diff --git a/src/libraries/System.Runtime.InteropServices/gen/ComInterfaceGenerator/VtableIndexStubGenerator.cs b/src/libraries/System.Runtime.InteropServices/gen/ComInterfaceGenerator/VtableIndexStubGenerator.cs
index f71d1432ee6ac..6a5c8f0d91b7c 100644
--- a/src/libraries/System.Runtime.InteropServices/gen/ComInterfaceGenerator/VtableIndexStubGenerator.cs
+++ b/src/libraries/System.Runtime.InteropServices/gen/ComInterfaceGenerator/VtableIndexStubGenerator.cs
@@ -103,16 +103,17 @@ public void Initialize(IncrementalGeneratorInitializationContext context)
                 )
                 .WithTrackingName(StepNames.CalculateStubInformation);
 
-            IncrementalValuesProvider<(MemberDeclarationSyntax, ImmutableArray<Diagnostic>)> generateSingleStub = generateStubInformation
+            IncrementalValuesProvider<(MemberDeclarationSyntax, ImmutableArray<Diagnostic>)> generateManagedToNativeStub = generateStubInformation
+                .Where(data => data.VtableIndexData.Direction.HasFlag(CustomTypeMarshallerDirection.In))
                 .Select(
                     static (data, ct) => GenerateManagedToNativeStub(data)
                 )
                 .WithComparer(Comparers.GeneratedSyntax)
                 .WithTrackingName(StepNames.GenerateManagedToNativeStub);
 
-            context.RegisterDiagnostics(generateSingleStub.SelectMany((stubInfo, ct) => stubInfo.Item2));
+            context.RegisterDiagnostics(generateManagedToNativeStub.SelectMany((stubInfo, ct) => stubInfo.Item2));
 
-            context.RegisterConcatenatedSyntaxOutputs(generateSingleStub.Select((data, ct) => data.Item1), "ManagedToNativeStubs.g.cs");
+            context.RegisterConcatenatedSyntaxOutputs(generateManagedToNativeStub.Select((data, ct) => data.Item1), "ManagedToNativeStubs.g.cs");
 
             IncrementalValuesProvider<MemberDeclarationSyntax> generateNativeInterface = generateStubInformation
                 .Select(static (context, ct) => context.ContainingSyntaxContext)
diff --git a/src/libraries/System.Runtime.InteropServices/gen/Microsoft.Interop.SourceGeneration/BoundGenerators.cs b/src/libraries/System.Runtime.InteropServices/gen/Microsoft.Interop.SourceGeneration/BoundGenerators.cs
index 90ca43a64abbd..b1f95c1903a1c 100644
--- a/src/libraries/System.Runtime.InteropServices/gen/Microsoft.Interop.SourceGeneration/BoundGenerators.cs
+++ b/src/libraries/System.Runtime.InteropServices/gen/Microsoft.Interop.SourceGeneration/BoundGenerators.cs
@@ -101,10 +101,16 @@ static IEnumerable<int> GetInfoDependencies(TypePositionInfo info)
 
             static int GetInfoIndex(TypePositionInfo info)
             {
+                // A TypePositionInfo needs to have either a managed or native index.
+                // We use negative values of the native index to distinguish them from the managed index.
                 if (info.ManagedIndex == TypePositionInfo.UnsetIndex)
                 {
-                    // A TypePositionInfo needs to have either a managed or native index.
-                    // We use negative values of the native index to distinguish them from the managed index.
+                    if (info.NativeIndex == 0)
+                    {
+                        // If we don't have a managed index and the native index is zero, use ReturnIndex + 1 as our
+                        // index to avoid conflict with managed parameter 0.
+                        return TypePositionInfo.ReturnIndex + 1;
+                    }
                     return -info.NativeIndex;
                 }
                 return info.ManagedIndex;
diff --git a/src/libraries/System.Runtime.InteropServices/tests/ComInterfaceGenerator.Unit.Tests/CodeSnippets.cs b/src/libraries/System.Runtime.InteropServices/tests/ComInterfaceGenerator.Unit.Tests/CodeSnippets.cs
index 78a5d550260d7..af21ae18bd081 100644
--- a/src/libraries/System.Runtime.InteropServices/tests/ComInterfaceGenerator.Unit.Tests/CodeSnippets.cs
+++ b/src/libraries/System.Runtime.InteropServices/tests/ComInterfaceGenerator.Unit.Tests/CodeSnippets.cs
@@ -85,5 +85,90 @@ sealed class NativeAPI : IUnmanagedVirtualMethodTableProvider<NoCasting>, INativ
 }
 ";
 
+        public static string BasicParametersAndModifiers(string typeName) => $@"
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
+
+[assembly:DisableRuntimeMarshalling]
+
+readonly record struct NoCasting {{}}
+partial interface INativeAPI
+{{
+    public static readonly NoCasting TypeKey = default;
+    [VirtualMethodIndex(0)]
+    {typeName} Method({typeName} value, in {typeName} inValue, ref {typeName} refValue, out {typeName} outValue);
+}}
+
+// Try using the generated native interface
+sealed class NativeAPI : IUnmanagedVirtualMethodTableProvider<NoCasting>, INativeAPI.Native
+{{
+    public VirtualMethodTableInfo GetFunctionPointerForIndex(NoCasting typeKey) => throw null;
+}}
+";
+
+        public static string BasicParametersAndModifiers<T>() => BasicParametersAndModifiers(typeof(T).FullName!);
+
+        public const string CustomTypeMarshallingTestsTypeName = "S";
+
+        public static string SimpleCustomTypeMarshallingDeclaration = $@"
+[NativeMarshalling(typeof(Marshaller))]
+struct {CustomTypeMarshallingTestsTypeName} {{}}
+
+[CustomTypeMarshaller(typeof({CustomTypeMarshallingTestsTypeName}))]
+struct Marshaller
+{{
+    public Marshaller({CustomTypeMarshallingTestsTypeName} managed) {{}}
+
+    public {CustomTypeMarshallingTestsTypeName} ToManaged() => throw null;
+}}
+";
+
+        public static string TwoStageCustomTypeMarshallingDeclaration = $@"
+[NativeMarshalling(typeof(Marshaller))]
+struct {CustomTypeMarshallingTestsTypeName} {{}}
+
+[CustomTypeMarshaller(typeof({CustomTypeMarshallingTestsTypeName}), Features = CustomTypeMarshallerFeatures.TwoStageMarshalling)]
+struct Marshaller
+{{
+    public Marshaller({CustomTypeMarshallingTestsTypeName} managed) {{}}
+
+    public {CustomTypeMarshallingTestsTypeName} ToManaged() => throw null;
+
+    public int ToNativeValue() => throw null;
+
+    public void FromNativeValue(int i) => throw null;
+}}
+";
+
+        public static string OptionalCallerAllocatedBufferMarshallingDeclaration = $@"
+[NativeMarshalling(typeof(Marshaller))]
+struct {CustomTypeMarshallingTestsTypeName}
+{{
+}}
+
+[CustomTypeMarshaller(typeof({CustomTypeMarshallingTestsTypeName}), Features = CustomTypeMarshallerFeatures.CallerAllocatedBuffer, BufferSize = 1)]
+struct Marshaller
+{{
+    public Marshaller({CustomTypeMarshallingTestsTypeName} s, System.Span<byte> b) {{}}
+    public Marshaller({CustomTypeMarshallingTestsTypeName} managed) {{}}
+
+    public {CustomTypeMarshallingTestsTypeName} ToManaged() => throw null;
+}}
+";
+
+        public static string UnmanagedResourcesCustomTypeMarshallingDeclaration = $@"
+[NativeMarshalling(typeof(Marshaller))]
+struct {CustomTypeMarshallingTestsTypeName} {{}}
+
+[CustomTypeMarshaller(typeof({CustomTypeMarshallingTestsTypeName}), Features = CustomTypeMarshallerFeatures.UnmanagedResources)]
+struct Marshaller
+{{
+    public Marshaller({CustomTypeMarshallingTestsTypeName} managed) {{}}
+
+    public {CustomTypeMarshallingTestsTypeName} ToManaged() => throw null;
+
+    public void FreeNative() {{}}
+}}
+";
     }
 }
diff --git a/src/libraries/System.Runtime.InteropServices/tests/ComInterfaceGenerator.Unit.Tests/Compiles.cs b/src/libraries/System.Runtime.InteropServices/tests/ComInterfaceGenerator.Unit.Tests/Compiles.cs
index ad1933c03563d..b395fae11037d 100644
--- a/src/libraries/System.Runtime.InteropServices/tests/ComInterfaceGenerator.Unit.Tests/Compiles.cs
+++ b/src/libraries/System.Runtime.InteropServices/tests/ComInterfaceGenerator.Unit.Tests/Compiles.cs
@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System;
 using System.Collections.Generic;
 using System.Threading.Tasks;
 using Microsoft.CodeAnalysis;
@@ -16,6 +17,29 @@ public static IEnumerable<object[]> VTableIndexCodeSnippetsToCompile()
             yield return new[] { CodeSnippets.SpecifiedMethodIndexNoExplicitParameters };
             yield return new[] { CodeSnippets.SpecifiedMethodIndexNoExplicitParametersNoImplicitThis };
             yield return new[] { CodeSnippets.SpecifiedMethodIndexNoExplicitParametersCallConvWithCallingConventions };
+
+            // Basic marshalling validation
+            yield return new[] { CodeSnippets.BasicParametersAndModifiers<byte>() };
+            yield return new[] { CodeSnippets.BasicParametersAndModifiers<sbyte>() };
+            yield return new[] { CodeSnippets.BasicParametersAndModifiers<short>() };
+            yield return new[] { CodeSnippets.BasicParametersAndModifiers<ushort>() };
+            yield return new[] { CodeSnippets.BasicParametersAndModifiers<int>() };
+            yield return new[] { CodeSnippets.BasicParametersAndModifiers<uint>() };
+            yield return new[] { CodeSnippets.BasicParametersAndModifiers<long>() };
+            yield return new[] { CodeSnippets.BasicParametersAndModifiers<ulong>() };
+            yield return new[] { CodeSnippets.BasicParametersAndModifiers<float>() };
+            yield return new[] { CodeSnippets.BasicParametersAndModifiers<double>() };
+            yield return new[] { CodeSnippets.BasicParametersAndModifiers<IntPtr>() };
+            yield return new[] { CodeSnippets.BasicParametersAndModifiers<UIntPtr>() };
+
+            // Attributed marshalling model validation
+            yield return new[] { CodeSnippets.BasicParametersAndModifiers(CodeSnippets.CustomTypeMarshallingTestsTypeName) + CodeSnippets.SimpleCustomTypeMarshallingDeclaration };
+            yield return new[] { CodeSnippets.BasicParametersAndModifiers(CodeSnippets.CustomTypeMarshallingTestsTypeName) + CodeSnippets.TwoStageCustomTypeMarshallingDeclaration };
+            yield return new[] { CodeSnippets.BasicParametersAndModifiers(CodeSnippets.CustomTypeMarshallingTestsTypeName) + CodeSnippets.OptionalCallerAllocatedBufferMarshallingDeclaration };
+            yield return new[] { CodeSnippets.BasicParametersAndModifiers(CodeSnippets.CustomTypeMarshallingTestsTypeName) + CodeSnippets.UnmanagedResourcesCustomTypeMarshallingDeclaration };
+
+            // SafeHandles
+            yield return new[] { CodeSnippets.BasicParametersAndModifiers("Microsoft.Win32.SafeHandles.SafeFileHandle") };
         }
 
         [Theory]