Does ICommand always requires an object as a parameter? - mvvm

Question

When I implement the ICommand interface, the following methods are created
#region ICommand Members
public bool CanExecute(object parameter)
{
}
public event EventHandler CanExecuteChanged;
public void Execute(object parameter)
{
}
#endregion
The interesting part is
public void Execute(object parameter)
{
}
Simply because it indicates that it expects 1 parameter. What if I don't need to pass a parameter? In my ViewModel I have the following code
public class DownloadViewModel : BaseViewModel
{
public ICommand BrowseForFile { get; set; }
public string File { get; set; }
public DownloadViewModel()
{
BrowseForFile = new RelayCommand(new Action<object>(OpenDialog));
}
private void OpenDialog(object o)
{
var dialog = new System.Windows.Forms.FolderBrowserDialog();
System.Windows.Forms.DialogResult result = dialog.ShowDialog();
File = dialog.SelectedPath;
}
}
The OpenDialog method does not require the parameter but it appears as if I have to just so I can satisfy the Interface.
Am I doing this right or have I missed the point?

Answer 1

Yes, ICommand always needs an object and RelayCommand too. If you don't need it, you pass null and don't use it in your method, which is ugly.
I would use Prism's DelegateCommand instead. This exists in a non-generic version, which doesn't take parameters:
Command = new DelegateCommand(DoSomething);
CommandWithParameter = new DelegateCommand<int>(DoSOmethingWithInt);
Its in the PRISM assembly, which you have to download and reference.
using Microsoft.Practices.Prism;
PRISM
Alternatively, use the MVVMLight toolkit, which provides a command class which does basically the same thing. There is no point in using MVVM without a MVVM framework anyway. I can recommend PRISM, also for it's basic stuff like the DelegateCommand or the EventAggregator.

Answer 2

The fact that Execute takes a parameter is irrelevant to the method from your ViewModel. The only thing that affects what parameters OpenDialog needs is your implementation of ICommand.
If your implementation is, for example:
public class MyRandomCommand : ICommand
{
private readonly Action _action;
public MyRandomCommand(Action action)
{
_action = action;
}
public void Execute(object parameter)
{
_action();
}
...
}
Then no parameters will be required for your OpenDialog method, as you can create a command as follows:
public ICommand Command { get { return new MyRandomCommand(OpenDialog); } }
You can, however, require any signature you like for the method you are passing to your command.
The most common, off-the-shelf implementations of RelayCommand can take methods with either 0 or 1 parameter and will be called from Execute appropriately.