Silverlight 4 and COM Interop

Question

I've created a ComVisible-class:

[Guid("73a3f91f-baa9-46ab-94b8-e526c22054a4"), ComVisible(true)]
public interface ITest
{
    void Foo();
}

[Guid("99f72d92-b302-4fde-89bb-2dac899f5a48"), ComVisible(true)]
public class Class1 : ITest
{
    public void Foo() { }
}

and registered it via

regasm ComClassTest.dll /tlb:ComClassTest.tlb

into the registry. When I try to call it in my Silverlight 4 out-of-browser, elevated trust application like this:

var foo = AutomationFactory.CreateObject("ComClassTest.Class1");

I get an exception "{System.Exception: Failed to create an object instance for the specified ProgID."

However, I am able to call AutomationFactory.CreateObject("Word.Application") without an Exception and to call Activator.CreateInstance(Type.GetTypeFromProgID("ComClassTest.Class1")) in a normal C#-console application if I copy the ComClassTest.dll into the bin-directory.

What have I forgotton?

Answer 1

First thing to do is test that you can create the object from somewhere else such as VBScript. Create a .vbs file with the content:-

o = CreateObject("ComClassTest.Class1")

If that doesn't generate an error then there is something specifically that SL OOB is upset with otherwise your problem isn't really related to Silverlight at all.

Consider making the following changes to your COM code.

• Its often easier to specify ComVisible(true) at the assembly level. You can do that from the application tab of the project properties on the Assembly Information dialog. You can also get Visual Studio to register the assembly with COM and build time using the option found on the build tab of the project properties.

• Its a good idea to be specific about the ComInterfaceType you want to expose.

• Things get really messy if you expose the class interface directly generally you only want the interface you have defined to be used and that this be the default interface for the class. In addition it probably better to stick to COM naming conventions for the default interface of a class.

• Finally (and possibly crucially in your case) its a good idea to be explicit about the ProgId to use for the class.

Applying the above and we get:-

[InterfaceType(ComInterfaceType.InterfaceIsDual)]
[Guid("73a3f91f-baa9-46ab-94b8-e526c22054a4")]
public interface _Class1
{
    void Foo();
}

[ClassInterface(ClassInterfaceType.None)] 
[Guid("99f72d92-b302-4fde-89bb-2dac899f5a48")]
[ProgId("ComClassTest.Class1")]
public class Class1 : _Class1
{
    public void Foo() { }
}