Question

(preliminary note: I’m not yet fully up to speed with the whole ‘interop’ thing…)

When using a COM library from within .NET, all HRESULT methods are wrapped into something that throws when the return code is not SUCCEEDED.

//ATL magic exluded
class C {
    HRESULT foo(){ return E_FAIL; }
};

// usage code:
if( SUCCEEDED( c.foo() ) ) {
   // success code
} else {
   // failure code
}

The .NET counterpart of this code reads:

try {
   c.foo();
   // success code
} catch ( Exception e ) {
   // failure code
}

Is there a way to access the COM return code directly in .NET, so that no exception handling is needed?

Answer 1

Yes, but you’ll have to manually define the interop interface (rather than use tlbimp.exe) and use the PreserveSig attribute on the methods in question.

For example:

[ComImport]
[Guid("your-guid-here")]
[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
interface IMyComInterface
{
     [PreserveSig]
     int DoSomething(out int result);
}

That is the equivalent of a COM method with the signature HRESULT DoSomething([out, retval] int *result);

If your interface is very complicated or you get stuck on how to define the interop interface, I recommend using tlbimp.exe, then using Reflector or ILSpy or something similar to decompile the generated interfaces, and then edit those to your liking. Saves work, too. 🙂