ByVal or ByRef - As Variant

Integers

When you declare a variable with an Integer data type it can be passed in using either ByVal or ByRef.
When you declare a variable with a Variant data it can be passed in using either ByVal or ByRef.

Arrays

There is a difference between declaring a variable with a Variant data type as opposed to an Array data type.
When you declare a variable with an Array data type it can only be passed in using ByRef.
When you declare a variable with a Variant data type it can be passed in using either ByVal or ByRef.

Sub MySubroutine1() 
   Dim var3 As Variant 
   var3 = Array(1, 2, 3, 4) 
    
   MyFunc_1a (var3) 
   Debug.Print var3(2)  '3  

   MyFunc_1b (var3) 
   Debug.Print var3(2)  '3  
End Sub 

Public Function MyFunc_1a(ByVal var4 As Variant) 
   var4(2) = 4 
End Function 
Public Function MyFunc_1b(ByRef var4 As Variant) 
   var4(2) = 8 
End Function 

Collections

Classes

When you declare a variable with a Class data type it can be passed in using either ByVal or ByRef.
When you declare a variable with a Variant data type it can only be passed in using either ByVal.

'myClass module
Private mstrPropertyName As String 

Property Get Property_Name() As String 
    Property_Name = mstrPropertyName 
End Property 

Property Let Property_Name(rData As String) 
    mstrPropertyName = rData 
End Property 

When you declare a variable with a Variant data type it can only be passed in using ByVal.

Sub MySubroutine3() 
   Dim var5 As Variant 
   Set var5 = New myClass 
   var5.Property_Name = "before" 
   Debug.Print var5.Property_Name 

   MyFunc_3a (var5) 
   Debug.Print var5.Property_Name 
End Sub 

'must be ByVal
Public Function MyFunc_3a(ByVal var6 As Variant) 
   var6.Property_Name = "after" 
End Function 

User Defined Types