Opt COM methods out of the new Windows instance-method handling. (#23974)

* Opt COM methods out of the new Windows instance-method handling.

* Add test for an HResult "struct" returned from a COM method.

* Update ErrorMarshalTesting.cs

* Update "is member function" check on the ilmarshalers.h side to only consider thiscall.

1 files changed, 4 insertions, 1 deletions

diff --git a/src/vm/dllimport.cpp b/src/vm/dllimport.cpp
index 924a50fe9d..93d0f79ca0 100644
--- a/src/vm/dllimport.cpp
+++ b/src/vm/dllimport.cpp
@@ -3956,7 +3956,10 @@ static void CreateNDirectStubWorker(StubState*         pss,
     BOOL fMarshalReturnValueFirst = FALSE;
 
     BOOL fReverseWithReturnBufferArg = FALSE;
-    bool isInstanceMethod = fStubNeedsCOM || fThisCall;
+    // Only consider ThisCall methods to be instance methods.
+    // Techinically COM methods are also instance methods, but we don't want to change the behavior of the built-in
+    // COM abi work because there are many users that rely on the current behavior (for example WPF).
+    bool isInstanceMethod = fThisCall;
     
     // We can only change fMarshalReturnValueFirst to true when we are NOT doing HRESULT-swapping!
     // When we are HRESULT-swapping, the managed return type is actually the type of the last parameter and not the return type.