SANS LES LAMBDAS :

public ICommand StartCommand { get; internal set; }

private initStartCommand()
{
    this.StartCommand = new RelayCommand<string>(execStartCommand, canExecuteStartCommand);
}

private void execStartCommand(string param)
{
    MessageBox.Show("Start");
}

private bool canExecuteStartCommand(string param)
{
    return true;
}


AVEC LES LAMBDAS :

public ICommand StartCommand { get; internal set; }

private initStartCommand()
{
    this.StartCommand = new RelayCommand<string>((param) => { MessageBox.Show("Start"); }, (param) => { return true; });
}