.Components;
 using MonoDevelop.Core;
 using MonoDevelop.Core.Execution;
-using MonoDevelop.Core.ProgressMonitoring;
-using MonoDevelop.Ide.ProgressMonitoring;
-using MonoDevelop.Ide.Gui.Dialogs;
+using MonoDevelop.Core.Instrumentation;
+using MonoDevelop.Core.Text;
+using MonoDevelop.Ide.Editor;
 using MonoDevelop.Ide.Gui;
+using MonoDevelop.Ide.Gui.Dialogs;
+using MonoDevelop.Ide.ProgressMonitoring;
 using MonoDevelop.Ide.Projects;
-using MonoDevelop.Core.Assemblies;
-using MonoDevelop.Core.Instrumentation;
-using System.Diagnostics;
-using System.Text;
-using MonoDevelop.Ide.TypeSystem;
-using System.Threading.Tasks;
-using System.Threading;
-using ExecutionContext = MonoDevelop.Projects.ExecutionContext;
 using MonoDevelop.Ide.Tasks;
+using MonoDevelop.Ide.TypeSystem;
+using MonoDevelop.Projects;
 using MonoDevelop.Projects.MSBuild;
-using System.Collections.Immutable;
-using MonoDevelop.Ide.Editor;
-using MonoDevelop.Core.Text;
-using MonoDevelop.Components.Extensions;
+using ExecutionContext = MonoDevelop.Projects.ExecutionContext;
 
 namespace MonoDevelop.Ide
-{
+{

 	/// <summary>
 	/// This is the basic interface to the workspace.
 	/// </summary>
-	public class ProjectOperations
+	public partial class ProjectOperations
 	{
 		AsyncOperation<BuildResult> currentBuildOperation = new AsyncOperation<BuildResult> (Task.FromResult (BuildResult.CreateSuccess ()), null);
 		MultipleAsyncOperation currentRunOperation = MultipleAsyncOperation.CompleteMultipleOperation;
@@ -1355,142 +1346,201 @@ namespace MonoDevelop.Ide
 			}
 		}
 
-		async Task<bool> CheckAndBuildForExecute (IBuildTarget executionTarget, ExecutionContext context, ConfigurationSelector configuration, RunConfiguration runConfiguration)
+		Task<bool> CheckAndBuildForExecute (IBuildTarget executionTarget, ExecutionContext context, ConfigurationSelector configuration, RunConfiguration runConfiguration)
 		{
+			var buildTarget = executionTarget;
+
+			// When executing a solution we are actually going to execute the startup project. So we only need to build that project.
+			// TODO: handle multi-startup solutions.
+			if (buildTarget is Solution sol && sol.StartupItem != null)
+				buildTarget = sol.StartupItem;
+
+			return CheckAndBuildForExecute (
+				new [] { buildTarget }, configuration,
+				IdeApp.Preferences.BuildBeforeExecuting, IdeApp.Preferences.RunWithWarnings,
+				(target, monitor) => {
+					if (target is IRunTarget)
+						return ((IRunTarget)target).PrepareExecution (monitor, context, configuration, runConfiguration);
+					return target.PrepareExecution (monitor, context, configuration);
+				}
+			);
+		}
+
+		/// <summary>
+		/// Prepares projects/solutions for execution by building them and their execution dependencies if necessary.
+		/// </summary>
+		/// <returns>Whether the operation was successful.</returns>
+		/// <param name="executionTargets">The projects and/or solution to build. If there are multiple projects, they must be in the same solution.</param>
+		/// <param name="configuration">The configuration selector.</param>
+		/// <param name="buildWithoutPrompting">Whether to prompt the user before building, when building is necessary.</param>
+		/// <param name="cancelOnWarning">Whether to cancel the execution operation if there is a build warning.</param>
+		/// <param name="createPrepareExecutionTask">
+		/// May be executed in parallel with the build to perform additional
+		/// preparation that does not depend on the build, such as launching a simulator.
+		/// There is no guaranteed this will be executed for any target.
+		/// </param>
+		public async Task<bool> CheckAndBuildForExecute (
+			ICollection<IBuildTarget> executionTargets, ConfigurationSelector configuration,
+			bool buildWithoutPrompting = true, bool cancelOnWarning = false,
+			Func<IBuildTarget, ProgressMonitor,Task> createPrepareExecutionTask = null,
+			CancellationToken? token = null)
+		{
+			if (executionTargets.Count == 0) {

+				throw new ArgumentException ("No execution targets specified", nameof (executionTargets)); ;

+			}

+

 			if (currentBuildOperation != null && !currentBuildOperation.IsCompleted) {
 				var bres = await currentBuildOperation.Task;
-				if (bres.HasErrors || !IdeApp.Preferences.RunWithWarnings && bres.HasWarnings)
+				if (bres.HasErrors || (cancelOnWarning && bres.HasWarnings))
 					return false;
 			}
 
 			//saves open documents since it may dirty the "needs building" check
 			var r = await DoBeforeCompileAction ();
 			if (r.Failed)
-				return false;
-
-			var buildTarget = executionTarget;
-
-			// When executing a solution we are actually going to execute the starup project. So we only need to build that project.
-			// TODO: handle multi-startup solutions.
-			var sol = buildTarget as Solution;
-			if (sol != null && sol.StartupItem != null)
-				buildTarget = sol.StartupItem;
-			
-			var buildDeps = buildTarget.GetExecutionDependencies ().ToList ();
-			if (buildDeps.Count > 1)
-				throw new NotImplementedException ("Multiple execution dependencies not yet supported");
-			if (buildDeps.Count != 0)
-				buildTarget = buildDeps [0];
-
-			bool needsBuild = FastCheckNeedsBuild (buildTarget, configuration);
-			if (!needsBuild) {
-				return true;
+				return false;

+

+			IBuildTarget buildTarget = SolutionItemBuildBatch.Create (executionTargets.SelectMany (et => et.GetExecutionDependencies ()));

+

+			if (!FastCheckNeedsBuild (buildTarget, configuration)) {

+				return true;

+			}
+
+			if (!buildWithoutPrompting) {

+				var ret = PromptToBuild ();
+				if (ret.HasValue) {

+					return ret.Value;

+				}
 			}
 
-			if (IdeApp.Preferences.BuildBeforeExecuting) {
-				// Building the project may take some time, so we call PrepareExecution so that the target can
-				// prepare the execution (for example, it could start a simulator).
-				var cs = new CancellationTokenSource ();
-				Task prepareExecution;
-				if (buildTarget is IRunTarget)
-					prepareExecution = ((IRunTarget)buildTarget).PrepareExecution (new ProgressMonitor ().WithCancellationSource (cs), context, configuration, runConfiguration);
-				else
-					prepareExecution = buildTarget.PrepareExecution (new ProgressMonitor ().WithCancellationSource (cs), context, configuration);
-				
-				var result = await Build (buildTarget, true).Task;
+			CancellationTokenSource prepareOpTokenSource = null;
 
-				if (result.HasErrors || (!IdeApp.Preferences.RunWithWarnings && result.HasWarnings)) {
-					cs.Cancel ();
-					return false;
-				}
-				else {
-					await prepareExecution;
-					return true;
-				}
+			// Building the project may take some time, so we call PrepareExecution so that the target can
+			// prepare the execution while the build is in progress (for example, it could start a simulator).
+			// As a simple way to avoid starvation, if there are multiple, we run them in sequence.
+			bool building = true;
+			Task prepareExecutionTask = null;
+			if (createPrepareExecutionTask != null) {
+				prepareOpTokenSource = token != null
+					? CancellationTokenSource.CreateLinkedTokenSource (token.Value)
+					: new CancellationTokenSource ();
+				prepareExecutionTask = RunPrepareExecutionTasks ();
 			}
 
-			var bBuild = new AlertButton (GettextCatalog.GetString ("Build"));
-			var bRun = new AlertButton (Gtk.Stock.Execute, true);
-			var res = MessageService.AskQuestion (
-				GettextCatalog.GetString ("Outdated Build"),
-				GettextCatalog.GetString ("The project you are executing has changes done after the last time it was compiled. Do you want to continue?"),
-				1,
-				AlertButton.Cancel,
-				bBuild,
-				bRun);
+			BuildResult result = await Build (buildTarget, token).Task;
 
-			// This call is a workaround for bug #6907. Without it, the main monodevelop window is left it a weird
-			// drawing state after the message dialog is shown. This may be a gtk/mac issue. Still under research.
-			DispatchService.RunPendingEvents ();
+			if (result.HasErrors || (cancelOnWarning && result.HasWarnings)) {
+				prepareOpTokenSource?.Cancel ();
+				return false;
+			}
 
-			if (res == bRun) {
-				return true;
+			building = false;
+			if (prepareExecutionTask != null) {
+				await prepareExecutionTask;
 			}
 
-			if (res == bBuild) {
-				// Building the project may take some time, so we call PrepareExecution so that the target can
-				// prepare the execution (for example, it could start a simulator).
-				var cs = new CancellationTokenSource ();
+			return true;
 
-				Task prepareExecution;
-				if (buildTarget is IRunTarget)
-					prepareExecution = ((IRunTarget)buildTarget).PrepareExecution (new ProgressMonitor ().WithCancellationSource (cs), context, configuration, runConfiguration);
-				else
-					prepareExecution = buildTarget.PrepareExecution (new ProgressMonitor ().WithCancellationSource (cs), context, configuration);
-				
-				var result = await Build (buildTarget, true).Task;
+			async Task RunPrepareExecutionTasks ()
+			{
+				var targetsToPrepare = new Queue<IBuildTarget> (executionTargets);
 
-				if (result.HasErrors || (!IdeApp.Preferences.RunWithWarnings && result.HasWarnings)) {
-					cs.Cancel ();
-					return false;
+				while (targetsToPrepare.Count > 0 && building) {
+					var target = targetsToPrepare.Dequeue ();
+					var monitor = new ProgressMonitor ().WithCancellationSource (prepareOpTokenSource);
+					await createPrepareExecutionTask (target, monitor);
 				}
-				else {
-					await prepareExecution;
-					return true;
+			}
+		}

+

+		/// <summary>

+		/// Prompts the user whether they want to build the project

+		/// </summary>

+		/// <returns>True to execute without building, false to cancel, null to build.</returns>

+		static bool? PromptToBuild ()

+		{

+			var bBuild = new AlertButton (GettextCatalog.GetString ("Build"));

+			var bRun = new AlertButton (Gtk.Stock.Execute, true);

+			var res = MessageService.AskQuestion (

+				GettextCatalog.GetString ("Outdated Build"),

+				GettextCatalog.GetString ("The project you are executing has changes done after the last time it was compiled. Do you want to continue?"),

+				1,

+				AlertButton.Cancel,

+				bBuild,

+				bRun);

+

+			// This call is a workaround for bug #6907. Without it, the main monodevelop window is left it a weird

+			// drawing state after the message dialog is shown. This may be a gtk/mac issue. Still under research.

+			DispatchService.RunPendingEvents ();

+

+			if (res == bRun) {

+				return true;

+			}

+

+			if (res == bBuild) {

+				return null;

+			}

+

+			return false;

+		}

+

+		/// <summary>
+		/// Given a build target, determines whether it or its dependencies needs to be built.
+		/// </summary>
+		/// <param name="target">The build target to check.</param>
+		/// <param name="configuration">The build configuration selector.</param>
+		static bool FastCheckNeedsBuild (IBuildTarget target, ConfigurationSelector configuration)
+		{
+			if (FastBuildCheckDisabled ()) {

+				return true;

+			}
+
+			IEnumerable<SolutionItem> items;
+
+			switch (target) {
+			case Project proj: {

+					var deps = new HashSet<SolutionItem> { proj };

+					CollectDependencies (proj, deps, configuration);

+					items = deps;

+					break;

+				}

+			case SolutionItemBuildBatch batch: {

+					var deps = new HashSet<SolutionItem> ();

+					foreach (var item in batch.Items) {

+						deps.Add (item);

+						CollectDependencies (item, deps, configuration);

+					}

+					items = deps;

+					break;

 				}
+			case Solution sln:
+				items = sln.GetAllSolutionItems ();
+				break;
+			default:
+				return true;
 			}
 
+			foreach (var item in items) {

+				if (!(item is Project p) || p.FastCheckNeedsBuild (configuration, InitOperationContext (target, new TargetEvaluationContext ()))) {

+					return true;

+				}

+			}

+

 			return false;
-		}
-			
-		bool FastCheckNeedsBuild (IBuildTarget target, ConfigurationSelector configuration)
-		{
+		}

+

+		static bool FastBuildCheckDisabled ()

+		{

 			var env = Environment.GetEnvironmentVariable ("DisableFastUpToDateCheck");
-			if (!string.IsNullOrEmpty (env) && env != "0" && !env.Equals ("false", StringComparison.OrdinalIgnoreCase))
-				return true;
-
-			var sei = target as Project;
-			if (sei != null) {
-				if (sei.FastCheckNeedsBuild (configuration, InitOperationContext (target, new TargetEvaluationContext ())))
-					return true;
-				//TODO: respect solution level dependencies
-				var deps = new HashSet<SolutionItem> ();
-				CollectReferencedItems (sei, deps, configuration);
-				foreach (var dep in deps.OfType<Project> ()) {
-					if (dep.FastCheckNeedsBuild (configuration, InitOperationContext (target, new TargetEvaluationContext ())))
-						return true;
-				}
-				return false;
-			}
-
-			var sln = target as Solution;
-			if (sln != null) {
-				foreach (var item in sln.GetAllProjects ()) {
-					if (item.FastCheckNeedsBuild (configuration, InitOperationContext (target, new TargetEvaluationContext ())))
-						return true;
-				}
-				return false;
-			}
-
-			//TODO: handle other IBuildTargets
-			return true;
-		}
-
-		void CollectReferencedItems (SolutionItem item, HashSet<SolutionItem> collected, ConfigurationSelector configuration)
+			return !string.IsNullOrEmpty (env) && env != "0" && !env.Equals ("false", StringComparison.OrdinalIgnoreCase);

+		}

+

+		//TODO: respect solution level dependencies

+		static void CollectDependencies (SolutionItem item, HashSet<SolutionItem> collected, ConfigurationSelector configuration)
 		{
 			foreach (var refItem in item.GetReferencedItems (configuration)) {
 				if (collected.Add (refItem)) {
-					CollectReferencedItems (refItem, collected, configuration);
+					CollectDependencies (refItem, collected, configuration);
 				}
 			}
 		}
@@ -1565,7 +1615,7 @@ namespace MonoDevelop.Ide
 		/// Initializes the context to be used for build operations. It currently just initializes
 		/// it with the currently selected execution target.
 		/// </summary>
-		T InitOperationContext<T> (IBuildTarget target, T context) where T:OperationContext
+		static T InitOperationContext<T> (IBuildTarget target, T context) where T:OperationContext
 		{
 			OperationContext ctx = context;
 			if (ctx == null)
diff --git a/main/tests/MonoDevelop.Core.Tests/MonoDevelop.Projects/ProjectBuildTests.cs b/main/tests/MonoDevelop.Core.Tests/MonoDevelop.Projects/ProjectBuildTests.cs
index 8a607ff712..e547f9deb2 100644
--- a/main/tests/MonoDevelop.Core.Tests/MonoDevelop.Projects/ProjectBuildTests.cs
+++ b/main/tests/MonoDevelop.Core.Tests/MonoDevelop.Projects/ProjectBuildTests.cs
@@ -319,12 +319,27 @@ namespace MonoDevelop.Projects
 			sol.Dispose();
 		}
 
+		static Solution WrapSolutionItem (SolutionItem item)
+		{
+			var sol = new Solution ();
+			var config = sol.AddConfiguration ("Debug", true);
+			sol.RootFolder.Items.Add (item);
+			config.GetEntryForItem (item).Build = true;
+			return sol;
+		}
+
+		async static Task<(Solution sln, T item)> LoadSampleSolutionItem<T> (params string [] projectName) where T : SolutionItem
+		{
+			string projFile = Util.GetSampleProject (projectName);
+			var item = (T) await Services.ProjectService.ReadSolutionItem (Util.GetMonitor (), projFile);
+			var sol = WrapSolutionItem (item);
+			return (sol, item);
+		}
+
 		[Test]
 		public async Task BuildWithCustomProps ()
 		{
-			string projFile = Util.GetSampleProject ("msbuild-tests", "project-with-custom-build-target.csproj");
-			var p = (Project)await Services.ProjectService.ReadSolutionItem (Util.GetMonitor (), projFile);
-
+			(var sol, var p) = await LoadSampleSolutionItem<Project> ("msbuild-tests", "project-with-custom-build-target.csproj");
 			var ctx = new ProjectOperationContext ();
 			ctx.GlobalProperties.SetValue ("TestProp", "foo");
 			var res = await p.Build (Util.GetMonitor (), p.Configurations [0].Selector, ctx);
@@ -340,6 +355,7 @@ namespace MonoDevelop.Projects
 			Assert.AreEqual ("Something failed: show", res.Errors [0].ErrorText);
 
 			p.Dispose ();
+			sol.Dispose ();
 		}
 
 		/// <summary>
@@ -350,8 +366,7 @@ namespace MonoDevelop.Projects
 		[Platform (Exclude = "Win")]
 		public async Task BuildWithCustomProps2 ()
 		{
-			string projFile = Util.GetSampleProject ("msbuild-tests", "project-with-custom-build-target2.csproj");
-			var p = (Project)await Services.ProjectService.ReadSolutionItem (Util.GetMonitor (), projFile);
+			(var sol, var p) = await LoadSampleSolutionItem<Project> ("msbuild-tests", "project-with-custom-build-target2.csproj");
 
 			var ctx = new ProjectOperationContext ();
 			ctx.GlobalProperties.SetValue ("TestProp", "foo");
@@ -368,6 +383,7 @@ namespace MonoDevelop.Projects
 			Assert.AreEqual ("Something failed (show.targets): show", res.Errors [0].ErrorText);
 
 			p.Dispose ();
+			sol.Dispose ();
 		}
 
 		/// <summary>
@@ -379,8 +395,7 @@ namespace MonoDevelop.Projects
 		[Platform (Exclude = "Win")]
 		public async Task BuildWithCustomProps3 ()
 		{
-			string projFile = Util.GetSampleProject ("msbuild-tests", "project-with-custom-build-target3.csproj");
-			var p = (Project)await Services.ProjectService.ReadSolutionItem (Util.GetMonitor (), projFile);
+			(var sol, var p) = await LoadSampleSolutionItem<Project> ("msbuild-tests", "project-with-custom-build-target3.csproj");
 
 			var ctx = new ProjectOperationContext ();
 			ctx.GlobalProperties.SetValue ("BuildingInsideVisualStudio", "false");
@@ -397,6 +412,7 @@ namespace MonoDevelop.Projects
 			Assert.AreEqual ("Something failed (true.targets): true", res.Errors [0].ErrorText);
 
 			p.Dispose ();
+			sol.Dispose ();
 		}
 
 		[Test]
@@ -509,7 +525,7 @@ namespace MonoDevelop.Projects
 		}
 
 		[Test ()]
-		public async Task ProjectReferencingDisabledProject_ProjectBuildFails ()
+		public async Task ProjectReferencingDisabledProject_ProjectBuildWorks ()
 		{
 			// If a project references another project that is disabled for the solution configuration, the referenced
 			// project should build when directly building the referencing project.
@@ -520,7 +536,7 @@ namespace MonoDevelop.Projects
 			var p = sol.Items.FirstOrDefault (pr => pr.Name == "ReferencingProject");
 
 			var res = await p.Build (Util.GetMonitor (), (SolutionConfigurationSelector)"Debug", true);
-			Assert.AreEqual (1, res.ErrorCount);
+			Assert.AreEqual (1, res.BuildCount);
 
 			sol.Dispose ();
 		}