Question

In following code (Josh Smith’s article on MVVM), can somebody give me some insight about
return _canExecute == null ? true : _canExecute(parameter); ?

it is a normal if/else statement but I’m not getting the last part of it.

public class RelayCommand : ICommand
{
    #region Fields

    readonly Action<object> _execute;
    readonly Predicate<object> _canExecute;        

    #endregion // Fields

    #region Constructors

    public RelayCommand(Action<object> execute)
    : this(execute, null)
    {
    }

    public RelayCommand(Action<object> execute, Predicate<object> canExecute)
    {
        if (execute == null)
            throw new ArgumentNullException("execute");

        _execute = execute;
        _canExecute = canExecute;           
    }
    #endregion // Constructors

    #region ICommand Members

    [DebuggerStepThrough]
    public bool CanExecute(object parameter)
    {
        return _canExecute == null ? true : _canExecute(parameter);
    }

    public event EventHandler CanExecuteChanged
    {
        add { CommandManager.RequerySuggested += value; }
        remove { CommandManager.RequerySuggested -= value; }
    }

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

    #endregion // ICommand Members
}

Thanks.

Answer 1

_canExecute is lambda function, and it can be null, depending on which constructor will instantiate RelayCommand object. Thus, the implementation of CanExecute method checks whether this function is set, and if it’s not, it returns true, wheras if there is function assigned, it evaluates it (given the parameter), and the evaluated value is returned as a result for CanExecute.

In a nutshell: CanExecute will be evaluated using whatever predicate is passed along in the constructor, and in case of lack one – will always return true.

You’ve asked that’s usually used for array/lists – it’s very similar situation. The predicate is just a function that you can pass around. When you pass such predicate to some method that is filtering a collection, the method is just calling this predicate as it would any other function.