CanExecuteChanged Event
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RoutedCommand.CanExecuteChanged Event


Occurs when changes to the command source are detected by the command manager. These changes often affect whether the command should execute on the current command target.

Namespace:   System.Windows.Input
Assembly:  PresentationCore (in PresentationCore.dll)

Public Event CanExecuteChanged As EventHandler

The RoutedCommand listens for the RequerySuggested event, which is raised by the CommandManager. The RequerySuggested event is raised whenever conditions are met that may change the whether the command should execute, such as a change in keyboard focus. When the command receives the RequerySuggested event, it raises the CanExecuteChanged event. Generally, the command source will listen for this event and query the RoutedCommand by way of the CanExecute method. Most command sources will disable themselves if the command cannot be executed, as part of their command binding. An example of this is when a MenuItem grays itself out when the command cannot be executed.

In some situations, the CommandManager is unaware of a change in conditions that alters the ability of a command to execute. In these cases, you can force the CommandManager to raises the RequerySuggested event by calling the InvalidateRequerySuggested method, this will in turn cause the RoutedCommand to raise the CanExecuteChanged event.

The following example is a CanExecuteChanged event handler from a custom implementation of ICommandSource.

this.Command in this example is the Command property on the ICommandSource. If the command is not null, the command is cast to a RoutedCommand. If the command is a RoutedCommand, then the CanExecute method is called passing the CommandTarget and the CommandParameter. If command is not a RoutedCommand, it is cast to an ICommand and the CanExecute method is called passing the CommandParameter.

If the CanExecute method returns true, then the control is enabled; otherwise, the control is disabled.

Private Sub CanExecuteChanged(ByVal sender As Object, ByVal e As EventArgs)

    If Me.Command IsNot Nothing Then
        Dim command As RoutedCommand = TryCast(Me.Command, RoutedCommand)

        ' If a RoutedCommand.
        If command IsNot Nothing Then
            If command.CanExecute(CommandParameter, CommandTarget) Then
                Me.IsEnabled = True
                Me.IsEnabled = False
            End If
            ' If a not RoutedCommand.
            If Me.Command.CanExecute(CommandParameter) Then
                Me.IsEnabled = True
                Me.IsEnabled = False
            End If
        End If
    End If
End Sub

.NET Framework
Available since 3.0
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