Question

My problem is that I would like to handle a commands in multiple places. For instance I have my custom UserControl where a Button is bound to some command. I have a command binding in that control but I also have a command binding in a Window which uses this control.

My aim is to perform some action inside the control while not interrupting handling of the command in the Window.

I tried experimenting with Executed and PreviewExecuted events but with no luck. Then I simulated the problem in a single window (code posted below).

<Window x:Class="CommandingEvents.Window1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:CommandingEvents="clr-namespace:CommandingEvents" 
    Title="Window1" Height="300" Width="300">
<Window.CommandBindings>
    <CommandBinding 
        Command="{x:Static CommandingEvents:Window1.Connect}"
        Executed="CommandBindingWindow_Executed"
        PreviewExecuted="CommandBindingWindow_PreviewExecuted"/>
</Window.CommandBindings>
<Grid>
    <Grid.CommandBindings>
        <CommandBinding 
        Command="{x:Static CommandingEvents:Window1.Connect}"
        Executed="CommandBindingGrid_Executed"
        PreviewExecuted="CommandBindingGrid_PreviewExecuted" />
    </Grid.CommandBindings>
    <Button Command="{x:Static CommandingEvents:Window1.Connect}" 
            CommandTarget="{Binding RelativeSource={RelativeSource Self}}"
            Content="Test" HorizontalAlignment="Center" VerticalAlignment="Center" />
</Grid>

namespace CommandingEvents
{
    public partial class Window1
    {
        public static readonly RoutedUICommand Connect = new
            RoutedUICommand("Connect", "Connect", typeof(Window1));

        public Window1()
        {
            InitializeComponent();
        }

        private void CommandBindingWindow_Executed(object sender, ExecutedRoutedEventArgs e)
        {
            Console.WriteLine("CommandBindingWindow_Executed");
            e.Handled = false;
        }

        private void CommandBindingGrid_Executed(object sender, ExecutedRoutedEventArgs e)
        {
            Console.WriteLine("CommandBindingGrid_Executed");
            e.Handled = false;
        }

        private void CommandBindingWindow_PreviewExecuted(object sender, ExecutedRoutedEventArgs e)
        {
            Console.WriteLine("CommandBindingWindow_PreviewExecuted");
            e.Handled = false;
        }

        private void CommandBindingGrid_PreviewExecuted(object sender, ExecutedRoutedEventArgs e)
        {
            Console.WriteLine("CommandBindingGrid_PreviewExecuted");
            e.Handled = false;
        }
    }
}

When I hit the button only “CommandBindingWindow_PreviewExecuted” is printed out. Why is that? I tried to set e.Handled to false but it doesn’t make a difference. Can anyone explain this behaviour?

Answer 1

I have no idea why that happens (and how it isn’t a bug) but here is what was written in the WPF wiki:

There is a particularity about
CommandBinding which is extremely
interesting and important to know.

The CommandManager uses routed events
to notify the different CommandBinding
objects that a command execution was
invoked (through default gestures,
input bindings, explicitly, etc.).

Up to now, this is fairly
straightforward. However, what is more
important is that CommandBinding will
mark the routed event from the
CommandManager as handled as soon as a
handler gets executed (either
PreviewExecuted or Executed).

Finally, even if your handler has a
prototype that matches a delegate
called ExecutedRoutedEventHandler, the
Executed event from the CommandBinding
is not a RoutedEvent but a normal CLR
event. Setting or leaving the
e.Handled flag to false will change
nothing.

Therefore, as soon as an Executed or
PreviewExecuted handler is invoked,
the RoutedCommand will halt its
routing.