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.
Assembly: PresentationCore (in PresentationCore.dll)
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 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 event.
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 Else Me.IsEnabled = False End If ' If a not RoutedCommand. Else If Me.Command.CanExecute(CommandParameter) Then Me.IsEnabled = True Else Me.IsEnabled = False End If End If End If End Sub
Available since 3.0