INFO: Variant Handling in an MFC Server w/ VB 4.0 as Its Client (142223)

SUMMARY

It is possible for Visual Basic 4.0 to pass parameters to automation methods either by value (ByVal) or by reference (ByRef). For a method that expects a non-VARIANT parameter passed by value, MFC's implementation of IDispatch will coerce a parameter passed by reference to a value. However, this is not the case with an automation method expecting a VARIANT parameter. The MFC framework cannot coerce a VARIANT parameter because it does not know what type is expected by the method. This can cause problems in a method that expects a VARIANT parameter to be passed by value. If the parameter is passed by reference and the VARIANT is referenced directly, the value obtained from the variant will be incorrect. Hence, an automation method that expects a VARIANT to be passed by value should check whether it was passed a reference and coerce the VARIANT if necessary. This article illustrates how you could implement this.

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
    ...

}