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private DelegateCommand loginCommand;

    public string Username { get; set; }
    public string Password { get; set; }


    public ICommand LoginCommand
    {
        get
        {
            if (loginCommand == null)
            {
                loginCommand = new DelegateCommand(
                    Login, CanLogin );
            }
            return loginCommand;
        }
    }

    private bool CanLogin()
    {
        return !string.IsNullOrEmpty(Username);
    }

    private void Login()
    {
        bool result = securityService.IsValidLogin(Username, Password);

        if (result) { }
        else { }
    }