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Tip of the Day
Language: VB5,VB6
Expertise: Intermediate
Apr 19, 2001

Implement the CallByName function using the TLI library

Thanks to the undocumented TlbInf32 library you can emulate the VB6 CallByName() function in VB5 too. First, add a reference to the "TypeLib Information" type library in your project. This library contains the InvokeHook function, which is very similar to CallByName.

Let's suppose you have an ActiveX DLL called MyServer.dll that exposes this function:


' Class.cls
Public Function MyFunc (FirstParam, SecondParam) As String
:
End Function
In VB6 you're able to call MyFunc() with:

Dim obj As Object
Dim First, Second
Dim obj As Object
Dim result As String

' create the object
Set obj = CreateObject("MyServer.Class")
' call MyFunc
result = CallByName(obj, "MyFunc", VbMethod, First, Second)
The following code demonstrates how to do the same in VB5:

Dim TLI As New TLIApplication
Dim obj As Object
Dim First, Second
Dim obj As Object
Dim result As String

' create the object
Set obj = CreateObject("MyServer.Class")
' call MyFunc
result = InvokeHook(obj, "MyFunc", INVOKE_FUNC, Second, First)
The sintax of InvokeHook() is simple: the first parameter is the object which you want to work, the second one indicates the name of the property or method to call, the third parameter is the type of call being made and can be one of the following values:

  • INVOKE_PROPERTYGET to assign a property value (vbGet in CallByName)
  • INVOKE_PROPERTYPUT to retrieve a property value (vbLet in CallByName)
  • INVOKE_PROPERTYPUTREF to assign an object reference to a property (vbSet in CallByName)
  • INVOKE_FUNC for a sub or function call (vbMethod in CallByName)
The last parameter is a ParamArray used to pass the list of parameters to the function being invoked: note that parameters must be in reverse order. Note that InvokeHook can be useful even under VB6, in that it lets you work around a bug of VB6's CallByName.
Alberto Falossi
 
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