Pass CommandParameter to Command in Silverlight using MVVM

Question

I'm just learning Silverlight and looking at MVVM and Commanding.

Ok, so I have seen the basic RelayCommand implementation:

public class RelayCommand : ICommand
{
    private readonly Action _handler;
    private bool _isEnabled;

    public RelayCommand(Action handler)
    {
        _handler = handler;
    }

    public bool IsEnabled
    {
        get { return _isEnabled; }
        set
        {
            if (value != _isEnabled)
            {
                _isEnabled = value;
                if (CanExecuteChanged != null)
                {
                    CanExecuteChanged(this, EventArgs.Empty);
                }
            }
        }
    }

    public bool CanExecute(object parameter)
    {
        return IsEnabled;
    }

    public event EventHandler CanExecuteChanged;

    public void Execute(object parameter)
    {
        _handler();
    }
}

How can I pass a parameter down with a Command using this?

I've seen that you can pass a CommandParameter like this:

<Button Command="{Binding SomeCommand}" CommandParameter="{Binding SomeCommandParameter}" ... />

In my ViewModel, I need to create the Command, but RelayCommand is expecting an Action delegate. Can I implement RelayCommand<T> using Action<T> - if so, how do I do it and how to I use it?

Can anyone give me any practical examples on CommandParameters with MVVM that don't involve using 3rd-party libraries (e.g. MVVM Light) as I want to understand it fully before using existing libraries.

Thanks.

Answer 1

public class Command : ICommand
{
    public event EventHandler CanExecuteChanged;

    Predicate<Object> _canExecute = null;
    Action<Object> _executeAction = null;

    public Command(Predicate<Object> canExecute, Action<object> executeAction)
    {
        _canExecute = canExecute;
        _executeAction = executeAction;
    }
    public bool CanExecute(object parameter)
    {
        if (_canExecute != null)
            return _canExecute(parameter);
        return true;
    }

    public void UpdateCanExecuteState()
    {
        if (CanExecuteChanged != null)
            CanExecuteChanged(this, new EventArgs());
    }

    public void Execute(object parameter)
    {
        if (_executeAction != null)
            _executeAction(parameter);
        UpdateCanExecuteState();
    }
}

Is the Base Class for Commands

And this is your Command Property in ViewModel:

 private ICommand yourCommand; ....

 public ICommand YourCommand
    {
        get
        {
            if (yourCommand == null)
            {
                yourCommand = new Command(  //class above
                    p => true,    // predicate to check "CanExecute" e.g. my_var != null
                    p => yourCommandFunction(param1, param2));
            }
            return yourCommand;
        }
    }

in XAML set Binding to Command Property like:

 <Button Command="{Binding Path=YourCommand}" .../>

Answer 2

Maybe this article explains what you're looking for. I also had the same problem just minutes ago.

http://www.eggheadcafe.com/sample-code/SilverlightWPFandXAML/76e6b583-edb1-4e23-95f6-7ad8510c0f88/pass-command-parameter-to-relaycommand.aspx