Unwrap nullable in IsValidParameterType (#33)

* first shot

* format

Author: jonathanalgar

src/CustomCode-Analyzer/Analyzer.cs

@@ -1373,6 +1373,16 @@ private static bool IsValidParameterType(ITypeSymbol typeSymbol, Compilation com
                 return false;
             }
 
+            // If the type is nullable, unwrap it and check the underlying type
+            if (
+                typeSymbol is INamedTypeSymbol named
+                && named.OriginalDefinition.SpecialType == SpecialType.System_Nullable_T
+                && named.TypeArguments.Length == 1
+            )
+            {
+                typeSymbol = named.TypeArguments[0];
+            }
+
             // Check for primitive or special types
             if (ValidParameterSpecialTypes.Contains(typeSymbol.SpecialType))
             {

tests/CustomCode-Analyzer.Tests/AnalyzerTests.cs

@@ -1136,6 +1136,30 @@ await CSharpAnalyzerVerifier<Analyzer>.VerifyAnalyzerAsync(
             );
         }
 
+        [TestMethod]
+        public async Task UnsupportedParameterTypeRule_InGlobalScope_PublicField_NullableStruct_NoWarning()
+        {
+            var test =
+                @"
+[OSStructure]
+public struct NestedStructure
+{
+    public int Value;
+}
+
+[OSStructure]
+public struct MyStructure
+{
+    public NestedStructure? NullableNested;    // Nullable struct OK
+}
+";
+            await CSharpAnalyzerVerifier<Analyzer>.VerifyAnalyzerAsync(
+                test,
+                TestContext,
+                skipSDKreference: false
+            );
+        }
+
         // --------------- ParameterByReferenceRule (OS-ELG-MODL-05016) ------------
         [TestMethod]
         public async Task ParameterByReferenceRule_InGlobalScope_ReportsWarning()