How to use IDispatch in plain C to call a COM object

Question

I need to compile some code of mine using the gcc compiler included in the R tools (R the statistical program for windows), the problem is that I need to use IDispatch in my code to create an access the methods of a COM object, and the gcc compiler doesn't support much of the code that I'm using to do so, which is basically C++ code.

So my question is how can I use IDispatch in C to create the COM object without having to depend on MFC, .NET, C#, WTL, or ATL. I believe that if I do so I will be able to compile my code without any problem.

Answer 1

In general, a C++ IDispatch interface is just a table of function pointers. In C, it would look something like:

typedef struct {
  HRESULT(*pQueryInterface)(void* this, REFIID riid, void **ppvObject);
  ULONG(*pAddRef)(void* this);
  ULONG(*pRelease)(void* this);
  HRESULT(*pGetTypeInfoCount)(void* this, unsigned int* pctInfo);
  HRESULT(*pGetTypeInfo)(void* this, unsigned int iTInfo,LCID lcid, ITypeInfo** ppTInfo);
  HRESULT(*pGetIDsOfNames)(void* this, REFIID riid, OLECHAR** rgszNames, unsigned int cNames, LCID lcid, DISPID* rgDispId);
 HRESULT(*pInvoke)(void* this, DISPID dispIdMember, REFIID riid, LCID lcid, WORD wFlags,DISPPARAMS* pDispParams, VARIANT* pVarResult, EXCEPINFO* pExcepInfo, unsigned int* puArgErr);
} IDispatch_in_C;

Note that every method has a THIS pointer as the first parameter, and that you will need to define more types, such as ITypeInfo, REFIID, DISPID, etc, etc.

So, its a big task. But it is possible to create C++ interfaces in pure C.

Answer 2

There is a great article on CodeProject entitled "COM in plain C".

Here is the link to Part 1.

There is a lot of very good info on working with COM in C in that article and the author's subsequent follow-ups (I think there are 3 or 4 in the series).

Edit:
I was wrong, there are 8 parts!

Part 2
Part 3
Part 4
Part 5
Part 6
Part 7
Part 8

Answer 3

Also you can use disphelper library.