Add test for specified behavior of a virtual method invocation (via a binary...

Add test for specified behavior of a virtual method invocation (via a binary compatibility scenario). (#16055)

* Add test for specified behavior of a virtual method invocation (via a binary compatibility scenario).

src/Compilers/CSharp/Test/Emit/CSharpCompilerEmitTest.csproj

@@ -75,6 +75,7 @@
     <Compile Include="Attributes\AttributeTests_Tuples.cs" />
     <Compile Include="Attributes\AttributeTests_WellKnownAttributes.cs" />
     <Compile Include="CodeGen\CodeGenCapturing.cs" />
+    <Compile Include="Emit\BinaryCompatibility.cs" />
     <Compile Include="Emit\DesktopStrongNameProviderTests.cs" />
     <Compile Include="Attributes\InternalsVisibleToAndStrongNameTests.cs" />
     <Compile Include="Attributes\WellKnownAttributesTestBase.cs" />

src/Compilers/CSharp/Test/Emit/Emit/BinaryCompatibility.cs

+// Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
+
+using Microsoft.CodeAnalysis.CSharp.Test.Utilities;
+using Microsoft.CodeAnalysis.Test.Utilities;
+using Xunit;
+
+namespace Microsoft.CodeAnalysis.CSharp.UnitTests
+{
+    public class BinaryCompatibility : EmitMetadataTestBase
+    {
+        [Fact]
+        public void InvokeVirtualBoundToOriginal()
+        {
+            // A method invocation of a virtual method is statically bound by the C# language to
+            // the original declaration of the virtual method, not the most derived override of
+            // that method. The difference between these two choices is visible in the binary
+            // compatibility behavior of the program at runtime (e.g. when executing the program
+            // against a modified library). This test checks that we bind the invocation to the
+            // virtual method, not the override.
+            var lib0 = @"
+public class Base
+{
+    public virtual void M() { System.Console.WriteLine(""Base0""); }
+}
+public class Derived : Base
+{
+    public override void M() { System.Console.WriteLine(""Derived0""); }
+}
+";
+            var lib0Image = CreateCompilationWithMscorlib46(lib0, options: TestOptions.ReleaseDll, assemblyName: "lib").EmitToImageReference();
+
+            var lib1 = @"
+public class Base
+{
+    public virtual void M() { System.Console.WriteLine(""Base1""); }
+}
+public class Derived : Base
+{
+    public new virtual void M() { System.Console.WriteLine(""Derived1""); }
+}
+";
+            var lib1Image = CreateCompilationWithMscorlib46(lib1, options: TestOptions.ReleaseDll, assemblyName: "lib").EmitToImageReference();
+
+            var client = @"
+public class Client
+{
+    public static void M()
+    {
+        Derived d = new Derived();
+        d.M();
+    }
+}
+";
+            var clientImage = CreateCompilationWithMscorlib46(client, references: new[] { lib0Image }, options: TestOptions.ReleaseDll).EmitToImageReference();
+
+            var program = @"
+public class Program
+{
+    public static void Main()
+    {
+        Client.M();
+    }
+}
+";
+            var compilation = CreateCompilationWithMscorlib46(program, references: new[] { lib1Image, clientImage }, options: TestOptions.ReleaseExe);
+            CompileAndVerify(compilation, expectedOutput: @"Base1");
+        }
+    }
+}