Use static-abstracts + DIMs to remove the extra shape validation.

docs/design/libraries/ComInterfaceGenerator/VTableStubs.md

@@ -57,7 +57,7 @@ public class VirtualMethodIndexAttribute : Attribute
 
 ```
 
-A new interface will be defined and used by the source generator to fetch the native `this` pointer and the vtable that the function pointer is stored in. This interface is designed to provide an API that various native platforms, like COM, WinRT, or Swift, could use to provide support for multiple managed interface wrappers from a single native object. In particular, this interface was designed to ensure it is possible support a managed gesture to do an unmanaged "type cast" (i.e., `QueryInterface` in the COM and WinRT worlds).
+New interfaces will be defined and used by the source generator to fetch the native `this` pointer and the vtable that the function pointer is stored in. These interfaces are designed to provide an API that various native platforms, like COM, WinRT, or Swift, could use to provide support for multiple managed interface wrappers from a single native object. In particular, these interfaces are designed to ensure it is possible support a managed gesture to do an unmanaged "type cast" (i.e., `QueryInterface` in the COM and WinRT worlds).
 
 ```csharp
 namespace System.Runtime.InteropServices;
@@ -82,13 +82,24 @@ public readonly ref struct VirtualMethodTableInfo
 
 public interface IUnmanagedVirtualMethodTableProvider<T> where T : IEquatable<T>
 {
-    VirtualMethodTableInfo GetVirtualMethodTableInfoForKey(T typeKey);
+    protected VirtualMethodTableInfo GetVirtualMethodTableInfoForKey(T typeKey);
+
+    public sealed VirtualMethodTableInfo GetVirtualMethodTableInfoForKey<TUnmanagedInterfaceType>()
+        where TUnmanagedInterfaceType : IUnmanagedInterfaceType<T>
+    {
+        return GetVirtualMethodTableInfoForKey(TUnmanagedInterfaceType.TypeKey);
+    }
+}
+
+public interface IUnmanagedInterfaceType<T> where T : IEquatable<T>
+{
+    public abstract static T TypeKey { get; }
 }
 ```
 
 ## Required API Shapes
 
-In addition to the provided APIs above, users will be required to add a `readonly static` field or `get`-able property to their user-defined interface type named `TypeKey`. The type of this member will be used as the `T` in `IUnmanagedVirtualMethodTableProvider<T>` and the value will be passed to `GetVirtualMethodTableInfoForKey`. This mechanism is designed to enable each native API platform to provide their own casting key, for example `IID`s in COM, without interfering with each other or requiring using reflection-based types like `System.Type`.
+The user will be required to implement `IUnmanagedVirtualMethodTableProvider<T>` on the type that provides the method tables, and `IUnmanagedInterfaceType<T>` on the type that defines the unmanaged interface. The `T` types must match between the two interfaces. This mechanism is designed to enable each native API platform to provide their own casting key, for example `IID`s in COM, without interfering with each other or requiring using reflection-based types like `System.Type`.
 
 ## Example Usage
 
@@ -149,11 +160,11 @@ using System.Runtime.InteropServices;
 [assembly:DisableRuntimeMarshalling]
 
 // Define the interface of the native API
-partial interface INativeAPI
+partial interface INativeAPI : IUnmanagedInterfaceType<NoCasting>
 {
     // There is no concept of casting for this API, but providing a type key is still required by the generator.
     // Use an empty readonly record struct to provide a type that implements IEquatable<T> but contains no data.
-    readonly static NoCasting TypeKey = default;
+    static NoCasting IUnmanagedInterfaceType.TypeKey => default;
 
     [VirtualMethodIndex(0, ImplicitThisParameter = false, Direction = CustomTypeMarshallerDirection.In)]
     int GetVersion();
@@ -218,7 +229,7 @@ partial interface INativeAPI
     {
         int INativeAPI.GetVersion()
         {
-            var (_, vtable) = ((IUnmanagedVirtualMethodTableProvider<NoCasting>)this).GetVirtualMethodTableInfoForKey(INativeAPI.TypeKey);
+            var (_, vtable) = ((IUnmanagedVirtualMethodTableProvider<NoCasting>)this).GetVirtualMethodTableInfoForKey<INativeAPI>();
             int retVal;
             retVal = ((delegate* unmanaged<int>)vtable[0])();
             return retVal;
@@ -231,7 +242,7 @@ partial interface INativeAPI
     {
         int INativeAPI.Add(int x, int y)
         {
-            var (_, vtable) = ((IUnmanagedVirtualMethodTableProvider<NoCasting>)this).GetVirtualMethodTableInfoForKey(INativeAPI.TypeKey);
+            var (_, vtable) = ((IUnmanagedVirtualMethodTableProvider<NoCasting>)this).GetVirtualMethodTableInfoForKey<INativeAPI>();
             int retVal;
             retVal = ((delegate* unmanaged<int, int, int>)vtable[1])(x, y);
             return retVal;
@@ -244,7 +255,7 @@ partial interface INativeAPI
     {
         int INativeAPI.Multiply(int x, int y)
         {
-            var (_, vtable) = ((IUnmanagedVirtualMethodTableProvider<NoCasting>)this).GetVirtualMethodTableInfoForKey(INativeAPI.TypeKey);
+            var (_, vtable) = ((IUnmanagedVirtualMethodTableProvider<NoCasting>)this).GetVirtualMethodTableInfoForKey<INativeAPI>();
             int retVal;
             retVal = ((delegate* unmanaged<int, int, int>)vtable[2])(x, y);
             return retVal;
@@ -279,9 +290,9 @@ struct IUnknown
 using System;
 using System.Runtime.InteropServices;
 
-interface IUnknown
+interface IUnknown: IUnmanagedInterfaceType<Guid>
 {
-    public static readonly Guid TypeKey = Guid.Parse("00000000-0000-0000-C000-000000000046");
+    static Guid IUnmanagedTypeInterfaceType<Guid>.TypeKey => Guid.Parse("00000000-0000-0000-C000-000000000046");
 
     [UnmanagedCallConv(CallConvs = new[] { typeof(CallConvStdcall), typeof(CallConvMemberFunction) })]
     [VirtualMethodIndex(0)]
@@ -347,7 +358,7 @@ partial interface IUnknown
     {
         int IUnknown.QueryInterface(in Guid riid, out IntPtr ppvObject)
         {
-            var (thisPtr, vtable) = ((IUnmanagedVirtualMethodTableProvider<Guid>)this).GetVirtualMethodTableInfoForKey(IUnknown.TypeKey);
+            var (thisPtr, vtable) = ((IUnmanagedVirtualMethodTableProvider<Guid>)this).GetVirtualMethodTableInfoForKey<IUnknown>();
             int retVal;
             fixed (Guid* riid__gen_native = &riid)
             fixed (IntPtr* ppvObject__gen_native = &ppvObject)
@@ -364,7 +375,7 @@ partial interface IUnknown
     {
         uint IUnknown.AddRef()
         {
-            var (thisPtr, vtable) = ((IUnmanagedVirtualMethodTableProvider<Guid>)this).GetVirtualMethodTableInfoForKey(IUnknown.TypeKey);
+            var (thisPtr, vtable) = ((IUnmanagedVirtualMethodTableProvider<Guid>)this).GetVirtualMethodTableInfoForKey<IUnknown>();
             uint retVal;
             retVal = ((delegate* unmanaged[Stdcall, MemberFunction]<IntPtr, uint>)vtable[1])(thisPtr);
             return retVal;
@@ -377,7 +388,7 @@ partial interface IUnknown
     {
         uint IUnknown.Release()
         {
-            var (thisPtr, vtable) = ((IUnmanagedVirtualMethodTableProvider<Guid>)this).GetVirtualMethodTableInfoForKey(IUnknown.TypeKey);
+            var (thisPtr, vtable) = ((IUnmanagedVirtualMethodTableProvider<Guid>)this).GetVirtualMethodTableInfoForKey<IUnknown>();
             uint retVal;
             retVal = ((delegate* unmanaged[Stdcall, MemberFunction]<IntPtr, uint>)vtable[2])(thisPtr);
             return retVal;

src/libraries/System.Runtime.InteropServices/gen/ComInterfaceGenerator/ManagedToNativeVTableMethodGenerator.cs

@@ -117,7 +117,7 @@ public BlockSyntax GenerateStubBody(int index, ImmutableArray<FunctionPointerUnm
         {
             var setupStatements = new List<StatementSyntax>
             {
-                // var (<thisParameter>, <virtualMethodTable>) = ((IUnmanagedVirtualMethodTableProvider<<typeKeyType>>)this).GetVirtualMethodTableInfoForKey(<containingTypeName>.TypeKey)
+                // var (<thisParameter>, <virtualMethodTable>) = ((IUnmanagedVirtualMethodTableProvider<<typeKeyType>>)this).GetVirtualMethodTableInfoForKey<<containingTypeName>>();
                 ExpressionStatement(
                     AssignmentExpression(
                         SyntaxKind.SimpleAssignmentExpression,
@@ -141,15 +141,12 @@ public BlockSyntax GenerateStubBody(int index, ImmutableArray<FunctionPointerUnm
                                             TypeArgumentList(
                                                 SingletonSeparatedList(typeKeyType.Syntax))),
                                         ThisExpression())),
-                                IdentifierName("GetVirtualMethodTableInfoForKey")))
+                                GenericName(
+                                    Identifier("GetVirtualMethodTableInfoForKey"),
+                                    TypeArgumentList(
+                                        SingletonSeparatedList(containingTypeName)))))
                         .WithArgumentList(
-                            ArgumentList(
-                                SingletonSeparatedList(
-                                    Argument(
-                                        MemberAccessExpression(
-                                            SyntaxKind.SimpleMemberAccessExpression,
-                                            containingTypeName,
-                                            IdentifierName("TypeKey"))))))))
+                            ArgumentList())))
             };
 
             GeneratedStatements statements = GeneratedStatements.Create(

src/libraries/System.Runtime.InteropServices/gen/ComInterfaceGenerator/VtableIndexStubGenerator.cs

@@ -228,6 +228,7 @@ private static IncrementalStubGenerationContext CalculateStubInformation(MethodD
             INamedTypeSymbol? lcidConversionAttrType = environment.Compilation.GetTypeByMetadataName(TypeNames.LCIDConversionAttribute);
             INamedTypeSymbol? suppressGCTransitionAttrType = environment.Compilation.GetTypeByMetadataName(TypeNames.SuppressGCTransitionAttribute);
             INamedTypeSymbol? unmanagedCallConvAttrType = environment.Compilation.GetTypeByMetadataName(TypeNames.UnmanagedCallConvAttribute);
+            INamedTypeSymbol iUnmanagedInterfaceTypeType = environment.Compilation.GetTypeByMetadataName(TypeNames.IUnmanagedInterfaceType_Metadata)!;
             // Get any attributes of interest on the method
             AttributeData? virtualMethodIndexAttr = null;
             AttributeData? lcidConversionAttr = null;
@@ -310,14 +311,14 @@ private static IncrementalStubGenerationContext CalculateStubInformation(MethodD
             var typeKeyOwner = ManagedTypeInfo.CreateTypeInfoForTypeSymbol(symbol.ContainingType);
             ManagedTypeInfo typeKeyType = SpecialTypeInfo.Byte;
 
-            IFieldSymbol? typeKeyField = symbol.ContainingType.GetMembers("TypeKey").OfType<IFieldSymbol>().FirstOrDefault(f => f.IsStatic);
-            if (typeKeyField is null)
+            INamedTypeSymbol? iUnmanagedInterfaceTypeInstantiation = symbol.ContainingType.AllInterfaces.FirstOrDefault(iface => SymbolEqualityComparer.Default.Equals(iface.OriginalDefinition, iUnmanagedInterfaceTypeType));
+            if (iUnmanagedInterfaceTypeInstantiation is null)
             {
                 // Report invalid configuration
             }
             else
             {
-                typeKeyType = ManagedTypeInfo.CreateTypeInfoForTypeSymbol(typeKeyField.Type);
+                typeKeyType = ManagedTypeInfo.CreateTypeInfoForTypeSymbol(iUnmanagedInterfaceTypeInstantiation.TypeArguments[0]);
             }
 
             return new IncrementalStubGenerationContext(

src/libraries/System.Runtime.InteropServices/gen/Microsoft.Interop.SourceGeneration/TypeNames.cs

@@ -32,6 +32,8 @@ public static class TypeNames
 
         public const string IUnmanagedVirtualMethodTableProvider = "System.Runtime.InteropServices.IUnmanagedVirtualMethodTableProvider";
 
+        public const string IUnmanagedInterfaceType_Metadata = "System.Runtime.InteropServices.IUnmanagedInterfaceType`1";
+
         public const string System_Span_Metadata = "System.Span`1";
         public const string System_Span = "System.Span";
         public const string System_ReadOnlySpan_Metadata = "System.ReadOnlySpan`1";

src/libraries/System.Runtime.InteropServices/tests/Ancillary.Interop/IUnmanagedVirtualMethodTableProvider.cs

@@ -29,7 +29,18 @@ public void Deconstruct(out IntPtr thisPointer, out ReadOnlySpan<IntPtr> virtual
 
     public interface IUnmanagedVirtualMethodTableProvider<T> where T : IEquatable<T>
     {
-        VirtualMethodTableInfo GetVirtualMethodTableInfoForKey(T typeKey);
+        protected VirtualMethodTableInfo GetVirtualMethodTableInfoForKey(T typeKey);
+
+        public sealed VirtualMethodTableInfo GetVirtualMethodTableInfoForKey<TUnmanagedInterfaceType>()
+            where TUnmanagedInterfaceType : IUnmanagedInterfaceType<T>
+        {
+            return GetVirtualMethodTableInfoForKey(TUnmanagedInterfaceType.TypeKey);
+        }
     }
 
+
+    public interface IUnmanagedInterfaceType<T> where T : IEquatable<T>
+    {
+        public abstract static T TypeKey { get; }
+    }
 }

src/libraries/System.Runtime.InteropServices/tests/ComInterfaceGenerator.Tests/ImplicitThisTests.cs

@@ -18,9 +18,9 @@ internal partial class ImplicitThis
         {
             public readonly record struct NoCasting;
 
-            internal partial interface INativeObject
+            internal partial interface INativeObject : IUnmanagedInterfaceType<NoCasting>
             {
-                public static readonly NoCasting TypeKey = default;
+                static NoCasting IUnmanagedInterfaceType<NoCasting>.TypeKey => default;
 
                 [VirtualMethodIndex(0, ImplicitThisParameter = true)]
                 int GetData();

src/libraries/System.Runtime.InteropServices/tests/ComInterfaceGenerator.Tests/NoImplicitThisTests.cs

@@ -18,9 +18,9 @@ internal partial class NoImplicitThis
         {
             public readonly record struct NoCasting;
 
-            internal partial interface IStaticMethodTable
+            internal partial interface IStaticMethodTable : IUnmanagedInterfaceType<NoCasting>
             {
-                public static readonly NoCasting TypeKey = default;
+                static NoCasting IUnmanagedInterfaceType<NoCasting>.TypeKey => default;
 
                 [VirtualMethodIndex(0, ImplicitThisParameter = false)]
                 int Add(int x, int y);