MVVM Light : RelayCommand : define it lazy or in constructor?

There are several examples on how to define a RelayCommand in the ViewModel:

Option 1 (lazy):

/// <summary>
/// Gets the LogOnCommand.
/// </summary>
/// <value>The LogOnCommand.</value>
public RelayCommand<LogOnUser> LogOnCommand
{
    get
    {
        if (this.logOnCommand == null)
        {
           开发者_运维技巧 this.logOnCommand = new RelayCommand<LogOnUser>(
                action =>
                {
                    // Action code...
                },
                g => g != null);
        }

        return this.logOnCommand;
    }
}

Option 2 (in constructor)

/// <summary>
/// Initializes a new instance of the <see cref="LogOnFormViewModel"/> class.
/// </summary>
public LogOnFormViewModel()
{
    this.logOnCommand = new RelayCommand<LogOnUser>(
                action =>
                {
                    // Action code...
                },
                g => g != null);
}

/// <summary>
/// Gets the LogOnCommand.
/// </summary>
/// <value>The LogOnCommand.</value>
public RelayCommand<LogOnUser> LogOnCommand {get; private set;}

What is the best / clearest design ?

It really depends on what style you prefer. Most people don't like having a bunch of logic in property getters if they can avoid it.

Personally, I prefer having a real method to call instead of an anonymous method. My ViewModels look something like this.

public class MyViewModel : ViewModelBase
{
    public RelayCommand<CommandParam> MyCommand { get; private set; }

    public MyViewModel()
    {
        CreateCommands();
    }

    private void CreateCommands()
    {
        MyCommand = new RelayCommand<CommandParam>(MyCommandExecute);
    }

    private void MyCommandExecute(CommandParam parm)
    {
        // Action code...
    }
}

Notice that if you're not using the enable command, you don't need to call the ctor overload that sets that.