PRB: ActiveX Controls Passing Variant* Back to VFP Cause Error (177575)

SYMPTOMS

When calling a method in an ActiveX control built with Microsoft Foundation Classes (MFC), if there is an out parameter in the method with a type variant, Visual FoxPro returns a type mismatch error.

CAUSE

When you call a method in the ActiveX control and pass it a variable by reference to receive a parameter value back from the ActiveX control, Microsoft Visual FoxPro seems to use a data type of BSTR. If the ActiveX method's parameter is defined as a Variant in the MFC based ActiveX control, this causes MFC to try and coerce the data types to match by using the VariantChangeType function. This function returns the type mismatch error.

RESOLUTION

Controls built with the ActiveX Template Library (ATL) do not have this problem. If you build the control in Microsoft Foundation Classes (MFC), avoid using out parameter data types of variant.

MORE INFORMATION

Steps to Reproduce Behavior

Creating the Server

Create an MFC ActiveX control using the control wizard.

Add a method to the control:

      void CMyCtrl::MyMethod(VARIANT FAR* m_pVariant)
      {
      }

Build and register the control.

Creating the Client

Create a new form.
Add an OLE container object.
When the dialog box displays, select the "Insert Control" option.
Select the control you just built with MFC.

Create a command button on the form and place the following code in the Click event:

****Begin Example Code************
      xy=space(0)
      THISFORM.OleControl1.mymethod(@xy)
      *****End Example Code**************

When the button is clicked, the type mismatch error occurs.

REFERENCES

For additional information, please see the following articles in the Microsoft Knowledge Base:

146219 How To Register an ActiveX Control (.ocx) Manually

139154 OLE Control Licensing in Visual FoxPro