Question

我有一个函数可以找到下周一的日期。如果没有传递可选的日期参数，则它将默认为零。我想我已经包含了解释问题的评论

Function NextMondayFromADateOrToday(Optional StartDate As Date) As Date
' objective: if date param not supplied should set StartDate to today
' ??? Problem: with no arg provided StartDate is zero = date value of 1/1/1900
' ??? how to add default to optional paramater of date type?
' ??? or, how to check if an arg was provided if the parameter is date type?
If Not (IsDate(StartDate)) Then StartDate = Date
Select Case Weekday(StartDate)
Case 1:     NextMondayFromADateOrToday = StartDate + 1
Case 2:     NextMondayFromADateOrToday = StartDate + 0
Case 3:     NextMondayFromADateOrToday = StartDate + 6
Case 4:     NextMondayFromADateOrToday = StartDate + 5
Case 5:     NextMondayFromADateOrToday = StartDate + 4
Case 6:     NextMondayFromADateOrToday = StartDate + 3
Case 7:     NextMondayFromADateOrToday = StartDate + 2
End Select
End Function

Answer 1

VBA变量通常具有默认值，并且任何可选参数都会自动设置为默认值，因此需要此行为。既然你已经知道可选参数默认为0（1/1/1900），如果没有通过，为什么不测试该值而不是测试是否传递日期？

另一方面，如果您认为某人可能需要在1/1/1900传递，那么您应该将可选参数设置为Variant类型。在可选地传入变量时，变量不会被初始化，因此它不具有默认值（与日期不同）。

Answer 2

日期不能为空，因此在这种情况下可能使用变体

Function NextMondayFromADateOrToday(Optional StartDate As Variant) As Date
' objective: if date param not supplied should set StartDate to today
' ??? Problem: with no arg provided StartDate is zero = date value of 1/1/1900
' ??? how to add default to optional paramater of date type?
' ??? or, how to check if an arg was provided if the parameter is date type?
If IsMissing(StartDate) Then StartDate = Date
Select Case Weekday(StartDate)
Case 1:     NextMondayFromADateOrToday = StartDate + 1
Case 2:     NextMondayFromADateOrToday = StartDate + 0
Case 3:     NextMondayFromADateOrToday = StartDate + 6
Case 4:     NextMondayFromADateOrToday = StartDate + 5
Case 5:     NextMondayFromADateOrToday = StartDate + 4
Case 6:     NextMondayFromADateOrToday = StartDate + 3
Case 7:     NextMondayFromADateOrToday = StartDate + 2
End Select
End Function

Answer 3

我知道有一个公认的答案，而且我迟到了，但是......

也许在函数声明中提供一个默认日期并测试它：

Function NextMondayFromADateOrToday(Optional StartDate As Date = #12/31/1592#) As Date
' objective: if date param not supplied should set StartDate to today
' ??? Problem: with no arg provided StartDate is zero = date value of 1/1/1900
' ??? how to add default to optional paramater of date type?
' ??? or, how to check if an arg was provided if the parameter is date type?
  If StartDate = #12/31/1592# Then
    StartDate = Date
  End If
  'If Not (IsDate(StartDate)) Then StartDate = Date
  Select Case Weekday(StartDate)
    Case 1:     NextMondayFromADateOrToday = StartDate + 1
    Case 2:     NextMondayFromADateOrToday = StartDate + 0
    Case 3:     NextMondayFromADateOrToday = StartDate + 6
    Case 4:     NextMondayFromADateOrToday = StartDate + 5
    Case 5:     NextMondayFromADateOrToday = StartDate + 4
    Case 6:     NextMondayFromADateOrToday = StartDate + 3
    Case 7:     NextMondayFromADateOrToday = StartDate + 2
  End Select
End Function

如果12/31/1592没有充分超出预期范围，您可以更早或更晚，直到您开始达到日期可以处理的限制 - 快速测试显示Excel 2010可以采用＃1 /的日期1/100＃和＃1/1 / 9999＃

功能可选参数DATE类型失败

3 个答案: