Passing argument via the calling procedure

[tiredofit]

Looking at this:

https://www.mrexcel.com/forum/excel-questions/573357-prevent-vba-editor-being-opened.html

in particular this code:

    CBTProc = CallNextHookEx _
    (lCBTHook, idHook, ByVal wParam, ByVal lParam)

What is the significance of adding ByVal?


My understanding is the passing of arguments, whether ByRef or ByVal is stipulated in the procedure being
called and not the calling procedure.

Thanks

 

[joeu2004]

I 'spose you could try reading the Call Statement help page in VBA.

Syntax

[Call] name [argumentlist]

The Call statement syntax has these parts:

Part Description 

Call -- Optional; keyword. If specified, you must enclose argumentlist in parentheses. For example: 

 Call MyProc(0) 
 
name -- Required. Name of the procedure to call. 

argumentlist -- Optional. Comma-delimited list of variables, arrays, or expressions to pass to the procedure.
 Components of argumentlist may include the keywords ByVal or ByRef to describe how the arguments are
 treated by the called procedure. However, ByVal and ByRef can be used with Call only when calling a DLL procedure.
 On the Macintosh, ByVal and ByRef can be used with Call when making a call to a Macintosh code resource. 

Remarks

You are not required to use the Call keyword when calling a procedure. However, if you use the Call keyword
 to call a procedure that requires arguments, argumentlist must be enclosed in parentheses. If you omit the
 Call keyword, you also must omit the parentheses around argumentlist. If you use either Call syntax to call
 any intrinsic or user-defined function, the function's return value is discarded.

To pass a whole array to a procedure, use the array name followed by empty parentheses.

 

[joeu2004]

PS.... The use of ByVal seems redundant, at best. In fact, it seems to cause errors (sometimes aborts) -- surprise! Perhaps I misunderstand its valid use or purpose.

Option Explicit

Public Declare Function QueryPerformanceFrequency Lib "kernel32" _
   (ByRef freq As Currency) As Long
   
Sub testit()
Dim f As Currency

' call by ref; f changed as expected
f = -1
QueryPerformanceFrequency f
MsgBox f

' call by val; f not changed, as expected
f = -1
QueryPerformanceFrequency (f)
MsgBox f

' call by ref; f changed as expected
f = -1
Call QueryPerformanceFrequency(f)
MsgBox f

' call by val; f not changed, as expected
f = -1
Call QueryPerformanceFrequency((f))
MsgBox f

' caveat: this sometimes aborts program
' on my computer (Excel 2010 with Win 7).
' otherwise, it just causes a runtime error

' call using ByVal syntax
' (note: redundant ByRef is not allowed!)
f = -1
Call QueryPerformanceFrequency(ByVal f)
MsgBox f
End Sub