Use ByVal to Pass a Control as an Argument to a DLL or VBX

This article was previously published under Q129827

In previous versions of Visual Basic, when you passed a control to a DLL or VBX, your function declaration looked like this:

   Declare Function fDoNothing Lib "MyFun.vbx" (ctlX as Control) As Integer
		

Now, in Microsoft Visual Basic version 4.0, the keyword ByVal must be used when passing the same control, so now your function declaration must look like this:

   Declare Function fDoNothing Lib "MyFun.vbx" (ByVal ctlX as Control)_
      As Integer
		

The Visual Basic API was changed. Now, functions expecting a control as a parameter require a handle to the control, an HCTL. Omitting ByVal within the parameter list causes a pointer to the HCTL to be passed instead of the HCTL itself. To ensure an HCTL is passed, use the ByVal keyword. This behavior is by design.