WPF CommandParameter is NULL first time CanExecute is called

.net-3.5commanddata-bindingwpfxaml

I have run into an issue with WPF and Commands that are bound to a Button inside the DataTemplate of an ItemsControl. The scenario is quite straight forward. The ItemsControl is bound to a list of objects, and I want to be able to remove each object in the list by clicking a Button. The Button executes a Command, and the Command takes care of the deletion. The CommandParameter is bound to the Object I want to delete. That way I know what the user clicked. A user should only be able to delete their "own" objects – so I need to do some checks in the "CanExecute" call of the Command to verify that the user has the right permissions.

The problem is that the parameter passed to CanExecute is NULL the first time it's called – so I can't run the logic to enable/disable the command. However, if I make it allways enabled, and then click the button to execute the command, the CommandParameter is passed in correctly. So that means that the binding against the CommandParameter is working.

The XAML for the ItemsControl and the DataTemplate looks like this:

<ItemsControl 
    x:Name="commentsList"
    ItemsSource="{Binding Path=SharedDataItemPM.Comments}"
    Width="Auto" Height="Auto">
    <ItemsControl.ItemTemplate>
        <DataTemplate>
            <StackPanel Orientation="Horizontal">
                <Button                             
                    Content="Delete"
                    FontSize="10"
                    Command="{Binding Path=DataContext.DeleteCommentCommand, ElementName=commentsList}" 
                    CommandParameter="{Binding}" />
            </StackPanel>                       
         </DataTemplate>
    </ItemsControl.ItemTemplate>
</ItemsControl>

So as you can see I have a list of Comments objects. I want the CommandParameter of the DeleteCommentCommand to be bound to the Command object.

So I guess my question is: have anyone experienced this problem before? CanExecute gets called on my Command, but the parameter is always NULL the first time – why is that?

Update: I was able to narrow the problem down a little. I added an empty Debug ValueConverter so that I could output a message when the CommandParameter is data bound. Turns out the problem is that the CanExecute method is executed before the CommandParameter is bound to the button. I have tried to set the CommandParameter before the Command (like suggested) – but it still doesn't work. Any tips on how to control it.

Update2: Is there any way to detect when the binding is "done", so that I can force re-evaluation of the command? Also – is it a problem that I have multiple Buttons (one for each item in the ItemsControl) that bind to the same instance of a Command-object?

Update3: I have uploaded a reproduction of the bug to my SkyDrive: http://cid-1a08c11c407c0d8e.skydrive.live.com/self.aspx/Code%20samples/CommandParameterBinding.zip

Best Solution

I was having this same issue while trying to bind to a command on my view model.

I changed it to use a relative source binding rather than referring to the element by name and that did the trick. Parameter binding didn't change.

Old Code:

Command="{Binding DataContext.MyCommand, ElementName=myWindow}"

New Code:

Command="{Binding DataContext.MyCommand, RelativeSource={RelativeSource AncestorType=Views:MyView}}"

Update: I just came across this issue without using ElementName, I'm binding to a command on my view model and my data context of the button is my view model. In this case I had to simply move the CommandParameter attribute before the Command attribute in the Button declaration (in XAML).

CommandParameter="{Binding Groups}"
Command="{Binding StartCommand}"