How do you call methods in a C# COM object from C

[richard190958]

Hi All,
I need your help...
I have a large application written in C (compiled using VS 6), which needs to access functions within a library written in C#. Having done some research and it appears that calling C# from C is possible, if the C# is assembled with the "Register for COM interop" setting.
I have installed Visual Studio 2005, and compiled a sample C# program, however, my C program needs a header file, with the function prototypes etc.
I understand that you use the MIDL compiler to create the required header files, however, this in turn requires a IDL file, which is not being created.
I am taking the correct approach?
What steps do I need to take to generate the header files needed by my C program?
Thanks In Advance
Richard

[Plater]

It was my understanding that in VB6 you would just use the CreateObject (or whatever it was called) function to create an instance of your C#(.Net) classes from your DLL that has the register for COM or whatever.

I have never done it though, so I am curious as to how it is done as well.

[richard190958]

Hi,
I have already found an example of creating an instance of an object in C. The following was taken from the MSDN library:

HRESULT hr;
struct IFoo *pIFoo;
struct IGoo *pIGoo;

printf("Hello, world!\n\n");
hr = CoInitialize(NULL);
if (FAILED(hr)) {
   printf("CoInitialize Failed: %x\n\n", hr);
   exit(1);
}
else {
   printf("CoInitialize succeeded\n");
}

hr = CoCreateInstance(&CLSID_MyObject, NULL, CLSCTX_ALL, 
            &IID_IFoo, (void **)&pIFoo);
if (FAILED(hr)) {
   printf("CoCreateInstance Failed: %x\n\n", hr);
   goto Uninit;
}
else {
   printf("CoCreateInstance succeeded\n");
}

Having created the object, the required functions can be called by using the pointer within the vTable like so:

pIFoo->lpVtbl->Func1(pIFoo);

What I am missing, is the header file, which would contain the structures needed by the compiler, which should look like:

typedef struct IFooVtbl
{
    BEGIN_INTERFACE
    
    HRESULT ( STDMETHODCALLTYPE __RPC_FAR *QueryInterface )( 
        IFoo __RPC_FAR * This,
        /* [in] */ REFIID riid,
        /* [iid_is][out] */ void __RPC_FAR *__RPC_FAR *ppvObject);
        
    ULONG ( STDMETHODCALLTYPE __RPC_FAR *AddRef )( 
        IFoo __RPC_FAR * This);
    
    ULONG ( STDMETHODCALLTYPE __RPC_FAR *Release )( 
        IFoo __RPC_FAR * This);
    
    HRESULT ( STDMETHODCALLTYPE __RPC_FAR *Func1 )( 
        IFoo __RPC_FAR * This);
    
    HRESULT ( STDMETHODCALLTYPE __RPC_FAR *Func2 )( 
        IFoo __RPC_FAR * This,
        int inonly);
    
    END_INTERFACE
} IFooVtbl;

According to the Microsoft documentation, the MIDL compiler can generate the headers, however, I do not know how this works with C#, as there is no IDL file. As I stated in my initial post, I have assembled a sample C# program with the "Register for COM interop", and trusted that there would be a method of creating the header file needed by my C program.