Calling method from COM Object using Reflection

In order to allow compatibility with another project that is written in .Net 2.0, we have had to come up with a COM interop (the newer application is in .Net 4.0). This would work because the 2.0 process would be able to use SxS execution with .Net 4.0. In order to have a COM interop from what I understand I have to do something like this:

Type myClass = Type.GetTypeFromProgID("Net4Assembly.Assembly4");
object myInstance = Activator.CreateInstance(myClass);
IAssembly4 assembly4Interface = (IAssembly4)myInstance;
assembly4Interface.CallMethod();

I have already created the COM component and registered it and this works fine. But the problem is that since the project written in 2.0 is outside our department, I want to find a way of doing the casting in line 3 above using reflection. So far I have found a suggestion in Invoke method using Reflection on COM Object

But this doesn't work for me since when I get all the methods of the object in "myInstance" which is of type COMObject, I can only see the methods that are mentioned in that link. I get this error:

Unknown name. (Exception from HRESULT: 0x80020006 (DISP_E_UNKNOWNNAME))

I think I should somehow cast the COMObject to the interface and then I would have access to the methods? Shouldn't I be able to extract the interface from the COMObject, then call the method using reflection? I tried GetInterfaces() from the COMObject but nothing is returned.

I am not sure if this will work but assuming you have No Misspelled Assembly Name try something like this

Type myClass  = Type.GetTypeFromProgID("Net4Assembly.Assembly4");
object myInstance = Activator.CreateInstance(myClass );
//object[] arguments = new object[] //add parameters if youre assembly expects them here
object result = myClass .InvokeMember("SubtractTwoNumbers", BindingFlags.InvokeMethod,
     null, myInstance, arguments);