Use 32bit COM Server from a 64bit .NET Program

Question

I'm having some trouble with COM Interop, the situation is as follows:

A 32-Bit COM Exe Server (that was programmed in C++) offers a class with some member functions that deal with 3rd party hardware (This hardware also ties the COM Exe Server to 32-Bit, since the manufacturer doesn't support 64-Bit).

I want to use the 32-Bit COM Exe Server in a 64-Bit .NET (C#) Application... At first I tried to add a reference to the Exe Server in Visual Studio 2010 and it created an Interop-DLL. This Interop-DLL provided me with the necessary functions, one of them being declared as:

int Initialize(ref string callingApplicationPath);

The original declaration in C++ looks like this:

LONG Class::Initialize(BSTR* callingApplicationPath)

...and like this in IDL:

[id(1)] LONG Initialize([in] BSTR* callingApplicationPath);

However, when I want to call this function from C# via the Interop-DLL, it throws a BadImageFormatException. Looks like the Interop-DLL is a 32-Bit DLL (Maybe there's a possibility to generate a 64-Bit-DLL?).

My next attempt was to instantiate the Exe Server with this code:

Type type = Type.GetTypeFromProgID("OurCompany.Class");
Object o = Activator.CreateInstance(type);
Object[] args = { Marshal.StringToBSTR(str) };
Object result = type.InvokeMember("Initialize", BindingFlags.InvokeMethod, null, o, args);

This code, on the other hand, throws a TargetInvocationException (More specifically: 0x80020005 (DISP_E_TYPEMISMATCH)) at my head. Unfortunately I was unable to find out what type I have to pass in to the function from C#... I tried all the StringToXXX-functions in the Marshal-class but nothing seems to work :/ I guess I'm missing something simple here, but I don't see what.

Any help is greatly appreciated!

Best Regards

Christian

Answer 1

The IDL declaration

[id(1)] LONG Initialize([in] BSTR* str);    

makes no sense. When you pass a BSTR as an in parameter just pass it "by value":

[id(1)] LONG Initialize([in] BSTR str);

then you will not need to do anything special in C# code - just pass string there and marshalling will be done automatically.

Of course you'll have to change the method implementation signature as well.