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Ivan Shcherbakov
Ivan Shcherbakov

Reputation: 2113

How to override casting from a managed object to a COM interface?

There is a 3rd-party .Net assembly that defines an imported COM interface, gets an object from another COM object and casts it to the imported interface:

[ComImport, Guid(...), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
private interface IMyInterface
{
    //...
}

void SomeMethod(object obj)
{
    IMyInterface iface = obj as IMyInterface ;
    if (iface == null)
        throw("Cannot get IMyInterface");
}

The method is public, while the imported COM interface is internal. How do I create my own managed object that implements that COM interface? The obvious solution of re-importing the same interface in my assembly does not work:

[ComImport, Guid(...), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
private interface IMyInterface
{
    //...
}

class MyClass : IMyInterface
{
}

SomeMethod(new MyClass());

Despite having the same GUID and being marked as a COM interface, the .Net runtime treats the imported interfaces as 2 different interfaces and does not cast my object to the interface declared in the other assembly.

I cannot reference the imported interface from the first assembly because it's not declared as public. Can I somehow instruct the .Net runtime to create an RCW for my managed object and hide the actual one or otherwise override the default casting behavior?

I am aware of the dynamic binder, however the code needs to run on .Net 2.0+, so it is unfortunately not an option.

Upvotes: 0

Views: 553

Answers (1)

Bryce Wagner
Bryce Wagner

Reputation: 2650

If you're stuck using .NET 2.0, your best would be to use reflection to grab the type from the other assembly and then use Marshal.CreateWrapperOfType().

For example, you could call it like this:

var t1 = Type.GetType(assemblyQualifiedName);
object properlyTypedObj = System.Runtime.InteropServices.Marshal.CreateWrapperOfType(myobj, t1);

Upvotes: 0

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