Why Use RelayCommand to Improve WPF MVVM Architecture?

Advantages of RelayCommand in WPF MVVM architecture

When building WPF applications, separating the concerns of the view and view model is crucial for maintainability and testability. RelayCommand is a commonly used command in MVVM (Model-View-ViewModel) architecture and plays a vital role in this separation.

Purpose of RelayCommand

Commands in MVVM separate the semantics and caller of an operation from the logic that performs the operation. This decoupling allows the business logic to be tested independently and keeps the UI loosely coupled to the logic.

Applicability across commands

RelayCommand applies to all commands in the form. WPF controls such as Button and MenuItem expose Command DependencyProperties so that commands declared in the view model can be bound. For events that are not bound through these properties, you can use interaction triggers to associate them with a RelayCommand.

Conditional button disabled

To disable a button based on the state of some textbox, you can use the overloaded constructor of RelayCommand to set the CanExecute predicate. In this predicate you can check if any bound property is null and return false which will disable the command and therefore the button.

Complete implementation of RelayCommand

<code class="language-c#">public class RelayCommand<T> : ICommand
{
    private readonly Action<T> _execute;
    private readonly Predicate<T> _canExecute;

    public RelayCommand(Action<T> execute, Predicate<T> canExecute = null)
    {
        _execute = execute ?? throw new ArgumentNullException(nameof(execute));
        _canExecute = canExecute;
    }

    public bool CanExecute(object parameter) => _canExecute == null || _canExecute((T)parameter);

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

    public void Execute(object parameter) => _execute((T)parameter);
}</code>

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