Question

I obviously don’t get this somewhere.

I have created a UserControl, the bare bones of which is:

private readonly DependencyProperty SaveCommandProperty =
      DependencyProperty.Register("SaveCommand", typeof(ICommand),
                                  typeof(ctlToolbarEdit));

private readonly DependencyProperty IsSaveEnabledProperty = 
      DependencyProperty.Register("IsSaveEnabled", typeof(bool), 
        typeof(ctlToolbarEdit), new PropertyMetadata(
           new PropertyChangedCallback(OnIsSaveEnabledChanged)));

public ctlToolbarEdit()
{
   InitializeComponent();
}

public bool IsSaveEnabled
{
   get { return (bool)GetValue(IsSaveEnabledProperty); }
   set { SetValue(IsSaveEnabledProperty, value); }
}

public static void OnIsSaveEnabledChanged(DependencyObject d,
   DependencyPropertyChangedEventArgs e)
{
   ((ctlToolbarEdit)d).cmdSave.IsEnabled = (bool)e.NewValue;
}

#region Command Handlers
public ICommand SaveCommand
{
   get { return (ICommand)GetValue(SaveCommandProperty); }
   set { SetValue(SaveCommandProperty, value); }
}

private void cmdSave_Click(object sender, RoutedEventArgs e)
{
   if (SaveCommand != null) 
      SaveCommand.Execute(null);
}

#endregion

Excellent. You can see what I am doing … handling the click event of the button, and basically firing up the command.

The form (lets call that Form1 for the time being … but note that this is actually a UserControl: common practice, I believe, in MVVM) that is hosting the control has the following line:

<ctl:ctlToolbarEdit HorizontalAlignment="Right" Grid.Row="1" 
   SaveCommand="{Binding Save}" IsSaveEnabled="{Binding IsValid}" />

This works great. I have an ICommand in my ViewModel called ‘Save’ and the ViewModel is correctly presenting the IsValid property.

So far so very good.

Now I want to have my new usercontrol also on Form2 (which is also a usercontrol – common practice, I believe, on MVVM). As it happens, Form1 and Form2 are on the screen at the same time.

It compiles, but I get a runtime exception:

‘SaveCommand’ property was already registered by ‘ctlToolbarEdit’.”

… leading me to believe that I don’t get ‘commands’ at all.

Why can I not use my usercontrol in more than one place?

If I cannot, what would you suggest is another way to do this?

Very frustrating!

Thanks for any help.

Answer 1

Try making your dependency properties static. Otherwise it is getting re-registered every time you instantiate a new control. Your usage of the MVVM commands looks good otherwise and sounds like you have a good grasp on it.