Fix nullable return type cref generation (issue #37) and add test coverage

NullableTypeSyntax (e.g. string?) was falling into the else branch of
BuildComment and calling ToFullString(), producing invalid XML like
<see cref="string?"/>. Fix recurses on the inner ElementType, stripping
the ? before cref generation.

Also updates MethodWithNullableReturnTestFixCode.cs to reflect the now-correct
output (MethodTester instead of MethodTester?) and adds a non-cref nullable
string return test case.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: d1820

.vscode/settings.json

@@ -0,0 +1,3 @@
+{
+  "dotnet.preferCSharpExtension": true
+}

CodeDocumentor.Common/Constructors/BaseReturnTypeCommentConstruction.cs

@@ -77,6 +77,10 @@ public virtual string BuildComment(TypeSyntax returnType, ReturnTypeBuilderOptio
             {
                 returnComment = GenerateGenericTypeComment(gst, options, wordMaps);
             }
+            else if (returnType is NullableTypeSyntax nts)
+            {
+                returnComment = BuildComment(nts.ElementType, options, wordMaps);
+            }
             else if (returnType is ArrayTypeSyntax ast)
             {
                 var comment = string.Format(ArrayCommentTemplate, DocumentationHeaderHelper.DetermineSpecificObjectName(ast.ElementType, wordMaps, options.TryToIncludeCrefsForReturnTypes));

CodeDocumentor.Test/CodeDocumentor.Test.csproj

@@ -79,6 +79,8 @@
     <Compile Remove="Methods\TestFiles\MethodWithMIxedExceptionTestFixCode.cs" />
     <Compile Remove="Methods\TestFiles\MethodWithNullableStructParameterTestCode.cs" />
     <Compile Remove="Methods\TestFiles\MethodWithNullableStructParameterTestFixCode.cs" />
+    <Compile Remove="Methods\TestFiles\MethodWithNullableStringReturnTestCode.cs" />
+    <Compile Remove="Methods\TestFiles\MethodWithNullableStringReturnTestFixCode.cs" />
     <Compile Remove="Methods\TestFiles\MethodWithObjectReturnTestCode.cs" />
     <Compile Remove="Methods\TestFiles\MethodWithObjectReturnTestFixCode.cs" />
     <Compile Remove="Methods\TestFiles\MethodWithParameterTestCode.cs" />
@@ -446,6 +448,12 @@
     <Content Include="Methods\TestFiles\MethodWithNullableStructParameterTestFixCode.cs">
       <CopyToOutputDirectory>Always</CopyToOutputDirectory>
     </Content>
+    <Content Include="Methods\TestFiles\MethodWithNullableStringReturnTestCode.cs">
+      <CopyToOutputDirectory>Always</CopyToOutputDirectory>
+    </Content>
+    <Content Include="Methods\TestFiles\MethodWithNullableStringReturnTestFixCode.cs">
+      <CopyToOutputDirectory>Always</CopyToOutputDirectory>
+    </Content>
     <Content Include="Methods\TestFiles\MethodWithObjectReturnTestCode.cs">
       <CopyToOutputDirectory>Always</CopyToOutputDirectory>
     </Content>

CodeDocumentor.Test/Methods/MethodUnitTests.cs

@@ -55,6 +55,7 @@ public async Task NoDiagnosticsShow(string testCode)
         [InlineData("MethodWithNullableStructParameterTestCode", "MethodWithNullableStructParameterTestFixCode", 9, 21)]
         [InlineData("MethodWithReturnTestCode", "MethodWithReturnTestFixCode", 9, 29)]
         [InlineData("MethodWithStringReturnTestCode", "MethodWithStringReturnTestFixCode", 9, 23)]
+        [InlineData("MethodWithNullableStringReturnTestCode", "MethodWithNullableStringReturnTestFixCode", 9, 24)]
         [InlineData("MethodWithObjectReturnTestCode", "MethodWithObjectReturnTestFixCode", 9, 23)]
         [InlineData("MethodWithIntReturnTestCode", "MethodWithIntReturnTestFixCode", 9, 20)]
         [InlineData("MethodWithListIntReturnTestCode", "MethodWithListIntReturnTestFixCode", 9, 26)]

CodeDocumentor.Test/Methods/TestFiles/Crefs/MethodWithNullableReturnTestFixCode.cs

@@ -9,7 +9,7 @@ public class MethodTester
         /// <summary>
         /// Show method with return tester.
         /// </summary>
-        /// <returns>A <see cref="MethodTester?"/></returns>
+        /// <returns>A <see cref="MethodTester"/></returns>
         public MethodTester? ShowMethodWithReturnTester()
         {
             return null;

CodeDocumentor.Test/Methods/TestFiles/Crefs/MethodWithNullableStringReturnTestCode.cs

@@ -6,7 +6,7 @@ namespace ConsoleApp40
 {
     public class MethodTester
     {
-        public string? ShowMethodWithStringReturnTester()
+        public string? ShowMethodWithNullableStringReturnTester()
         {
             return null;
         }

CodeDocumentor.Test/Methods/TestFiles/Crefs/MethodWithNullableStringReturnTestFixCode.cs

@@ -7,10 +7,10 @@ namespace ConsoleApp40
     public class MethodTester
     {
         /// <summary>
-        /// Show method with string return tester.
+        /// Show method with nullable string return tester.
         /// </summary>
-        /// <returns>A <see cref="string?"/></returns>
-        public string? ShowMethodWithStringReturnTester()
+        /// <returns>A <see cref="string"/></returns>
+        public string? ShowMethodWithNullableStringReturnTester()
         {
             return null;
         }

CodeDocumentor.Test/Methods/TestFiles/MethodWithNullableStringReturnTestCode.cs

@@ -0,0 +1,14 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace ConsoleApp4
+{
+    public class MethodTester
+    {
+        public string? ShowMethodWithNullableStringReturnTester()
+        {
+            return null;
+        }
+    }
+}

CodeDocumentor.Test/Methods/TestFiles/MethodWithNullableStringReturnTestFixCode.cs

@@ -0,0 +1,18 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace ConsoleApp4
+{
+    public class MethodTester
+    {
+        /// <summary>
+        /// Show method with nullable string return tester.
+        /// </summary>
+        /// <returns>A string</returns>
+        public string? ShowMethodWithNullableStringReturnTester()
+        {
+            return null;
+        }
+    }
+}