CODEWITHSUNDEEP
.netwpfprismweak-referencesdelegatecommand
dmigo
dmigo

Reputation: 3029

Can't subscribe to DelegateCommand.CanExecuteChanged

I have two ViewModels and one contains another. The inner one has a Microsoft.Practices.Prism.Commands.DelegateCommand called PrintCommand. It is desirable to subscribe to the CanExecuteChanged event of this command. This part is implemented as usual: 

OneViewModel.PrintCommand.CanExecuteChanged += CanExecuteChangedHandler;

The problem is that this subscription doesn't work. Decompiled CanExecuteChanged looks like this: 

public event EventHandler CanExecuteChanged  
{  
  add  
  {  
    WeakEventHandlerManager.AddWeakReferenceHandler(ref this._canExecuteChangedHandlers, value, 2);  
  }  
  remove  
  {  
    WeakEventHandlerManager.RemoveWeakReferenceHandler(this._canExecuteChangedHandlers, value);  
  }  
}

When I debug, after a couple of steps after subscription the _canExecuteChangedHandlers doesn't seem to contain any alive handlers, even though subscriber object still exists.
I'm kind of curious, why is it happening?

Upvotes: 2

Views: 399

Answers (1)

dmigo
dmigo

Reputation: 3029

The answer was found in the description of CanExecuteChanged. Here it is: 

/// When subscribing to the <see cref="E:System.Windows.Input.ICommand.CanExecuteChanged"/> event using
///             code (not when binding using XAML) will need to keep a hard reference to the event handler. This is to prevent
///             garbage collection of the event handler because the command implements the Weak Event pattern so it does not have
///             a hard reference to this handler. An example implementation can be seen in the CompositeCommand and CommandBehaviorBase
///             classes. In most scenarios, there is no reason to sign up to the CanExecuteChanged event directly, but if you do, you
///             are responsible for maintaining the reference.

This means that we should have a hard reference:

private readonly EventHandler commandCanExecuteChangedHandler;

And use it like this:

this.commandCanExecuteChangedHandler = new EventHandler(this.CommandCanExecuteChanged);
this.command.CanExecuteChanged += this.commandCanExecuteChangedHandler;

Upvotes: 2

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