freshmvvm命令参数

Question

我的页面中有一个带CommandParameter的Command，但是当PageModel的触发器值为null时，我就没有了。 我有一个模型“UserLogin”：

[AddINotifyPropertyChangedInterface]
public class UserLogin
{
    public string Username { get; set; }
    public string Password { get; set; }

    public UserLogin(string username, string password)
    {
       Username = username;
       Password = Password;
    }
    public UserLogin()
    {

    }
}

我的各个PageModel是“LoginPageModel”：

    [AddINotifyPropertyChangedInterface]
    public class LoginPageModel : FreshBasePageModel
    {
    IDatabaseService _databaseService;
    IUserDialogs _userDialogs;
    IRestService _restService;

     public LoginPageModel(IDatabaseService databaseService, IRestService restService, IUserDialogs userDialogs) //injected from IOC
        {
            _databaseService = databaseService;
            _restService = restService;
            _userDialogs = userDialogs;
        }

        private UserLogin _userLogin;
        public UserLogin UserLogin
        {
            get
            {
                return _userLogin;
            }
            set
            {
                _userLogin = value;
                if (value != null)
                    RaisePropertyChanged();
            }
        }

    public Command LoginCommand
    {
    get
    {
     return new Command<UserLogin>(async (log) => 
    {
    ....
}

我的相应页面是“LoginPage.xaml”：

<ContentPage.Content>
     <StackLayout Padding="20" Spacing="5">

            <Entry x:Name="Username" Text="{Binding Username}" Keyboard="Email" Placeholder="Username"  />

            <Entry x:Name="Password" Text="{Binding Password}" Placeholder="Password" IsPassword="true" />

            <Button x:Name="btn_Login" Command="{Binding LoginCommand}" CommandParameter="{Binding UserLogin}" Text="Login" />         

    </StackLayout>       
</ContentPage.Content>

我的参数代码出错了吗？

谢谢人