Relaycommand and parameterless execute method

Question

I\'m currently learning WPF and MVVM, I think I get most of it and how it works but I\'ve come across something on using the RelayCommand (or DelegateCommand) that I don\'t unde

Answer 1

The constructor parameter is a delegate that has the following signature:

void MethodName(T parameter)

where parameter is of type T (in the case of the RelayCommand this will be of type system.Object.

This code:

param => this.UpdateTextContentCommand_Execute()

is a lambda expression that essentially expands to this:

void AnonymousMethod(object param)
{
    this.UpdateTextContentCommand_Execute();
}

So in this case you are passing in a parameter (param) you are just not using it. If you understand this then you should now realise why your other examples behave the way they do.

Example 1

if (updateTextContentCommand == null)
{
    updateTextContentCommand = new RelayCommand(
        this.UpdateTextContentCommand_Execute());
}

Here you are calling the method which returns void. The constructor is expecting something that matches the Action<T> delegate hence the error.

Example 2

If you then remove the brackets like this:

if (updateTextContentCommand == null)
{
    updateTextContentCommand = new RelayCommand(
        this.UpdateTextContentCommand_Execute);
}

Think of this as being a bit like subscribing to an event:

myObject.myevent += new Action<object>(this.UpdateTextContentCommand_Execute);

which can be shortened to:

myObject.myevent += this.UpdateTextContentCommand_Execute;

So the constructor accepts any method that has a signature that matches the Action<T> delegates signature i.e.

void UpdateTextContentCommand_Execute(object parameter)

Your method has the following signature:

void UpdateTextContentCommand_Execute()

As you can see the signatures don't match so the compiler complains.

When you update your UpdateTextContentCommand_Execute method to accept an object parameter it's signature now matches which is why it now works.