INFO: Variant Handling in an MFC Server w/ VB 4.0 as Its Client

MORE INFORMATION

Consider the following method named Add exposed by a MFC automation server with a ProgID TestVar.Document:

   LPDISPATCH Add(const VARIANT FAR& varTest);
				

Given the following code in Visual Basic, it may pass the parameter either by reference or by value:

   Dim doc As Object
   Set doc = CreateObject("TestVar.Document")

   Dim docDispatch As Object
   Dim varParam As Variant
   varParam = 2
				

Visual Basic will pass the parameter by reference in these cases:

   Set docDispatch = doc.Add(varParam)
   doc.Add varParam
				

Visual Basic will pass the parameter by value in these cases:

   Set docDispatch = doc.Add((varParam))
   doc.Add (varParam)
   Set docDispatch = doc.Add(2)
   doc.Add 2
				

From these examples, you may notice that Visual Basic will pass all variables by reference unless the () operator is used to indicate that the variable should be passed by value. Moreover, Visual Basic will pass all constants by value.

Because of the possibility of the parameter being passed by reference, it is necessary to coerce the parameter to a value before using it in the automation server. Making a copy of the VARIANT parameter using the VariantCopyInd function will perform the necessary indirection, if the source VARIANT parameter passed is specified to be VT_BYREF. The following code will properly handle a VARIANT passed either by reference or by value.

LPDISPATCH CTestVarDoc::Add(const VARIANT FAR& varTest)
{
    HRESULT hr;
    VARIANT var;
    VariantInit(&var);

    hr = VariantCopyInd(&var, (LPVARIANT)&varTest);

    if (FAILED(hr))
       return NULL;

    // Now use var instead of varTest
    ...

}