Excel 函数可选参数日期类型失败

我有一个函数，可以查找下一个星期一作为日期。如果没有传递可选的日期参数，则默认为零。我想我已经加入了一些评论来解释这个问题

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

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日期不能为null，因此在这种情况下可能使用变量

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

---

日期不能为null，因此在这种情况下可能使用变量

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

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VBA变量通常有一个默认值，任何可选参数都会自动设置为其默认值，因此这种行为是预期的。既然您已经知道，如果未通过，可选参数将默认为0（1/1/1900），为什么不测试该值，而不是测试是否传入了日期

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另一方面，如果您认为某人可能需要传入1/1/1900，则应将可选参数设置为

变量
类型。可选传入变量时不会初始化变量，因此它不会有默认值（与日期不同）。
VBA变量通常有默认值，并且任何可选参数都会自动设置为默认值，因此这种行为是预期的。既然您已经知道，如果未通过，可选参数将默认为0（1/1/1900），为什么不测试该值，而不是测试是否传入了日期

另一方面，如果您认为某人可能需要传入1/1/1900，则应将可选参数设置为
变量
类型。可选传入变量时不会初始化变量，因此它不会有默认值（与日期不同）。
我知道有一个可接受的答案，而且我迟到了，但是

可能在函数声明和测试中提供默认日期：

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1592年12月31日还没有完全超出预期范围，你可以提前或推迟，直到你开始达到一个日期可以处理的极限-一个快速测试显示Excel 2010可以将日期定为#1/1/100#和#1/1/9999#
我知道有一个公认的答案，但是我迟到了

可能在函数声明和测试中提供默认日期：

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1592年12月31日还没有完全超出预期范围，你可以提前或推迟，直到你开始达到一个日期可以处理的极限-一个快速测试显示Excel 2010可以将日期定为#1/1/100#和#1/1/9999#
非常感谢。我希望有更优雅的东西，特别是一种为约会设置默认值的方法。但是你说的很有道理，我已经试过了，效果很好。非常感谢。很高兴它起作用了。如果您想在没有日期传递的情况下设置默认日期，则需要使用变量，然后像ooo编写的那样对其进行测试
如果IsMissing（StartDate），则…
。无论你选择哪一个答案，别忘了把它标记为已接受。非常感谢。我希望有更优雅的东西，特别是一种为约会设置默认值的方法。但是你说的很有道理，我已经试过了，效果很好。非常感谢。很高兴它起作用了。如果您想在没有日期传递的情况下设置默认日期，则需要使用变量，然后像ooo编写的那样对其进行测试
如果IsMissing（StartDate），则…
。无论你选择哪一个答案，别忘了把它标为已接受。