Early-binding to a non-existent COM object

In my previous article "Getting the most out of IDispatch", I introduced a C++ class, XYDispDriver, which can be used to create any COM object (well, almost any) and call its methods. With this class, we no longer need a separate wrapper class for each type of COM object we want to use. For those of you who are familiar with the #import directive, we don't need that either (not that it is not useful).

To call a method in a COM object using XYDispDriver, we only need to know the parameter types and return type (although we can get this info dynamically). In other words, we need the COM interface, not the COM object itself. As I understand it, a COM interface is just a contract between the users and the implementer of the COM object. So there is nothing to prevent us from writing code that uses a COM object before the COM object is actually implemented, and you can do that easily with the help of XYDispDriver. Of course, you have to create a dummy COM object to test the code or wait until after the COM object is implemented to test.

As some readers have pointed out, XYDispDriver uses IDispatch::Invoke to call COM methods indirectly, which is called late-binding. Late-binding is not as efficient as early-binding, which calls the COM methods directly. If you have looked at the .tlh files generated by the #import directive, you will see that it uses both early-binding and late-binding. To do early-binding, at least as I thought previously, the COM object has to be implemented first. But I was wrong. If we already know the COM interface (the contract), early-binding is actually possible without the COM object being implemented.

I wrote the following console application that early-binds to a non-existent COM object. We assume that the prog id of the non-existent COM object is TestObj.1, the COM interface is derived from IDispatch, and there is only one method called Test which takes an input string parameter and an output string parameter. As you can see, it is very easy to do early-binding. Here is the code.

#include <stdio.h>
#include <oaidl.h>

// first define a com interface with the Testmethod
// (the order and signatures of methods have to be
// the same as in the future com object)
interface ITestObj : public
IDispatch
{
public:
    virtual HRESULT STDMETHODCALLTYPE Test(BSTR strInput, BSTR* pOutput) = 0;
};

// the main function that uses a non-existent com object
void main()
{
    // initialize com
    ::CoInitialize(NULL);
    // find the class id of the com object from its prog id
    CLSID clsidObj;
    HRESULT hRet = ::CLSIDFromProgID(L"TestObj.1", &clsidObj);
    if(hRet==S_OK)
    {
        // declare a pointer to the com interface
        ITestObj* pDisp = NULL;
        // create the com object
        hRet = ::CoCreateInstance(clsidObj,NULL,CLSCTX_ALL,IID_IDispatch,(void**)&pDisp);
        if(hRet==S_OK)
        {
           // declare input and output parameters
           BSTR strInput = ::SysAllocString(L"This is test 3");
           BSTR strOutput = NULL;
           // call the Test method (early-binding!!!)
           hRet = pDisp->Test(strInput,&strOutput);
           if(hRet==S_OK)
           {
               // print the output string
               wprintf(L"%s\n",strOutput);
               // free the output string
               ::SysFreeString(strOutput);
           }
           else printf("Error: %s",hRet);
           // free the input string
           ::SysFreeString(strInput);
           // release the com interface
           pDisp->Release();
        }
        else printf("Error: %x",hRet);
    }
    else printf("Error: %x",hRet);
    // uninitialize com
    ::CoUninitialize();
}

The above console app will compile fine without the actual COM object. Now you can use the ATL COM Wizard to generate a project for the COM object and implement the Test method as specified in the COM interface. After building the COM object, run the console app to verify that early-binding works! You can, of course, use the same technique for more complicated COM interfaces.

The code in this article is mainly for educational and entertainment purposes. You can find more code and articles from my home page. Thank you.

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