INFO: Type of OCX Property in Typelib Should Match Its RetVal

ID: Q153632

The information in this article applies to:

• Microsoft Visual Basic for Windows Learning, Professional, and Enterprise Editions, version 6.0
• Control Creation, Learning, Professional, and Enterprise Editions of Microsoft Visual Basic for Windows, version 5.0
• Standard, Professional, and Enterprise Editions of Microsoft Visual Basic for Windows, 32-bit only, version 4.0

SUMMARY

An ActiveX control can cause an unhandled exception in VB32.exe if the type of any of its Properties as specified in its ODL file differs from the actual type that the property handler returns.

MORE INFORMATION

For example, this will happen if, in an ActiveX control's type library, a property is marked as type String (VT_BSTR), and the Get method for this property returns an integer (VT_I4). This is because when Visual Basic calls IDispatch::Invoke() to get that property value, the control fills in the Result variant parameter of Invoke() with the type VT_I4, but Visual Basic believing it is a string (from the Typelib definition), tries to dereference the integer and hence the general protection (GP) fault. Visual Basic assumes that the type returned by Invoke() will be the same as the type stated in the type library.

This situation can be prevented by fixing the type library of the custom OLE control or the property handler for the related property, whichever is wrong.

Additional reference words: kbVBp400 kbVBp500 kbVBp600 kbVBp kbdsd kbDSupport kbActiveX kbControl

Platform          : NT WINDOWS
Issue type        : kbinfo

Last Reviewed: August 7, 1998