ICommand with Parameter

Question

I found this: Close Window from ViewModel which gets me started down the path of modifying my DelegateCommand class to handle parameters. But I am not able to get the syntax worked out.

Here is my DelegateCommand class, and the DelegateCommand class that I'm trying to create with little success:

    public class DelegateCommand : ICommand
    {
        private readonly Action _action;

        public DelegateCommand(Action action)
        {
            _action = action;
        }

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

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

#pragma warning disable 67
        public event EventHandler CanExecuteChanged { add { } remove { } }
#pragma warning restore 67
    }

    public class DelegateCommand<T> : ICommand
    {
        private readonly Action _action;

        public DelegateCommand(Action action)
        {
            _action = action;
        }

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

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

#pragma warning disable 67
        public event EventHandler CanExecuteChanged { add { } remove { } }
#pragma warning restore 67
    }

And here is what I do in the viewmodel:

public ICommand RowEditEndingAction
{
    get { return new DelegateCommand(RowEditEnding); }
}

public ICommand UpdateDatabaseClick
{
    get { return new DelegateCommand<object>(UpdateDatabase); } //THIS LINE HAS THE ERROR
}

And the actual method that will get called:

public void UpdateDatabase(object parameter)
{
    Window w = (Window)parameter;
    // A bunch of stuff that works is removed for brevity
    w.Close();
}

The compiler does not like my UpdateDatabaseClick, specifically saying there is something wrong with the arguments to DelegateCommand, but I am at a loss as to what I am doing wrong (though I am thinking it is syntax . . . ). What do I need to change? This all worked before I added the parameter to UpdateDatabase, and just had the DelegateCommand (no template class). But in that case I could not close the window.

Answer 1

Here's the constructor of your DelegateCommand<T> class that you are calling:

public DelegateCommand(Action action)

And here is how you call it:

new DelegateCommand<object>(UpdateDatabase)

In there, UpdateDatabase is declared as follows:

public void UpdateDatabase(object parameter)

Now, the constructor you are invoking expects an Action. That is a parameterless method without a return value.

However, you are passing in a method with one parameter. That is what the compiler complains about.

What you actually probably want to do is to accept any method with one parameter - for that, you can use the type Action<T>. As your parameter should presumeably have the same type that is passed as a type argument to your DelegateCommand<T> class, you can declare your constructor like this:

public DelegateCommand(Action<T> action)

Now, you also need to update the type of your backing field where you store the action:

private readonly Action<T> _action;

Lastly, as _action now expects an argument, you need to pass that argument when invoking _action in DelegateCommand<T>.Execute. Normally, you want to hand over the parameter object you receive as an argument to the Execute method. However, that value is always typed to object, whereas you want to work with a strongly-typed value of type T in your method. Therefore, you will also have to add an additional cast:

public void Execute(object parameter)
{
    _action((T)parameter);
}

Answer 2

I suggest you try Prism framework. It has all the component, tools, you need to work with WPF application with MVVM model.