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Disable marshalling delegates as _Delegate and enable marshalling delegates as IDispatch.#23738
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…egates as IDispatch.
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…egates as IDispatch. (dotnet/coreclr#23738) * Disable marshalling delegates as _Delegate and enable marshalling delegates as IDispatch. * ifdef out the new IDispatch marshalling on non-COM-supporting platforms. * PR feedback. Commit migrated from dotnet/coreclr@8241971
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As part of the transition to .NET Core, we dropped the concept of the mscorlib tlb interfaces. However, in the marshalling space we continued to imply that their usage was safe, particularly with delegates. In .NET Framework, it is possible to marshal a delegate as a COM object implementing the
_Delegateinterface provided by themscorlib.tlbtype library. Since we did not make many significant changes to theSystem.Delegatetype in .NET Core, this behavior continued to work in .NET Core. However, when theSystem.Delegatetype was moved to the shared partition in #23552, the change to the compiled type's surface was large enough to change how the dispid's were assigned, breaking the marshalling to the Framework_Delegateinterface and resulting in bad runtime errors (crashes, incorrect return values depending on exactly how the interface pointer is generated).We've decided that since the
_DelegateCOM interface is a .NET Framework type, that we are not going to try to add back support to marshalling to it. Instead, we are changing behavior as follows:[MarshalAs(UnmanagedType.Interface)]on a delegate-typed field, parameter, or return value is illegal.[MarshalAs(UnmanagedType.Interface)].[MarshalAs(UnmanagedType.IDispatch)]on a delegate-typed field, parameter or return value is now legal and is the suggested change for consumers. Additionally, these consumers will have to change their native code to take anIDispatchpointer and operate on the delegate's COM wrapper via theIDispatchinterface instead of the_Delegateinterface.Fixes #23730.