How does the method work ?

[Mohammad Dayyan]

Hello there, Would you please explain , how the below method (that is bold) works ?

public class RelayCommand : ICommand
{
readonly Action<object> _execute;
readonly Predicate<object> _canExecute;

**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;           
}

\[DebuggerStepThrough\]
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);
}

}

I mean , what's the meaning of : this(execute, null) ?

[Steve Westbrook]

It calls the constructor below it: it's like base(whatever). The point is to avoid duplicating whatever functionality is in the two-parameter method.

[Mohammad Dayyan]

Thanks, What's the name of these methods ? Is there a specific name for these methods ?

[Luc Pattyn]

It works on constructors only, i.e. methods that: - have no return type; - have a name identical to the class name; - get invoked using the new keyword. :)

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modified on Friday, February 5, 2010 4:29 PM

[Mohammad Dayyan]

Got it ;)

[Mohammad Dayyan]

So, this(execute, null) means something like base(execute, null) :confused:

[Luc Pattyn]

No, they are different. use base to refer to the base class, the class from which the current class derives. this refers to some other constructor of the current class; the normal use is: - one implements the "full" constructor, the one taking most parameters; - then one refers all other constructors to the first one, by providing default parameters for the missing ones. See your own example. :)

Luc Pattyn [Forum Guidelines] [Why QA sucks] [My Articles]

---

I only read code that is properly formatted, adding PRE tags is the easiest way to obtain that.
[The QA section does it automatically now, I hope we soon get it on regular forums as well]

---

modified on Friday, February 5, 2010 4:28 PM

[Pete OHanlon]

this(execute, null) passes the call through to the second constructor - setting the predicate to null. As you can't call a constructor by name, you need to do this to actually call the second constructor. As you know, in a method, you can do the following:

public void DoThis(int index)
{
DoThis(index, null);
}

public void DoThis(int index, string message)
{
}

While it would be nice to do this with a constructor, you can't:

public MyClass()
{
MyClass(null);
}

public MyClass(string message)
{
}

In order to get the same effect, you modify the constructor definition instead.

public MyClass() : this(null)
{
}

public MyClass(string message)
{
}

Now, this differs from calling base which invokes a method on the parent class, rather than this class. What you can't do, is combine this and base in a constructor - it's one or the other.

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[Mohammad Dayyan]

excellent, I've got it. :rose:

[Mohammad Dayyan]

very good, Thanks dude ;)