[Mono-dev] COM Interop Patch

Rafael Teixeira monoman at gmail.com
Fri Jul 14 10:42:39 EDT 2006

Very nice Jon. Bravo!!!

On 7/12/06, Jon Chambers <joncham at gmail.com> wrote:
> Here is another attempt at a COM Interop patch. First, all
> changes/contributions are MIT X11. Now, for a brief overview ;-).
> I implemented COM Interop on top of the current remoting infrastructure.
> This allows for two main things. 1. Forwarding of method calls to the
> underlying unmanaged COM object. 2. Casting RCW (Runtime Callable Wrappers -
> the managed wrapper around the unmanaged COM object) to interfaces not
> specified as implemented in metadata. This can occur if a QueryInterface for
> that interface's Guid (IID) succeeds on the underlying object.
> So, when a user says "MyComObj obj = new MyComObj()" the runtime creates a
> ComInteropProxy and returns it's TransparentProxy. However, instead of
> forwarding methods via the remoting invoke mechanism using messages, I
> shortcut to a Com Interop invoke. The is a great performance boost (by a few
> orders of magnitude if I recall). I first emit the invoke call, which
> transitions the call from a call on the transparent proxy, into a call on
> the actuall RCW.
> The method implementation is done by cominterop_get_native_wrapper. This
> emits a method whose signature matches the managed method. The emitted
> method calls a final emitted method created by
> cominterop_get_native_wrapper_adjusted. The two methods,
> created by cominterop_get_native_wrapper and
> cominterop_get_native_wrapper_adjusted are 1-1. The
> adjusted method matches the unmanaged signature, and as thus can reuse all
> the existing unmanaged calling functionality provided by
> mono_marshal_emit_native_wrapper. The only small change to
> mono_marshal_emit_native_wrapper was that the function
> pointer is push onto the stack at call time, rather than when the method is
> emitted since the function pointer depends on the object making the call
> (different objects implementing the same interface could have different
> implementations, and thus vtables, thus function pointers).
> Another minor note is the internal calls added to __ComObject. This is so I
> can store a hashtable of COM interfaces and later release them in the
> finalizer to ensure proper reference counting. I had a Hashtable in managed
> code intially but couldn't access it in my finalizer.
> I also implemented a series of COM Interop related methods in the Marshal
> class.
> On windows you should be able to run some basic tests. I added a few to the
> cominterop.cs test file but disabled them for the moment. COM is a binary
> standard and I wasn't sure what vtable layouts would be like on Solaris,
> ARM, etc. (I know HP is different) and didn't want to cause regressions on
> those platforms. But, on windows/linux x86/64 you can enable them and run
> them. Or more excitingly you can add a reference to an interop assembly on
> MS and test run real COM objects (Internet Explorer for example). The
> largest thing missing right now is the marshalling of com objects. So, you
> can't call any methods that have parameters/return value of COM objects.
> This will be the next thing I work on.
> I'm sure there are some issues, so please review and I'll fix them ASAP.
> Thanks,
> Jonathan
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Rafael "Monoman" Teixeira
"The reasonable man adapts himself to the world; the unreasonable one
persists in trying to adapt the world to himself. Therefore all
progress depends on the unreasonable man." George Bernard Shaw

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