Passing strongly typed arguments in .NET COM interop

Question

I have two .NET classes exposed via COM interop - let's say Foo and Bar, and I need to pass an argument of type Foo to a method defined in Bar. Something like this:

[ComVisible(true)]
public class Foo
{
    // whatever
}

[ComVisible(true)]
public class Bar
{
    public void Method(Foo fff)
    {
        // do something with fff
    }
}

When I run the following VBS (using cscript.exe):

set foo = CreateObject("TestCSProject.Foo")
set bar = CreateObject("TestCSProject.Bar")
call bar.Method(foo)

I get an error:

D:\test.vbs(3, 1) Microsoft VBScript runtime error: Invalid procedure call or argument: 'bar.Method'

However, if I change the Method declaration to this:

    public void Method(object o)
    {
        Foo fff = (Foo)o;
        // do something with fff
    }

everything works. I tried some magic with interfaces, attributes, etc. but no luck so far.

Any insight?

Many thanks

Answer 1

Make sure, you define a GUID attribute, this is necessary if you make a QueryInterface (VB does probably). You have to generate a new unique GUID for every comvisible class.

[Guid("77777777-3333-40df-9C0D-2B580E7E1F3B")]
[ComVisible(true)]
public class Foo
{
}

Then i would strongly recommend to write interfaces for your COM objects, and set the ClassInterface to None, so no internals are revealed. Your typelibrary will be much cleaner this way.

[Guid("88888888-ABCD-458c-AB4C-B14AF7283A6B")]
[ComVisible(true)]
public interface IFoo
{
}

[ClassInterface(ClassInterfaceType.None)]
[Guid("77777777-3333-40df-9C0D-2B580E7E1F3B")]
[ComVisible(true)]
public class Foo : IFoo
{
}

Answer 2

After struggling with this same issue for a while, I found that is was having issues with passing argments by reference instead of by value. See here:

http://msdn.microsoft.com/en-us/library/ee478101.aspx

So I just added round brackets to the passed argument in VB Script, and it seemed to have solved the problem. So in your example, just doing this:

Set foo = CreateObject("TestCSProject.Foo")
Set bar = CreateObject("TestCSProject.Bar")
Call bar.Method((foo))

Should work as expected, without having to set the ClassInterface attribute, and without using Interfaces.