Some messages contain parameters declared as Any. It means this parameter can be a variety of types (you may pass an integer, a string, or a user-defined type, or else). So, here is an example of a function (SendMessage) which contains a parameter of type Any:

Public Declare Function SendMessage Lib "User32" Alias "SendMessageA" (ByVal Hwnd as Long, ByVal wMsg as Long, ByVal wParam as Long, lParam as Any) as Long

lParam is declared ByRef (default) and As Any. Now, here are some rules to follow when passing different type of values to this function as lParam.

 

If the value is	Pass it As
numeric	ByVal (As Long, or As Any)
Null	ByVal (As Long, or As Any)
String	ByRef (As String, or As Any)
Type	ByRef (As Any)
array of Type	ByRef (As Any)

If your function declaration looks like the one above, and you need to pass a Long, write something like:

Call SendMessage(Me.Hwnd, WM_XXXX, 0&, ByVal LongValue)

Note that there is nothing in front of the first three parameter although they are numeric values. This is so, because in the function declaration they are declared as ByVal. The fourth parameter, though, is declared ByRef (VB doesn't know what kind of values you are going to pass) and you must explicitly specify ByVal in front of it.

Sometimes it's much simpler to just declare several versions of one function and use a different one for different calls. You may declare something like:

Public Declare Function SendMessageLng Lib "User32" Alias "SendMessageA" (ByVal Hwnd as Long, ByVal wMsg as Long, ByVal wParam as Long, ByVal lParam as Long) as Long

or

Public Declare Function SendMessageStr Lib "User32" Alias "SendMessageA" (ByVal Hwnd as Long, ByVal wMsg as Long, ByVal wParam as Long, lParam as String) as Long

Notice that the parameter type does not change for API. The fourth parameter is always a 4-byte Long value. When you pass a Long or Null ByVal, a the 4-byte value is passed directly to the function. If you pass a String or else, you pass it ByRef and so VB actually passes the address of you variable, and it is a 4-byte value again.

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Copyright (c) 1998, Billy&George Software and Peter Dimitrov
Revised March 2000