范围和变量之间的差异作为范围变暗

Question

I've built a class and need to record the data into a cell. Therefore I write a function doing this. Codes below:

Option Explicit

Private sName As String

Public Property Let Name(ByVal strValue As String)
    sName = strValue
End Property
Public Property Get Name() As String
    Name = sName
End Property

Public Function ItemToCell(ByRef tgtCell As Range)
    tgtCell = sName
End Function

And I also set a button to trigger this process:

Private Sub CommandButton1_Click()
    Dim tmpData As New MyClass
    tmpData.Name = "Tom"

    Dim tgtCell As Range
    Set tgtCell = Worksheets("Sheet1").Range("A1")

    'Method 1, this failed with error 424
    tmpData.ItemToCell (tgtCell)

    'Method 2, it works
    tmpData.ItemToCell (Worksheets("Sheet1").Range("B1"))
End Sub

I thought that these two methods were the same, but apparently they are not. Why? Isn't the variable tgtCell an object?

Answer 1

Note that method 1A below, with the parentheses removed, DOES work as expected:

Public Sub CommandButton1_Click()
    On Error GoTo EH
    Dim tmpData As New MyClass
    tmpData.Name = "Tom"

    Dim tgtCell As Range
    Set tgtCell = Worksheets("Sheet1").Range("A1")

    'Method 1, this failed with error 424
    tmpData.ItemToCell (tgtCell)

    'Method 1A, this works
    tmpData.ItemToCell tgtCell

    'Method 2, it works
    tmpData.ItemToCell (Worksheets("Sheet1").Range("B1"))
XT: Exit Sub
EH: MsgBox Err.Description, vbOKOnly, Err.Source
    Resume Next
End Sub

The difficulty arises because the call to tmpData.ItemToCell requires an l-value (ie it is a ByRef argument) but the invocation statement IS NOT a function call, and so the parentheses are not the parentheses of invocation , but rather the parentheses of grouping . This can be a confusing issue in VBA.

The effect of the parentheses of grouping is to return the value of the variable tgtCell rather than its storage location , and implicitly evaluating the default member Value of the Range object . However, as you stumbled into with Method 2, there are circumstances where VBA does not implicitly evaluate the default member. Yes, it's confusing to everyone; don't feel alone.

One way to minimize the occurrence of this annoyance is to explicitly specify parameters for Functions and Subs (And Set/Let properties) as ByVal unless you actually desire to pass back a changed value to the caller. This wins by:

• eliminating many instances of this annoying feature;
• eliminating many subtle bugs when you treat the parameter as a local variable, and change its value expecting those changes to be local in scope, when they are actually non-local.

However, your circumstance is the rare one where this does not help. In these case it is imply best to not add parentheses to method calls until VBA complains of their absence, which is generally just for Functions rather than Subs and Property Setters/Letters.

In summary:
- Parameters should be explicitly specified as ByVal (rather than the default of ByRef) unless you really are passing back a calculated value (in which case a Function is a better implementation and usually sufficient) or when the language requires you to pass a ByRef argument.
- Parentheses should only be used in method invocations when VBA complains of their absence.