C++ IDispatch Invoke with output parameter

Question

I am working on an application, which connects to a COM object and calls methods and gets properties from this object etc. I can connect and call the members, that is not the issue. I cannot however figure out, how to call a method, which has an output parameter, for instance (pseudo code):

int GetAppVersion(bsRetMsg [out, optional]). 

This functions return int as the version and can also return a string representation of the version via the output parameter.

What I've tried:

(1)

        VARIANT result;
        DISPPARAMS params = {NULL, NULL, 0, 0};
        VARIANTARG args[1];

        BSTR str = SysAllocString(L"longerfoostring");
        VariantInit(&args[0]);
        args[0].vt = VT_BSTR | VT_BYREF;
        args[0].bstrVal = str;
        params.rgvarg = args;
        params.cArgs = 1;

        res = dispatch->Invoke(dispid, IID_NULL, LOCALE_USER_DEFAULT, DISPATCH_METHOD, &params, &result, NULL,
                               NULL);

        if (SUCCEEDED(res)) {    // here it failed: Not enough storage is available to complete this operation.
            std::cout << result.intVal << std::endl;
...

(2)

        VARIANT result;
        DISPPARAMS params = {NULL, NULL, 0, 0};
        VARIANTARG args[1];

        BSTR str = SysAllocString(L"");    // change: empty string
        VariantInit(&args[0]);
        args[0].vt = VT_BSTR | VT_BYREF;
        args[0].bstrVal = str;
        params.rgvarg = args;
        params.cArgs = 1;

        res = dispatch->Invoke(dispid, IID_NULL, LOCALE_USER_DEFAULT, DISPATCH_METHOD, &params, &result, NULL,
                               NULL);

        if (SUCCEEDED(res)) {    // here it failed: Not enough storage is available to complete this operation.
            std::cout << result.intVal << std::endl;
...

(3)

...
BSTR str = SysAllocString(L"longerfoostring");
VariantInit(&args[0]);
args[0].vt = VT_BSTR;    // change: no BYREF
args[0].bstrVal = str;
params.rgvarg = args;
params.cArgs = 1;
... invoke is the same...
res = dispatch->Invoke(dispid, IID_NULL, LOCALE_USER_DEFAULT, DISPATCH_METHOD, &params, &result, NULL,
                               NULL);
if (SUCCEEDED(res)) {
    std::cout << result.intVal << std::endl;    // this number is correct - version in int
    printf("'%S'", params.rgvarg[0].bstrVal);    // prints 'longerfoostring' instead of version, ie. '2.0.5...'

(4)

BSTR *str;
VariantInit(&args[0]);
args[0].vt = VT_BSTR | VT_PTR;    // change:  different VT
args[0].pbstrVal = str;    //change: different type
params.rgvarg = args;
params.cArgs = 1;
... invoke failes with Bad variable type.

So the question is: How to pass a string (or any type) as an input/output parameter to a COM method and correctly get the output from this parameter?

Answer 1

Since you want to get back the data, you have to provide the address of a BSTR

BSTR str;
VariantInit(&args[0]);
args[0].vt = VT_BSTR | VT_BYREF;    // it's a BSTR and it's by ref
args[0].pbstrVal = &str;    // give address of variable
params.rgvarg = args;
params.cArgs = 1;

now call invoke and don't forget to free the returned BSTR.