How to prevent crashing if com dll isnt registered

Question

Hi

From some old c++ code im trying to use a com dll, it works fine when the dll is registered, but it crahses if the dll isnt registered.

// Initialize COM. HRESULT hr = CoInitialize(NULL);

IGetTestPtr ptest(__uuidof(tester));

"Use method from the dll"

// Uninitialize COM. CoUninitialize();

Is it anyway to check if the dll has been registered, before calling IGetTestPtr ptest(__uuidof(tester))?

Or what is the correct way to prevent the crash?

Answer 1

If the DLL is registered, there is a record in HKCR/CLSID/{uuidof(tester)} (curly brackets do matter).

Actually, if it's not, then CoCreateInstance will return an error. Check for this error before using the pointer.

Answer 2

Calling CreateInstance on your object will return an HRESULT that can be tested for success:

IGetTestPtr p = null; 
HRESULT hRes = p.CreateInstance( __uuidof(tester) );
bool bSuccess = SUCCEEDED(hRes);

This assumes you've created an interface wrapper around your type library using Visual Studio, where COM Smart Pointers are used in the interface (this gives you the CreateInstance method).

Answer 3

If the COM class is not registered then CoCreateInstance will return REGDB_E_CLASSNOTREG. You should check for general success or failure by using the SUCCEEDED() or FAILED() macros. You also need to create the object properly - see MDSN for an introduction to COM. For example:

HRESULT hr = CoInitialize(NULL);
if (SUCCEEDED(hr))
{
    IGetTestPtr ptr = NULL;
    hr = CoCreateInstance(CLSID_MyObjectNULL, CLSCTX_INPROC_SERVER, IID_IMyInterface, ptr)
    if (SUCCEEDED(hr))
    {
        Do something with your COM object
        ...

        // Don't forget to release your interface pointer or the object will leak
        ptr->Release();
    }

    hr = CoUninitialize();
}
return hr;

Answer 4

If I recall correctly this "#import" styled COM framework throws exceptions. Add a try-catch block around the code. Google tells me the exceptions are of type _com_error.

Unless you twiddle the params on #import to prevent this, all failing COM calls are going to throw exceptions. You're going to have to turn this off (#import "raw" interfaces I think does it - look at the #import docs) or get real familiar with these exceptions.