如何檢測何時可選的ByRef參數未通過

Question

我想檢測是否通過了opational參數，但由於某些原因，所有常用函數（IsMissing（）/ IsEmpty（）/ IsNull（））總是返回false。

這就是我想：

Public Sub SetValue(Key As String, Optional ByRef ws As Worksheet) 
    If IsMissing(ws) Or IsEmpty(ws) Or IsNull(ws) Then 
     ws = ThisWorkbook.Sheets(SheetName) 
    End If 

我也嘗試設置WS爲Nothing或Null，但結果是一樣的：

Public Sub SetValue(Key As String, Optional ByRef ws As Worksheet = Nothing) 
    If IsMissing(ws) Or IsEmpty(ws) Or IsNull(ws) Then 
     ws = ThisWorkbook.Sheets(SheetName) 
    End If 

任何想法，爲什麼這可能是happenning ？

Answer 1

嘗試is nothing

Private Sub CommandButton1_Click() 
    Dim ws As Excel.Worksheet 
    Set ws = ActiveWorkbook.Sheets("sheet1") 

    'Call the sub both ways. 
    SetValue "a" 
    SetValue "a", ws 
End sub 

Public Sub SetValue(Key As String, Optional ByRef ws As Worksheet) 
    If ws Is Nothing Then 
     'We got a sheet 
     MsgBox "We got no sheet" 
    End If 

    If Not ws Is Nothing Then 
     'We got a sheet 
     MsgBox ws.name 
     MsgBox "We got a sheet" 
    End If 
End sub 

Answer 2

建設是任何方法都行不通。我在所有參數中都使用它。

Public Sub SetValue(ByRef Key As String, _ 
        Optional ByRef ws As Worksheet = Nothing) 

    'Make sure we have the object 
    If ws Is Nothing Then 
     Set ws = ThisWorkbook.Sheets(SheetName) 
    End If 

End Sub 

希望這有助於。 :)