2

I took some code from a C# project and put it into a converter. The original code was:

(Nullable<bool>)false

and the converter said the VB equivalent is:

DirectCast(False, Nullable(Of Boolean))

I even compiled the C# project and looked at it in Reflector. It gave the same VB code as above, but this generates the error:

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

How do I cast this properly?

More Code as requested:

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
4

2 回答 2

1

看起来您可以删除 DirectCast。从我所见，您将 false 传递给具有布尔值的函数/方法？（或 Nullable(of Boolean)）作为参数。VB 不需要 C# 所需的显式转换（尽管在某些情况下这不是一个坏主意）。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

顺便说一句，如果它混淆了 Nullable(of Boolean) 和 Boolean？意思是一样的，它是一个布尔变量，它的值可以是真、假或什么都没有。

于 2011-04-08T14:06:53.437 回答
1

TKTS 是正确的，VB 通常不需要显式转换，但是，为了完整起见，“正确”的转换将是：

New Boolean?(False)
于 2013-02-02T16:23:16.770 回答