有条件地跳过案例

Question

在Select...Case语句中，有没有一种方法可以基于前置条件跳过案例？

我现在在做什么，用一个非常愚蠢的例子：

Private Sub PrintNumbers(includeEvenNumbers As Boolean, includeOddNumbers As Boolean)
    For number As Integer = 0 To 9
        Select Case number
            Case 0, 2, 4, 6, 8
                If includeEvenNumbers Then
                    Console.WriteLine(number)
                End If
            Case 1, 3, 5, 7, 9
                If includeOddNumbers Then
                    Console.WriteLine(number)
                End If
        End Select
    Next
End Sub

有时我甚至会把我的案例写出来：

Private Sub PrintNumbers(includeEvenNumbers As Boolean, includeOddNumbers As Boolean)
    For number As Integer = 0 To 9
        Select Case True
            Case includeEvenNumbers
                If number Mod 2 = 0 Then
                    Console.WriteLine(number)
                End If
            Case includeOddNumbers
                If number Mod 2 <> 0 Then
                    Console.WriteLine(number)
                End If
        End Select
    Next
End Sub

我真正想做的是：

Private Sub PrintNumbers(includeEvenNumbers As Boolean, includeOddNumbers As Boolean)
    For number As Integer = 0 To 9
        Select Case number
            Case 0, 2, 4, 6, 8 When includeEvenNumbers
                Console.WriteLine(number)
            Case 1, 3, 5, 7, 9 When includeOddNumbers
                Console.WriteLine(number)
        End Select
    Next
End Sub

请注意，我使用了When关键字，该关键字目前仅在Try...Catch块中使用。

这能做到吗？我该和谁谈谈才能做到这一点？

编辑（1/2）

重要的是，此代码将首先评估When <expression>。只有当计算结果为True时，它才会继续计算Case <expression>。

我想这样做的主要原因是，如果情况是对的（或错的，取决于你如何看待它），我想写测试条件抛出异常的案例。如果前提条件是真的，我想跳过这些案例。

编辑（2/2）

现在很清楚，在当前的VB迭代中，我所要求的是不可能的。所以我寻找了一个地方向.NET开发团队提交功能请求，然后发现他们有一个用户语音平台。

长话短说：如果你想看到这一点的实现，点击此页面上的投票按钮：https://visualstudio.uservoice.com/forums/121579-visual-studio/suggestions/4274712-add-when-keyword-support-to-select-case-stat

Answer 1

linq:示例

    Dim condition As Predicate(Of Integer) = Nothing
    If includeEvenNumbers Then
        condition = Function(x) x Mod 2 = 0
    Else
        condition = Function(x) x Mod 2 = 1
    End If
    For Each item In Enumerable.Range(0, 8).Where(condition)
        ' do what ever you like here...
    Next

编辑：简化：

For Each item In Enumerable.Range(0, 8).Where(Function(x) x Mod 2 = If(includeEvenNumbers, 0, 1))
    ' do what ever you like here...
Next

Answer 2

通过使用枚举而不是2个布尔值，您只需要一个if语句即可完成2个条件的检查。类似这样的东西：

Public Enum Include
    Odd = 1
    Even = 2
    Both = 0
End Enum
Private Sub PrintNumbers(includenumbers As Include)
    For I = 1 To 10
        If includenumbers = Include.Both OrElse I Mod 2 = includenumbers Mod 2 Then
            TextBox2.AppendText(I.ToString)
        End If
    Next
End Sub

你可以这样称呼它：

PrintNumbers(Include.Odd)