Calling DCOM/COM+ methods in C.

dcom.c

#define _CRT_SECURE_NO_WARNINGS
#include <ole2.h> // OLE2 Definitions
#include <Windows.h>
#include <stdio.h>

HRESULT AutoWrap(int autoType, VARIANT *pvResult, IDispatch *pDisp, LPOLESTR ptName, int cArgs...) {
	va_list marker;
	va_start(marker, cArgs);

	if (!pDisp) {
		_exit(0);
	}

	DISPPARAMS dp = { NULL, NULL, 0, 0 };
	DISPID dispidNamed = DISPID_PROPERTYPUT;
	DISPID dispID;
	HRESULT hr;
	char buf[200];
	char szName[200];

	WideCharToMultiByte(CP_ACP, 0, ptName, -1, szName, 256, NULL, NULL);

	hr = pDisp->GetIDsOfNames(IID_NULL, &ptName, 1, LOCALE_USER_DEFAULT, &dispID);
	if (FAILED(hr)) {
		_exit(0);
		return hr;
	}

	VARIANT *pArgs = new VARIANT[cArgs + 1];

	for (int i = 0; i < cArgs; i++) {
		pArgs[i] = va_arg(marker, VARIANT);
	}

	dp.cArgs = cArgs;
	dp.rgvarg = pArgs;

	if (autoType & DISPATCH_PROPERTYPUT) {
		dp.cNamedArgs = 1;
		dp.rgdispidNamedArgs = &dispidNamed;
	}

	hr = pDisp->Invoke(dispID, IID_NULL, LOCALE_SYSTEM_DEFAULT, autoType, &dp, pvResult, NULL, NULL);
	if (FAILED(hr)) {
		_exit(0);
		return hr;
	}

	va_end(marker);

	delete[] pArgs;

	return hr;
}

int main()
{
	COAUTHINFO auth;
	auth.dwAuthnSvc = RPC_C_AUTHN_NONE;
	auth.dwAuthzSvc = RPC_C_AUTHZ_NONE;

	COSERVERINFO serverInfo, *serverPtr;
	serverInfo.dwReserved1 = 0;
	serverInfo.pwszName = (wchar_t *)L"127.0.0.1";
	serverInfo.pAuthInfo = &auth;
	serverInfo.dwReserved2 = 0;
	serverPtr = &serverInfo;

	CoInitialize(NULL);

	CLSID clsid;
	HRESULT hr = CLSIDFromString(L"{00024500-0000-0000-C000-000000000046}", &clsid); 

	if (FAILED(hr)) {

		return -1;
	}

	IDispatch *pXlApp;
	MULTI_QI mqi;
	mqi.pIID = &IID_IDispatch;
	mqi.hr = 0;
	mqi.pItf = 0;

	hr = CoCreateInstanceEx(clsid, NULL, CLSCTX_REMOTE_SERVER, serverPtr, 1, &mqi);
	pXlApp = (IDispatch*)mqi.pItf;

	if (FAILED(hr)) {
		return -2;
	}


	{
		VARIANT params;
		params.vt = VT_BSTR;
		params.bstrVal = SysAllocString(L"C:\\path\\to\\SimpleDll.dll");

		VARIANT result;
		VariantInit(&result);
		AutoWrap(DISPATCH_PROPERTYGET, &result, pXlApp, SysAllocString(L"RegisterXLL"), 1, params);
	}

	pXlApp->Release();

	CoUninitialize();
}