Add support for setting AppContext switches

Adds an `--appcontextswitch` argument to the compiler (and accompanying `AppContextSwitchOverrides` MSBuild `ItemGroup`) to influence class library behaviors.

3 files changed, 107 insertions, 0 deletions

diff --git a/src/ILCompiler.Compiler/src/IL/Stubs/StartupCode/AppContextInitializerMethod.Sorting.cs b/src/ILCompiler.Compiler/src/IL/Stubs/StartupCode/AppContextInitializerMethod.Sorting.cs
new file mode 100644
index 000000000..67a2b612c
--- /dev/null
+++ b/src/ILCompiler.Compiler/src/IL/Stubs/StartupCode/AppContextInitializerMethod.Sorting.cs
@@ -0,0 +1,22 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using Internal.TypeSystem;
+
+using Debug = System.Diagnostics.Debug;
+
+namespace Internal.IL.Stubs.StartupCode
+{
+    partial class AppContextInitializerMethod
+    {
+        protected override int ClassCode => 15749517;
+
+        protected override int CompareToImpl(MethodDesc other, TypeSystemComparer comparer)
+        {
+            // Should be a singleton
+            Debug.Assert(this == other);
+            return 0;
+        }
+    }
+}
diff --git a/src/ILCompiler.Compiler/src/IL/Stubs/StartupCode/AppContextInitializerMethod.cs b/src/ILCompiler.Compiler/src/IL/Stubs/StartupCode/AppContextInitializerMethod.cs
new file mode 100644
index 000000000..2c2d3a51a
--- /dev/null
+++ b/src/ILCompiler.Compiler/src/IL/Stubs/StartupCode/AppContextInitializerMethod.cs
@@ -0,0 +1,83 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.Collections.Generic;
+
+using Internal.TypeSystem;
+
+namespace Internal.IL.Stubs.StartupCode
+{
+    public sealed partial class AppContextInitializerMethod : ILStubMethod
+    {
+        private TypeDesc _owningType;
+        private MethodSignature _signature;
+        private IReadOnlyCollection<string> _switches;
+
+        public AppContextInitializerMethod(TypeDesc owningType, IEnumerable<string> switches)
+        {
+            _owningType = owningType;
+            _switches = new List<string>(switches);
+        }
+
+        public override TypeSystemContext Context
+        {
+            get
+            {
+                return _owningType.Context;
+            }
+        }
+
+        public override TypeDesc OwningType
+        {
+            get
+            {
+                return _owningType;
+            }
+        }
+
+        public override string Name
+        {
+            get
+            {
+                return "SetAppContextSwitches";
+            }
+        }
+
+        public override MethodIL EmitIL()
+        {
+            ILEmitter emitter = new ILEmitter();
+            ILCodeStream codeStream = emitter.NewCodeStream();
+
+            MetadataType appContextType = Context.SystemModule.GetKnownType("System", "AppContext");
+            MethodDesc setSwitchMethod = appContextType.GetKnownMethod("SetSwitch", null);
+            ILToken setSwitchToken = emitter.NewToken(setSwitchMethod);
+
+            foreach (string switchName in _switches)
+            {
+                codeStream.Emit(ILOpcode.ldstr, emitter.NewToken(switchName));
+                codeStream.EmitLdc(1);
+                codeStream.Emit(ILOpcode.call, setSwitchToken);
+            }
+
+            codeStream.Emit(ILOpcode.ret);
+
+            return emitter.Link(this);
+        }
+
+        public override MethodSignature Signature
+        {
+            get
+            {
+                if (_signature == null)
+                {
+                    _signature = new MethodSignature(MethodSignatureFlags.Static, 0,
+                            Context.GetWellKnownType(WellKnownType.Void),
+                            TypeDesc.EmptyTypes);
+                }
+
+                return _signature;
+            }
+        }
+    }
+}
diff --git a/src/ILCompiler.Compiler/src/ILCompiler.Compiler.csproj b/src/ILCompiler.Compiler/src/ILCompiler.Compiler.csproj
index 9744fa933..b86472d82 100644
--- a/src/ILCompiler.Compiler/src/ILCompiler.Compiler.csproj
+++ b/src/ILCompiler.Compiler/src/ILCompiler.Compiler.csproj
@@ -352,6 +352,8 @@
     <Compile Include="Compiler\WindowsNodeMangler.cs" />
     <Compile Include="IL\ILImporter.Scanner.cs" />
     <Compile Include="IL\Stubs\PInvokeILProvider.cs" />
+    <Compile Include="IL\Stubs\StartupCode\AppContextInitializerMethod.cs" />
+    <Compile Include="IL\Stubs\StartupCode\AppContextInitializerMethod.Sorting.cs" />
     <Compile Include="IL\Stubs\StartupCode\StartupCodeMainMethod.cs" />
     <Compile Include="IL\Stubs\StartupCode\StartupCodeMainMethod.Sorting.cs" />
     <Compile Include="IL\Stubs\StartupCode\NativeLibraryStartupMethod.cs" />