vb.net - 如何在 VB.NET 中创建 Nullable Optional Numeric (integer/double) 参数？

Question

如何在VB.NET中创建可为空的数字可选参数？

Answer 1

编辑：根据这篇博文，这在 VB.NET 10 中应该是可能的。如果您正在使用它，那么您可以：

Public Sub DoSomething(Optional ByVal someInteger As Integer? = Nothing)
    Console.WriteLine("Result: {0} - {1}", someInteger.HasValue, someInteger)
End Sub

' use it
DoSomething(Nothing)
DoSomething(20)

---

对于 VB.NET 10 以外的版本：

你的要求是不可能的。您应该使用可选参数或可为空的。此签名无效：

Public Sub DoSomething(Optional ByVal someInteger As Nullable(Of Integer) _
                        = Nothing)

你会得到这个编译错误：“可选参数不能有结构类型。”

如果您使用的是可为空的，则如果您不想向其传递值，请将其设置为 Nothing。在这些选项之间进行选择：

Public Sub DoSomething(ByVal someInteger As Nullable(Of Integer))
    Console.WriteLine("Result: {0} - {1}", someInteger.HasValue, someInteger)
End Sub

或者

Public Sub DoSomething(Optional ByVal someInteger As Integer = 42)
    Console.WriteLine("Result: {0}", someInteger)
End Sub

Answer 2

你不能，所以你将不得不使用重载来代替：

Public Sub Method()
  Method(Nothing) ' or Method(45), depending on what you wanted default to be
End Sub

Public Sub Method(value as Nullable(Of Integer))
  ' Do stuff...
End Sub

Answer 3

您还可以使用对象：

Public Sub DoSomething(Optional ByVal someInteger As Object = Nothing)
If someInteger IsNot Nothing Then
  ... Convert.ToInt32(someInteger)
End If

结束子

Answer 4

我在VS2012版本中弄清楚了

Private _LodgingItemId As Integer?

Public Property LodgingItemId() As Integer?
        Get
            Return _LodgingItemId
        End Get
        Set(ByVal Value As Integer?)
            _LodgingItemId = Value
        End Set
    End Property

Public Sub New(ByVal lodgingItem As LodgingItem, user As String)
        Me._LodgingItem = lodgingItem
        If (lodgingItem.LodgingItemId.HasValue) Then
            LoadLodgingItemStatus(lodgingItem.LodgingItemId)
        Else
            LoadLodgingItemStatus()
        End If
        Me._UpdatedBy = user
    End Sub

Private Sub LoadLodgingItemStatus(Optional ByVal lodgingItemId As Integer? = Nothing)
    ''''statement 
End Sub