C#: Add a couple of unit tests.

Author: michaelnebel

csharp/extractor/Semmle.Extraction.Tests/Semmle.Extraction.Tests.csproj

@@ -16,6 +16,7 @@
 		<PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.4.0" />
 	</ItemGroup>
 	<ItemGroup>
+		<ProjectReference Include="..\Semmle.Extraction.CSharp.StubGenerator\Semmle.Extraction.CSharp.StubGenerator.csproj" />
 		<ProjectReference Include="..\Semmle.Extraction.CSharp.Standalone\Semmle.Extraction.CSharp.Standalone.csproj" />
 		<ProjectReference Include="..\Semmle.Extraction.CSharp\Semmle.Extraction.CSharp.csproj" />
 		<ProjectReference Include="..\Semmle.Extraction\Semmle.Extraction.csproj" />

csharp/extractor/Semmle.Extraction.Tests/StubGenerator.cs

@@ -0,0 +1,74 @@
+using Xunit;
+using System.IO;
+using System.Text;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp;
+using Semmle.Extraction.CSharp.StubGenerator;
+
+namespace Semmle.Extraction.Tests;
+/// <summary>
+/// Tests for the stub generator.
+/// 
+/// These tests can be used to more easily step debug the stub generator SymbolVisitor.
+/// </summary>
+public class StubGeneratorTests
+{
+    [Fact]
+    public void StubGeneratorFieldTest()
+    {
+        // Setup
+        const string source = @"
+public class MyTest {
+    public static readonly int MyField1;
+    public const string MyField2 = ""hello"";
+    }
+";
+
+        // Execute
+        var stub = GenerateStub(source);
+
+        // Verify
+        const string expected = @"public class MyTest {
+public static readonly int MyField1;
+public const string MyField2 = default;
+}
+";
+        Assert.Equal(expected, stub);
+    }
+
+    [Fact]
+    public void StubGeneratorMethodTest()
+    {
+        // Setup
+        const string source = @"
+public class MyTest {
+    public int M1(string arg1) { return 0;}
+}";
+
+        // Execute
+        var stub = GenerateStub(source);
+
+        // Verify
+        const string expected = @"public class MyTest {
+public int M1(string arg1) => throw null;
+}
+";
+        Assert.Equal(expected, stub);
+    }
+
+    private static string GenerateStub(string source)
+    {
+        var st = CSharpSyntaxTree.ParseText(source);
+        var compilation = CSharpCompilation.Create(null, new[] { st });
+        var sb = new StringBuilder();
+        var visitor = new StubVisitor(new StringWriter(sb), new RelevantSymbolStub());
+        compilation.GlobalNamespace.Accept(visitor);
+        return sb.ToString();
+    }
+
+    private class RelevantSymbolStub : IRelevantSymbol
+    {
+        public bool IsRelevantNamedType(INamedTypeSymbol symbol) => true;
+        public bool IsRelevantNamespace(INamespaceSymbol symbol) => true;
+    }
+}