[Core] Make MSBuildProject thread safe for reading

12 files changed, 145 insertions, 79 deletions

diff --git a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildChoose.cs b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildChoose.cs
index 036680196d..dec11b4c0a 100644
--- a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildChoose.cs
+++ b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildChoose.cs
@@ -42,7 +42,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 			if (op != null) {
 				op.ParentNode = this;
 				op.Read (reader);
-				ChildNodes.Add (op);
+				ChildNodes = ChildNodes.Add (op);
 			} else
 				base.ReadChildElement (reader);
 		}
@@ -86,7 +86,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 			if (ob != null) {
 				ob.ParentNode = this;
 				ob.Read (reader);
-				ChildNodes.Add (ob);
+				ChildNodes = ChildNodes.Add (ob);
 			} else
 				reader.Read ();
 		}
diff --git a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildImport.cs b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildImport.cs
index 5db3d39c11..91bcbeaa9a 100644
--- a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildImport.cs
+++ b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildImport.cs
@@ -64,7 +64,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 
 		public string Project {
 			get { return target; }
-			set { target = value; NotifyChanged (); }
+			set { AssertCanModify (); target = value; NotifyChanged (); }
 		}
 
 		public string EvaluatedProject {
diff --git a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildImportGroup.cs b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildImportGroup.cs
index 58b801e693..afc3d07757 100644
--- a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildImportGroup.cs
+++ b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildImportGroup.cs
@@ -38,7 +38,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 				var item = new MSBuildImport ();
 				item.ParentNode = this;
 				item.Read (reader);
-				ChildNodes.Add (item);
+				ChildNodes = ChildNodes.Add (item);
 			} else
 				base.ReadChildElement (reader);
 		}
@@ -55,6 +55,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 
 		public MSBuildImport AddNewImport (string name, string condition = null, MSBuildImport beforeImport = null)
 		{
+			AssertCanModify ();
 			var import = new MSBuildImport ();
 			import.Project = name;
 			import.Condition = condition;
@@ -64,9 +65,9 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 				insertIndex = ChildNodes.IndexOf (beforeImport);
 
 			if (insertIndex != -1)
-				ChildNodes.Insert (insertIndex, import);
+				ChildNodes = ChildNodes.Insert (insertIndex, import);
 			else
-				ChildNodes.Add (import);
+				ChildNodes = ChildNodes.Add (import);
 
 			import.ResetIndent (false);
 			NotifyChanged ();
@@ -75,9 +76,10 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 
 		public void RemoveImport (MSBuildImport import)
 		{
+			AssertCanModify ();
 			if (import.ParentObject == this) {
 				import.RemoveIndent ();
-				ChildNodes.Remove (import);
+				ChildNodes = ChildNodes.Remove (import);
 				NotifyChanged ();
 			}
 		}
diff --git a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildItem.cs b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildItem.cs
index c0a1a5ec9a..59b1a416b7 100644
--- a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildItem.cs
+++ b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildItem.cs
@@ -35,7 +35,6 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 	public class MSBuildItem: MSBuildElement
 	{
 		MSBuildPropertyGroup metadata;
-		MSBuildPropertyGroupEvaluated evaluatedMetadata;
 		string name;
 		string include;
 		string exclude;
@@ -113,6 +112,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 		public string Include {
 			get { return include; }
 			set {
+				AssertCanModify ();
 				include = value;
 				NotifyChanged ();
 			}
@@ -121,6 +121,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 		public string Exclude {
 			get { return exclude; }
 			set {
+				AssertCanModify ();
 				exclude = value;
 				NotifyChanged ();
 			}
@@ -141,14 +142,6 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 			}
 		}
 
-		public IMSBuildPropertyGroupEvaluated EvaluatedMetadata {
-			get {
-				if (evaluatedMetadata == null)
-					evaluatedMetadata = new MSBuildPropertyGroupEvaluated (ParentProject);
-				return evaluatedMetadata; 
-			}
-		}
-
 		internal int EvaluatedItemCount { get; set; }
 
 		internal bool IsWildcardItem {
@@ -169,6 +162,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 			this.include = include;
 			this.evaluatedInclude = evaluatedInclude;
 			this.parent = parent;
+			metadata = new MSBuildPropertyGroupEvaluated (parent);
 			Name = name;
 		}
 
@@ -193,8 +187,6 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 
 		public IMSBuildPropertyGroupEvaluated Metadata {
 			get {
-				if (metadata == null)
-					metadata = new MSBuildPropertyGroupEvaluated (parent);
 				return metadata; 
 			}
 		}
diff --git a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildItemGroup.cs b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildItemGroup.cs
index 6fde109101..37bd43bea2 100644
--- a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildItemGroup.cs
+++ b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildItemGroup.cs
@@ -38,7 +38,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 			var item = new MSBuildItem ();
 			item.ParentNode = this;
 			item.Read (reader);
-			ChildNodes.Add (item);
+			ChildNodes = ChildNodes.Add (item);
 		}
 
 		internal override string GetElementName ()
@@ -53,6 +53,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 		
 		public MSBuildItem AddNewItem (string name, string include)
 		{
+			AssertCanModify ();
 			var it = new MSBuildItem (name);
 			it.Include = include;
 			AddItem (it);
@@ -61,8 +62,9 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 
 		public void AddItem (MSBuildItem item)
 		{
-			ChildNodes.Add (item);
+			AssertCanModify ();
 			item.ParentNode = this;
+			ChildNodes = ChildNodes.Add (item);
 			item.ResetIndent (false);
 			if (ParentProject != null)
 				ParentProject.NotifyChanged ();
@@ -76,9 +78,10 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 
 		internal void RemoveItem (MSBuildItem item)
 		{
+			AssertCanModify ();
 			if (ChildNodes.Contains (item)) {
 				item.RemoveIndent ();
-				ChildNodes.Remove (item);
+				ChildNodes = ChildNodes.Remove (item);
 				NotifyChanged ();
 			}
 		}
diff --git a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildObject.cs b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildObject.cs
index d86e28ee0b..565de09aa3 100644
--- a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildObject.cs
+++ b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildObject.cs
@@ -28,6 +28,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Collections.Immutable;

 using System.Linq;
 using System.Xml;
 
@@ -38,7 +39,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 	{
 		UnknownAttribute[] unknownAttributes;
 		string [] attributeOrder;
-		List<MSBuildNode> children;
+		ImmutableList<MSBuildNode> children = ImmutableList<MSBuildNode>.Empty;
 		EmptyElementMode emptyElementMode;
 
 		enum EmptyElementMode : byte
@@ -185,7 +186,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 					}
 					var tn = new MSBuildXmlTextNode ();
 					tn.Read (reader);
-					ChildNodes.Add (tn);
+					ChildNodes = ChildNodes.Add (tn);
 				} else if (reader.NodeType == XmlNodeType.CDATA) {
 					if (!childFound) {
 						childFound = true;
@@ -193,7 +194,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 					}
 					var tn = new MSBuildXmlCDataNode ();
 					tn.Read (reader);
-					ChildNodes.Add (tn);
+					ChildNodes = ChildNodes.Add (tn);
 				} else if (reader.NodeType == XmlNodeType.Comment) {
 					if (!childFound) {
 						childFound = true;
@@ -201,7 +202,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 					}
 					var tn = new MSBuildXmlCommentNode ();
 					tn.Read (reader);
-					ChildNodes.Add (tn);
+					ChildNodes = ChildNodes.Add (tn);
 				} else if (reader.IsWhitespace) {
 					reader.ReadAndStoreWhitespace ();
 				} else if (reader.EOF)
@@ -311,19 +312,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 				var n = new MSBuildXmlElement ();
 				n.Read (reader);
 				n.ParentNode = this;
-				ChildNodes.Add (n);
-			}
-		}
-
-		internal void RemoveUnknownAttribute (string name)
-		{
-			if (unknownAttributes == null)
-				return;
-			var list = new List<UnknownAttribute> (unknownAttributes);
-			int i = list.FindIndex (a => a.LocalName == name);
-			if (i != -1) {
-				list.RemoveAt (i);
-				unknownAttributes = list.ToArray ();
+				ChildNodes = ChildNodes.Add (n);
 			}
 		}
 
@@ -342,17 +331,18 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 
 		internal abstract string GetElementName ();
 
-		internal virtual List<MSBuildNode> ChildNodes {
+		internal virtual ImmutableList<MSBuildNode> ChildNodes {
 			get {
-				if (children == null)
-					children = new List<MSBuildNode> ();
 				return children;
 			}
+			set {
+				children = value;
+			}
 		}
 
 		internal override IEnumerable<MSBuildNode> GetChildren ()
 		{
-			return children != null ? children : Enumerable.Empty<MSBuildNode> ();
+			return children;
 		}
 
 		internal void ResetIndent (bool closeInNewLine)
diff --git a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildProject.cs b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildProject.cs
index 5743c14722..7dcb4bef45 100644
--- a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildProject.cs
+++ b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildProject.cs
@@ -46,6 +46,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 		MSBuildProjectInstance mainProjectInstance;
 		int changeStamp;
 		bool hadXmlDeclaration;
+		bool isShared;
 
 		MSBuildEngineManager engineManager;
 		bool engineManagerIsLocal;
@@ -74,7 +75,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 		public FilePath FileName
 		{
 			get { return file; }
-			set { file = value; }
+			set { AssertCanModify (); file = value; }
 		}
 
 		public FilePath BaseDirectory
@@ -140,6 +141,30 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 			}
 		}
 
+		internal override void AssertCanModify ()
+		{
+			if (isShared)
+				Runtime.AssertMainThread ();
+		}
+
+		/// <summary>
+		/// Gets a value indicating whether this instance is shared.
+		/// </summary>
+		/// <remarks>Shared objects can only be modified in the main thread</remarks>
+		public bool IsShared {
+			get { return isShared; }
+		}
+
+		/// <summary>
+		/// Sets this object as shared, which means that it is accessible from several threads for reading,
+		/// but it can only be modified in the main thread
+		/// </summary>
+		public void SetShared ()
+		{
+			isShared = true;
+		}
+
+
 		void EnableChangeTracking ()
 		{
 		}
@@ -176,6 +201,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 
 		internal void Load (string file, MSBuildXmlReader reader)
 		{
+			AssertCanModify ();
 			try {
 				this.file = file;
 				IsNewProject = false;
@@ -199,6 +225,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 
 		internal void LoadXml (string xml, MSBuildXmlReader reader)
 		{
+			AssertCanModify ();
 			try {
 				DisableChangeTracking ();
 				var xr = new XmlTextReader (new StringReader (xml));
@@ -215,8 +242,9 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 
 		void LoadFromXml (MSBuildXmlReader reader)
 		{
+			AssertCanModify ();
 			DisposeMainInstance ();
-			ChildNodes.Clear ();
+			ChildNodes = ChildNodes.Clear ();
 			bestGroups = null;
 			hadXmlDeclaration = false;
 			initialWhitespace = null;
@@ -292,7 +320,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 			if (ob != null) {
 				ob.ParentNode = this;
 				ob.Read (reader);
-				ChildNodes.Add (ob);
+				ChildNodes = ChildNodes.Add (ob);
 			} else
 				base.ReadChildElement (reader);
 		}
@@ -452,7 +480,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 		public string DefaultTargets
 		{
 			get { return defaultTargets; }
-			set { defaultTargets = value; NotifyChanged (); }
+			set { AssertCanModify (); defaultTargets = value; NotifyChanged (); }
 		}
 
 		string toolsVersion;
@@ -462,6 +490,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 			get { return toolsVersion; }
 			set
 			{
+				AssertCanModify ();
 				toolsVersion = value;
 				NotifyChanged ();
 			}
@@ -500,6 +529,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 
 		public MSBuildImport AddNewImport (string name, string condition = null, MSBuildObject beforeObject = null)
 		{
+			AssertCanModify ();
 			var import = new MSBuildImport {
 				Project = name,
 				Condition = condition
@@ -513,12 +543,14 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 				if (index != -1)
 					index++;
 			}
+
+			import.ParentNode = this;
+
 			if (index != -1)
-				ChildNodes.Insert (index, import);
+				ChildNodes = ChildNodes.Insert (index, import);
 			else
-				ChildNodes.Add (import);
+				ChildNodes = ChildNodes.Add (import);
 
-			import.ParentNode = this;
 			import.ResetIndent (false);
 			NotifyChanged ();
 			return import;
@@ -531,22 +563,24 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 
 		public void RemoveImport (string name, string condition = null)
 		{
+			AssertCanModify ();
 			var i = GetImport (name, condition);
 			if (i != null) {
 				i.RemoveIndent ();
-				ChildNodes.Remove (i);
+				ChildNodes = ChildNodes.Remove (i);
 				NotifyChanged ();
 			}
 		}
 
 		public void RemoveImport (MSBuildImport import)
 		{
+			AssertCanModify ();
 			if (import.ParentProject != this)
 				throw new InvalidOperationException ("Import object does not belong to this project");
 			
 			if (import.ParentObject == this) {
 				import.RemoveIndent ();
-				ChildNodes.Remove (import);
+				ChildNodes = ChildNodes.Remove (import);
 				NotifyChanged ();
 			} else
 				((MSBuildImportGroup)import.ParentObject).RemoveImport (import);
@@ -595,20 +629,23 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 
 		public void AddPropertyGroup (MSBuildPropertyGroup group, bool insertAtEnd)
 		{
+			AssertCanModify ();
 			if (group.ParentProject != null)
 				throw new InvalidOperationException ("Group already belongs to a project");
 
+			group.ParentNode = this;
+
 			bool added = false;
 			if (insertAtEnd) {
 				var last = ChildNodes.FindLastIndex (g => g is MSBuildPropertyGroup);
 				if (last != -1) {
-					ChildNodes.Insert (last + 1, group);
+					ChildNodes = ChildNodes.Insert (last + 1, group);
 					added = true;
 				}
 			} else {
 				var first = ChildNodes.FindIndex (g => g is MSBuildPropertyGroup);
 				if (first != -1) {
-					ChildNodes.Insert (first, group);
+					ChildNodes = ChildNodes.Insert (first, group);
 					added = true;
 				}
 			}
@@ -616,12 +653,11 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 			if (!added) {
 				var first = ChildNodes.FindIndex (g => g is MSBuildItemGroup);
 				if (first != -1)
-					ChildNodes.Insert (first, group);
+					ChildNodes = ChildNodes.Insert (first, group);
 				else
-					ChildNodes.Add (group);
+					ChildNodes = ChildNodes.Add (group);
 			}
 
-			group.ParentNode = this;
 			group.ResetIndent (true);
 			NotifyChanged ();
 		}
@@ -678,6 +714,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 
 		public MSBuildItemGroup AddNewItemGroup ()
 		{
+			AssertCanModify ();
 			var group = new MSBuildItemGroup ();
 
 			MSBuildObject refNode = null;
@@ -689,12 +726,13 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 				if (g != null)
 					refNode = g;
 			}
+
+			group.ParentNode = this;
 			if (refNode != null)
-				ChildNodes.Insert (ChildNodes.IndexOf (refNode) + 1, group);
+				ChildNodes = ChildNodes.Insert (ChildNodes.IndexOf (refNode) + 1, group);
 			else
-				ChildNodes.Add (group);
+				ChildNodes = ChildNodes.Add (group);
 
-			group.ParentNode = this;
 			group.ResetIndent (true);
 			NotifyChanged ();
 			return group;
@@ -764,11 +802,12 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 
 		public void SetProjectExtension (XmlElement value)
 		{
+			AssertCanModify ();
 			var ext = (MSBuildProjectExtensions)ChildNodes.FirstOrDefault (ob => ob is MSBuildProjectExtensions);
 			if (ext == null) {
 				ext = new MSBuildProjectExtensions ();
 				ext.ParentNode = this;
-				ChildNodes.Add (ext);
+				ChildNodes = ChildNodes.Add (ext);
 				ext.ResetIndent (false);
 			}
 			ext.SetProjectExtension (value);
@@ -777,6 +816,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 
 		public void SetMonoDevelopProjectExtension (string section, XmlElement value)
 		{
+			AssertCanModify ();
 			var elem = GetProjectExtension ("MonoDevelop");
 			if (elem == null) {
 				XmlDocument doc = new XmlDocument ();
@@ -807,6 +847,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 
 		public void RemoveProjectExtension (string section)
 		{
+			AssertCanModify ();
 			var ext = (MSBuildProjectExtensions)ChildNodes.FirstOrDefault (ob => ob is MSBuildProjectExtensions);
 			if (ext != null) {
 				ext.RemoveProjectExtension (section);
@@ -817,6 +858,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 
 		public void RemoveMonoDevelopProjectExtension (string section)
 		{
+			AssertCanModify ();
 			var md = GetProjectExtension ("MonoDevelop");
 			if (md == null)
 				return;
@@ -834,14 +876,16 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 
 		public void Remove (MSBuildObject ob)
 		{
+			AssertCanModify ();
 			if (ob.ParentObject == this) {
 				ob.RemoveIndent ();
-				ChildNodes.Remove (ob);
+				ChildNodes = ChildNodes.Remove (ob);
 			}
 		}
 
 		public void RemoveItem (MSBuildItem item)
 		{
+			AssertCanModify ();
 			if (item.ParentGroup != null) {
 				item.RemoveIndent ();
 				var g = item.ParentGroup;
diff --git a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildProjectExtensions.cs b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildProjectExtensions.cs
index 9c93dd64d3..dfed253c66 100644
--- a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildProjectExtensions.cs
+++ b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildProjectExtensions.cs
@@ -72,6 +72,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 
 		public void SetProjectExtension (XmlElement value)
 		{
+			AssertCanModify ();
 			var sr = new StringReader (value.OuterXml);
 			var xr = new XmlTextReader (sr);
 			xr.MoveToContent ();
@@ -83,10 +84,10 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 
 			int i = ChildNodes.FindIndex (n => (n is MSBuildXmlElement) && ((MSBuildXmlElement)n).Name == section);
 			if (i == -1)
-				ChildNodes.Add (elem);
+				ChildNodes = ChildNodes.Add (elem);
 			else {
-				ChildNodes.RemoveAt (i);
-				ChildNodes.Insert (i, elem);
+				ChildNodes = ChildNodes.RemoveAt (i);
+				ChildNodes = ChildNodes.Insert (i, elem);
 			}
 			elem.ParentNode = this;
 			elem.ResetIndent (false);
@@ -95,9 +96,10 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 
 		public void RemoveProjectExtension (string section)
 		{
+			AssertCanModify ();
 			int i = ChildNodes.FindIndex (n => (n is MSBuildXmlElement) && ((MSBuildXmlElement)n).Name == section);
 			if (i != -1) {
-				ChildNodes.RemoveAt (i);
+				ChildNodes = ChildNodes.RemoveAt (i);
 				NotifyChanged ();
 			}
 		}
diff --git a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildProperty.cs b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildProperty.cs
index f9f25b388c..2b49640aac 100644
--- a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildProperty.cs
+++ b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildProperty.cs
@@ -209,6 +209,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 
 		public void SetValue (string value, bool preserveCase = false, bool mergeToMainGroup = false)
 		{
+			AssertCanModify ();
 			MergeToMainGroup = mergeToMainGroup;
 			this.preserverCase = preserveCase;
 			valueType = preserveCase ? MSBuildValueType.Default : MSBuildValueType.DefaultPreserveCase;
@@ -230,6 +231,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 
 		public void SetValue (FilePath value, bool relativeToProject = true, FilePath relativeToPath = default(FilePath), bool mergeToMainGroup = false)
 		{
+			AssertCanModify ();
 			MergeToMainGroup = mergeToMainGroup;
 			this.preserverCase = false;
 			valueType = MSBuildValueType.Path;
@@ -268,6 +270,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 
 		public void SetValue (object value, bool mergeToMainGroup = false)
 		{
+			AssertCanModify ();
 			if (value is bool) {
 				if (Owner != null && Owner.UppercaseBools)
 					SetValue ((bool)value ? "True" : "False", preserveCase: true, mergeToMainGroup: mergeToMainGroup);
diff --git a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildPropertyGroup.cs b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildPropertyGroup.cs
index 365eee48af..0ec4ea07bb 100644
--- a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildPropertyGroup.cs
+++ b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildPropertyGroup.cs
@@ -32,7 +32,8 @@ using System.Xml;
 using MonoDevelop.Core;
 using System.Xml.Linq;
 using Microsoft.Build.BuildEngine;
-
+using System.Collections.Immutable;

+

 namespace MonoDevelop.Projects.Formats.MSBuild
 {
 	public class MSBuildPropertyGroup: MSBuildElement, IMSBuildPropertySet, IMSBuildEvaluatedPropertyCollection
@@ -43,12 +44,17 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 		{
 		}
 
-		internal override List<MSBuildNode> ChildNodes {
+		internal override ImmutableList<MSBuildNode> ChildNodes {
 			get {
 				if (ParentNode is MSBuildItem)
 					return ((MSBuildItem)ParentNode).ChildNodes;
 				return base.ChildNodes;
 			}
+			set {
+				if (ParentNode is MSBuildItem)
+					((MSBuildItem)ParentNode).ChildNodes = value;
+				base.ChildNodes = value;
+			}
 		}
 
 		internal MSBuildObject PropertiesParent {
@@ -67,7 +73,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 			prop.ParentNode = PropertiesParent;
 			prop.Owner = this;
 			prop.Read (reader);
-			ChildNodes.Add (prop);
+			ChildNodes = ChildNodes.Add (prop);
 			properties [prop.Name] = prop; // If a property is defined more than once, we only care about the last registered value
 		}
 
@@ -85,6 +91,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 
 		internal void CopyFrom (MSBuildPropertyGroup other)
 		{
+			AssertCanModify ();
 			foreach (var node in other.ChildNodes) {
 				var prop = node as MSBuildProperty;
 				if (prop != null) {
@@ -92,16 +99,16 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 					var currentPropIndex = ChildNodes.FindIndex (p => (p is MSBuildProperty) && ((MSBuildProperty)p).Name == prop.Name);
 					if (currentPropIndex != -1) {
 						var currentProp = (MSBuildProperty) ChildNodes [currentPropIndex];
-						ChildNodes [currentPropIndex] = cp;
+						ChildNodes = ChildNodes.SetItem (currentPropIndex, cp);
 					} else {
-						ChildNodes.Add (cp);
+						ChildNodes = ChildNodes.Add (cp);
 					}
 					properties [cp.Name] = cp;
 					cp.ParentNode = PropertiesParent;
 					cp.Owner = this;
 					cp.ResetIndent (false);
 				} else
-					ChildNodes.Add (node);
+					ChildNodes = ChildNodes.Add (node);
 			}
 			foreach (var prop in ChildNodes.OfType<MSBuildProperty> ().ToArray ()) {
 				if (!other.HasProperty (prop.Name))
@@ -214,6 +221,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 
 		MSBuildProperty AddProperty (string name, string condition = null)
 		{
+			AssertCanModify ();
 			int i = propertyOrder.IndexOf (name);
 			int insertIndex = -1;
 			if (i != -1) {
@@ -233,9 +241,9 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 			properties [name] = prop;
 
 			if (insertIndex != -1)
-				ChildNodes.Insert (insertIndex, prop);
+				ChildNodes = ChildNodes.Insert (insertIndex, prop);
 			else
-				ChildNodes.Add (prop);
+				ChildNodes = ChildNodes.Add (prop);
 
 			if (condition != null)
 				prop.Condition = condition;
@@ -272,6 +280,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 
 		public void SetValue (string name, string value, string defaultValue = null, bool preserveExistingCase = false, bool mergeToMainGroup = false, string condition = null)
 		{
+			AssertCanModify ();
 			if (value == null && defaultValue == "")
 				value = "";
 			var prop = GetProperty (name, condition);
@@ -295,6 +304,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 
 		public void SetValue (string name, FilePath value, FilePath defaultValue = default(FilePath), bool relativeToProject = true, FilePath relativeToPath = default(FilePath), bool mergeToMainGroup = false, string condition = null)
 		{
+			AssertCanModify ();
 			var prop = GetProperty (name, condition);
 			var isDefault = value.CanonicalPath == defaultValue.CanonicalPath;
 			if (isDefault && !mergeToMainGroup) {
@@ -316,6 +326,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 
 		public void SetValue (string name, object value, object defaultValue = null, bool mergeToMainGroup = false, string condition = null)
 		{
+			AssertCanModify ();
 			var prop = GetProperty (name, condition);
 			var isDefault = object.Equals (value, defaultValue);
 			if (isDefault && !mergeToMainGroup) {
@@ -347,21 +358,24 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 
 		public void RemoveProperty (MSBuildProperty prop)
 		{
+			AssertCanModify ();
 			prop.RemoveIndent ();
 			properties.Remove (prop.Name);
-			ChildNodes.Remove (prop);
+			ChildNodes = ChildNodes.Remove (prop);
 			NotifyChanged ();
 		}
 
 		public void RemoveAllProperties ()
 		{
+			AssertCanModify ();
 			properties.Clear ();
-			ChildNodes.Clear ();
+			ChildNodes = ChildNodes.Clear ();
 			NotifyChanged ();
 		}
 
 		public void UnMerge (IMSBuildPropertySet baseGrp, ISet<string> propsToExclude)
 		{
+			AssertCanModify ();
 			HashSet<string> baseProps = new HashSet<string> ();
 			foreach (MSBuildProperty prop in baseGrp.GetProperties ()) {
 				if (propsToExclude != null && propsToExclude.Contains (prop.Name))
@@ -398,6 +412,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 
 		public void SetPropertyOrder (params string[] propertyNames)
 		{
+			AssertCanModify ();
 			int i = 0;
 			foreach (var name in propertyNames) {
 				if (i < propertyOrder.Count) {
diff --git a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildTarget.cs b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildTarget.cs
index 269086c445..8405304be5 100644
--- a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildTarget.cs
+++ b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildTarget.cs
@@ -62,6 +62,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 				return this.afterTargets;
 			}
 			set {
+				AssertCanModify ();
 				this.afterTargets = value;
 				NotifyChanged ();
 			}
@@ -72,6 +73,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 				return this.inputs;
 			}
 			set {
+				AssertCanModify ();
 				this.inputs = value;
 				NotifyChanged ();
 			}
@@ -82,6 +84,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 				return this.outputs;
 			}
 			set {
+				AssertCanModify ();
 				this.outputs = value;
 				NotifyChanged ();
 			}
@@ -92,6 +95,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 				return this.beforeTargets;
 			}
 			set {
+				AssertCanModify ();
 				this.beforeTargets = value;
 				NotifyChanged ();
 			}
@@ -103,6 +107,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 				return this.dependsOnTargets;
 			}
 			set {
+				AssertCanModify ();
 				this.dependsOnTargets = value;
 				NotifyChanged ();
 			}
@@ -113,6 +118,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 				return this.returns;
 			}
 			set {
+				AssertCanModify ();
 				this.returns = value;
 				NotifyChanged ();
 			}
@@ -123,6 +129,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 				return this.keepDuplicateOutputs;
 			}
 			set {
+				AssertCanModify ();
 				this.keepDuplicateOutputs = value;
 				NotifyChanged ();
 			}
@@ -173,14 +180,14 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 			if (ob != null) {
 				ob.ParentNode = this;
 				ob.Read (reader);
-				ChildNodes.Add (ob);
+				ChildNodes = ChildNodes.Add (ob);
 				return;
 			}
 
 			var task = new MSBuildTask ();
 			task.ParentNode = this;
 			task.Read (reader);
-			ChildNodes.Add (task);
+			ChildNodes = ChildNodes.Add (task);
 		}
 
 		internal override void Write (XmlWriter writer, WriteContext context)
@@ -195,7 +202,7 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 		public MSBuildTarget (string name, IEnumerable<MSBuildTask> tasks)
 		{
 			this.name = name;
-			ChildNodes.AddRange (tasks);
+			ChildNodes = ChildNodes.AddRange (tasks);
 		}
 
 		public string Name {
@@ -210,10 +217,11 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 
 		public void RemoveTask (MSBuildTask task)
 		{
+			AssertCanModify ();
 			if (task.ParentObject != this)
 				throw new InvalidOperationException ("Task doesn't belong to the target");
 			task.RemoveIndent ();
-			ChildNodes.Remove (task);
+			ChildNodes = ChildNodes.Remove (task);
 		}
 	}
 	
diff --git a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildXmlNode.cs b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildXmlNode.cs
index b146ee7f56..89db16f5a0 100644
--- a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildXmlNode.cs
+++ b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.Formats.MSBuild/MSBuildXmlNode.cs
@@ -51,6 +51,13 @@ namespace MonoDevelop.Projects.Formats.MSBuild
 		{
 		}
 
+		internal virtual void AssertCanModify ()
+		{
+			var pp = ParentProject;
+			if (pp != null)
+				pp.AssertCanModify ();
+		}
+
 		internal MSBuildNode ParentNode {
 			get {
 				return parentObject;