Why Use RelayCommand in WPF for MVVM Command Handling?

Advantages of RelayCommand

One of the core principles of the Model-View-ViewModel (MVVM) architecture is the separation of the user interface (View) from the application logic (ViewModel). This separation ensures loose coupling and testability of the application. One way to achieve this separation is through commands, which allow you to delegate event handling to the ViewModel.

The role of RelayCommand

RelayCommand is a concrete implementation of the ICommand interface that simplifies the process of creating and using commands in WPF applications. It encapsulates the logic to determine whether a command can be executed (CanExecute) and perform the required action (Execute).

When to use RelayCommand

RelayCommand can be used for any command in a form that needs to be separated from the UI. For example, you can use it to perform operations such as saving data, opening files, or validating input.

Disable button based on text box content

To disable a button based on the contents of the text box, you can bind the button's IsEnabled property to the output of the CanExecute delegate in the RelayCommand. In the CanExecute delegate, you can check if any bound textbox properties are empty or invalid and return false to disable the button.

Example implementation

The following code shows a basic implementation of a RelayCommand with its CanExecute delegate checking whether the text box is empty:

<code class="language-csharp">public class TextBoxEmptyCommand : RelayCommand<TextBox>
{
    public TextBoxEmptyCommand(Action<TextBox> execute) : base(execute, (tb) => tb.Text.Length == 0)
    {
    }
}</code>

You can bind this command to the Button's IsEnabled property like this:

<code class="language-xaml"><Button Command="{Binding TextBoxEmptyCommand}" Content="Submit" /></code>

(Please note that the second code block is empty in the input, I have not added anything since the original article does not provide a complete second code block either.)

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