Passing Arguments ByRef or ByVal

Introduction#

The ByRef and ByVal modifiers are part of a procedure’s signature and indicate how an argument is passed to a procedure. In VBA a parameter is passed ByRef unless specified otherwise (i.e. ByRef is implicit if absent).

Note In many other programming languages (including VB.NET), parameters are implicitly passed by value if no modifier is specified: consider specifying ByRef modifiers explicitly to avoid possible confusion.

Remarks#

Passing arrays

Arrays must be passed by reference. This code compiles, but raises run-time error 424 “Object Required”:

Public Sub Test()
    DoSomething Array(1, 2, 3)
End Sub

Private Sub DoSomething(ByVal foo As Variant)
    foo.Add 42
End Sub

This code does not compile:

Private Sub DoSomething(ByVal foo() As Variant) 'ByVal is illegal for arrays
    foo.Add 42
End Sub

Passing Simple Variables ByRef And ByVal

Passing ByRef or ByVal indicates whether the actual value of an argument is passed to the CalledProcedure by the CallingProcedure, or whether a reference (called a pointer in some other languages) is passed to the CalledProcedure.

If an argument is passed ByRef, the memory address of the argument is passed to the CalledProcedure and any modification to that parameter by the CalledProcedure is made to the value in the CallingProcedure.

If an argument is passed ByVal, the actual value, not a reference to the variable, is passed to the CalledProcedure.

A simple example will illustrate this clearly:

Sub CalledProcedure(ByRef X As Long, ByVal Y As Long)
    X = 321
    Y = 654
End Sub

Sub CallingProcedure()
    Dim A As Long
    Dim B As Long
    A = 123
    B = 456

    Debug.Print "BEFORE CALL => A: " & CStr(A), "B: " & CStr(B)
    ''Result : BEFORE CALL => A: 123 B: 456

    CalledProcedure X:=A, Y:=B

    Debug.Print "AFTER CALL =  A: " & CStr(A), "B: " & CStr(B)
    ''Result : AFTER CALL => A: 321 B: 456
End Sub

Another example:

Sub Main()
    Dim IntVarByVal As Integer
    Dim IntVarByRef As Integer
    
    IntVarByVal = 5
    IntVarByRef = 10
    
    SubChangeArguments IntVarByVal, IntVarByRef '5 goes in as a "copy". 10 goes in as a reference
    Debug.Print "IntVarByVal: " & IntVarByVal 'prints 5 (no change made by SubChangeArguments)
    Debug.Print "IntVarByRef: " & IntVarByRef 'prints 99 (the variable was changed in SubChangeArguments)
End Sub

Sub SubChangeArguments(ByVal ParameterByVal As Integer, ByRef ParameterByRef As Integer)
    ParameterByVal = ParameterByVal + 2 ' 5 + 2 = 7 (changed only inside this Sub)
    ParameterByRef = ParameterByRef + 89 ' 10 + 89 = 99 (changes the IntVarByRef itself - in the Main Sub)
End Sub

ByRef

Default modifier

If no modifier is specified for a parameter, that parameter is implicitly passed by reference.

Public Sub DoSomething1(foo As Long)
End Sub

Public Sub DoSomething2(ByRef foo As Long)
End Sub

The foo parameter is passed ByRef in both DoSomething1 and DoSomething2.

Watch out! If you’re coming to VBA with experience from other languages, this is very likely the exact opposite behavior to the one you’re used to. In many other programming languages (including VB.NET), the implicit/default modifier passes parameters by value.

Passing by reference

• When a value is passed ByRef, the procedure receives a reference to the value.

  Public Sub Test()
      Dim foo As Long
      foo = 42
      DoSomething foo
      Debug.Print foo
  End Sub
  
  Private Sub DoSomething(ByRef foo As Long)
      foo = foo * 2
  End Sub

  Calling the above Test procedure outputs 84. DoSomething is given foo and receives a reference to the value, and therefore works with the same memory address as the caller.

• When a reference is passed ByRef, the procedure receives a reference to the pointer.

  Public Sub Test()
      Dim foo As Collection
      Set foo = New Collection
      DoSomething foo
      Debug.Print foo.Count
  End Sub
  
  Private Sub DoSomething(ByRef foo As Collection)
      foo.Add 42
      Set foo = Nothing
  End Sub

  The above code raises run-time error 91, because the caller is calling the Count member of an object that no longer exists, because DoSomething was given a reference to the object pointer and assigned it to Nothing before returning.

Forcing ByVal at call site

Using parentheses at the call site, you can override ByRef and force an argument to be passed ByVal:

Public Sub Test()
    Dim foo As Long
    foo = 42
    DoSomething (foo)
    Debug.Print foo
End Sub

Private Sub DoSomething(ByRef foo As Long)
    foo = foo * 2
End Sub

The above code outputs 42, regardless of whether ByRef is specified implicitly or explicitly.

Watch out! Because of this, using extraneous parentheses in procedure calls can easily introduce bugs. Pay attention to the whitespace between the procedure name and the argument list:

bar = DoSomething(foo) 'function call, no whitespace; parens are part of args list
DoSomething (foo) 'procedure call, notice whitespace; parens are NOT part of args list
DoSomething foo 'procedure call does not force the foo parameter to be ByVal

ByVal

Passing by value

• When a value is passed ByVal, the procedure receives a copy of the value.

  Public Sub Test()
      Dim foo As Long
      foo = 42
      DoSomething foo
      Debug.Print foo
  End Sub
  
  Private Sub DoSomething(ByVal foo As Long)
      foo = foo * 2
  End Sub

  Calling the above Test procedure outputs 42. DoSomething is given foo and receives a copy of the value. The copy is multiplied by 2, and then discarded when the procedure exits; the caller’s copy was never altered.

• When a reference is passed ByVal, the procedure receives a copy of the pointer.

  Public Sub Test()
      Dim foo As Collection
      Set foo = New Collection
      DoSomething foo
      Debug.Print foo.Count
  End Sub
  
  Private Sub DoSomething(ByVal foo As Collection)
      foo.Add 42
      Set foo = Nothing
  End Sub

  Calling the above Test procedure outputs 1. DoSomething is given foo and receives a copy of the pointer to the Collection object. Because the foo object variable in the Test scope points to the same object, adding an item in DoSomething adds the item to the same object. Because it’s a copy of the pointer, setting its reference to Nothing does not affect the caller’s own copy.