DirectCast（False，Nullable（Of Boolean））错误

Question

我从C＃项目中提取了一些代码并将其放入转换器中。原来的代码是：

(Nullable<bool>)false 

和转换器说VB相当于是：

DirectCast(False, Nullable(Of Boolean)) 

我甚至编译C＃项目，并在反射看着它。它给出了与上面相同的VB代码，但是这会产生错误：

Value of type 'Boolean' cannot be converted to 'Boolean?' 

如何正确投射？根据要求

更多的代码：

Imports System.Windows 
Imports System.Windows.Controls.Primitives 
Imports System.Windows.Input 

Public Class VirtualToggleButton 

    Public Shared ReadOnly IsCheckedProperty As DependencyProperty = DependencyProperty.RegisterAttached("IsChecked", _ 
      GetType(Nullable(Of Boolean)), _ 
      GetType(VirtualToggleButton), _ 
      New FrameworkPropertyMetadata(DirectCast(False, Nullable(Of Boolean)), _ 
      FrameworkPropertyMetadataOptions.BindsTwoWayByDefault Or _ 
      FrameworkPropertyMetadataOptions.Journal, _ 
      New PropertyChangedCallback(AddressOf OnIsCheckedChanged))) 

      Public Shared Function GetIsChecked(ByVal d As DependencyObject) As Nullable(Of Boolean) 
     Return DirectCast(d.GetValue(IsCheckedProperty), Nullable(Of Boolean)) 
    End Function 

      Public Shared Sub SetIsChecked(ByVal d As DependencyObject, ByVal value As Nullable(Of Boolean)) 
     d.SetValue(IsCheckedProperty, value) 
    End Sub 

      Private Shared Sub OnIsCheckedChanged(ByVal d As DependencyObject, ByVal e As DependencyPropertyChangedEventArgs) 
     Dim pseudobutton As UIElement = TryCast(d, UIElement) 
     If pseudobutton IsNot Nothing Then 
      Dim newValue As Nullable(Of Boolean) = DirectCast(e.NewValue, Nullable(Of Boolean)) 
      If newValue = True Then 
       RaiseCheckedEvent(pseudobutton) 
      ElseIf newValue = False Then 
       RaiseUncheckedEvent(pseudobutton) 
      Else 
       RaiseIndeterminateEvent(pseudobutton) 
      End If 
     End If 
    End Sub 


      Public Shared ReadOnly IsThreeStateProperty As DependencyProperty = DependencyProperty.RegisterAttached("IsThreeState", _ 
          GetType(Boolean), _ 
          GetType(VirtualToggleButton), _ 
          New FrameworkPropertyMetadata(CBool(False))) 

      Public Shared Function GetIsThreeState(ByVal d As DependencyObject) As Boolean 
     Return CBool(d.GetValue(IsThreeStateProperty)) 
    End Function 

      Public Shared Sub SetIsThreeState(ByVal d As DependencyObject, ByVal value As Boolean) 
     d.SetValue(IsThreeStateProperty, value) 
    End Sub 


      Public Shared ReadOnly IsVirtualToggleButtonProperty As DependencyProperty = DependencyProperty.RegisterAttached("IsVirtualToggleButton", _ 
           GetType(Boolean), _ 
           GetType(VirtualToggleButton), _ 
           New FrameworkPropertyMetadata(CBool(False), _ 
             New PropertyChangedCallback(AddressOf OnIsVirtualToggleButtonChanged))) 

      Public Shared Function GetIsVirtualToggleButton(ByVal d As DependencyObject) As Boolean 
     Return CBool(d.GetValue(IsVirtualToggleButtonProperty)) 
    End Function 

      Public Shared Sub SetIsVirtualToggleButton(ByVal d As DependencyObject, ByVal value As Boolean) 
     d.SetValue(IsVirtualToggleButtonProperty, value) 
    End Sub 

      Private Shared Sub OnIsVirtualToggleButtonChanged(ByVal d As DependencyObject, ByVal e As DependencyPropertyChangedEventArgs) 
     Dim element As IInputElement = TryCast(d, IInputElement) 
     If element IsNot Nothing Then 
      If CBool(e.NewValue) Then 
       AddHandler element.MouseLeftButtonDown, New MouseButtonEventHandler(AddressOf VirtualToggleButton.OnMouseLeftButtonDown) 
       AddHandler element.KeyDown, New KeyEventHandler(AddressOf VirtualToggleButton.OnKeyDown) 
      Else 
       RemoveHandler element.MouseLeftButtonDown, New MouseButtonEventHandler(AddressOf VirtualToggleButton.OnMouseLeftButtonDown) 
       RemoveHandler element.KeyDown, New KeyEventHandler(AddressOf VirtualToggleButton.OnKeyDown) 
      End If 
     End If 
    End Sub 


      Friend Shared Function RaiseCheckedEvent(ByVal target As UIElement) As RoutedEventArgs 
     If target Is Nothing Then 
      Return Nothing 
     End If 

     Dim args As New RoutedEventArgs() 
     args.RoutedEvent = ToggleButton.CheckedEvent 
     [RaiseEvent](target, args) 
     Return args 
    End Function 

      Friend Shared Function RaiseUncheckedEvent(ByVal target As UIElement) As RoutedEventArgs 
     If target Is Nothing Then 
      Return Nothing 
     End If 

     Dim args As New RoutedEventArgs() 
     args.RoutedEvent = ToggleButton.UncheckedEvent 
     [RaiseEvent](target, args) 
     Return args 
    End Function 

      Friend Shared Function RaiseIndeterminateEvent(ByVal target As UIElement) As RoutedEventArgs 
     If target Is Nothing Then 
      Return Nothing 
     End If 

     Dim args As New RoutedEventArgs() 
     args.RoutedEvent = ToggleButton.IndeterminateEvent 
     [RaiseEvent](target, args) 
     Return args 
    End Function 


    Private Shared Sub OnMouseLeftButtonDown(ByVal sender As Object, ByVal e As MouseButtonEventArgs) 
     e.Handled = True 
     UpdateIsChecked(TryCast(sender, DependencyObject)) 
    End Sub 

    Private Shared Sub OnKeyDown(ByVal sender As Object, ByVal e As KeyEventArgs) 
     If e.OriginalSource Is sender Then 
      If e.Key = Key.Space Then 
            If (Keyboard.Modifiers And ModifierKeys.Alt) = ModifierKeys.Alt Then 
        Return 
       End If 

       UpdateIsChecked(TryCast(sender, DependencyObject)) 

       e.Handled = True 
      ElseIf e.Key = Key.Enter AndAlso CBool(TryCast(sender, DependencyObject).GetValue(KeyboardNavigation.AcceptsReturnProperty)) Then 
       UpdateIsChecked(TryCast(sender, DependencyObject)) 
       e.Handled = True 
      End If 
     End If 
    End Sub 

    Private Shared Sub UpdateIsChecked(ByVal d As DependencyObject) 
     Dim isChecked As Nullable(Of Boolean) = GetIsChecked(d) 
     If isChecked = True Then 
      SetIsChecked(d, If(GetIsThreeState(d), DirectCast(Nothing, Nullable(Of Boolean)), DirectCast(False, Nullable(Of Boolean)))) 
     Else 
      SetIsChecked(d, isChecked.HasValue) 
     End If 
    End Sub 

    Private Shared Sub [RaiseEvent](ByVal target As DependencyObject, ByVal args As RoutedEventArgs) 
     If TypeOf target Is UIElement Then 
      TryCast(target, UIElement).[RaiseEvent](args) 
     ElseIf TypeOf target Is ContentElement Then 
      TryCast(target, ContentElement).[RaiseEvent](args) 
     End If 
    End Sub 

End Class 

Answer 1

看起来你可以只取出DirectCast。从我看到你传递一个false到一个布尔函数/方法？ （或Nullable（布尔值））作为参数。 VB不需要C＃需要的显式转换（尽管在某些情况下它不是一个坏主意）。举个简单的例子，

Private Function DoSomething(byval param as Boolean?) as Boolean? 
    'do something and return a Nullable(of Boolean) 
End Function 

DoSomething(false) 'is just fine, no DirectCast needed 
DoSomething(nothing) 'is also fine 
DoSomething(true) 'fine 

DoSomething(DirectCast(false, Nullable(of Boolean)) 'will give you the error you described 

正如一个侧面说明，在情况下，它是混淆了可空（布尔）和布尔？意味着它是一个布尔变量，它可以具有true，false或nothing的值。

Answer 2

TKTS是正确的，VB，通常并不需要显式强制转换，但是，对于完整性，“正确”的转换将是：

New Boolean?(False)