Question

我有一个页面-页面A，该页面具有订阅另一页面-页面B上的事件的方法。我认为在页面A中的方法订阅页面A之前，可以在页面A的代码中实例化页面B。 B页中的事件，然后最终将B页推入导航堆栈。

不幸的是，当我在iOS上测试我的应用程序时，在该方法订阅事件的那一行上，我不断收到NullReferenceException。当我将其作为Android应用程序进行部署和测试时，代码运行得非常好，但是我总是在iOS上收到NullReferenceException。是什么导致引发此异常，我该如何解决？为什么平台专用于iOS？

页面A上的代码

var confirmationPage = new EmailConfirmationPage(username);
confirmationPage.EmailConfirmed += this.OnEmailConfirmed;
await this.Navigation.PushModalAsync(confirmationPage);

...

private void OnEmailConfirmed(object source, EventArgs args)
{
    this.LabelMessage.Text = "Email Confirmed!";
}

B页上的代码

using System;
using Xamarin.Forms;
using Xamarin.Forms.Xaml;

namespace appFBLA2019
{
    [XamlCompilation(XamlCompilationOptions.Compile)]
    public partial class EmailConfirmationPage : ContentPage
    {
        private string username;
        private string email;

        public delegate void EmailConfirmedEventHandler(object source, EventArgs args);
        public event EmailConfirmedEventHandler EmailConfirmed;

        public EmailConfirmationPage(string username)
        {
            InitializeComponent();
            this.username = username;
            this.LabelTitle.Text = "Loading...";
            GetEmail();
        }

        private void GetEmail()
        {
            try
            {
                ServerConnector.QueryDB($"getEmail/{this.username}/-");
                this.email = ServerConnector.ReceiveFromDB();
                this.LabelTitle.Text = $"Enter the confirmation code sent to {this.email.Split('/')[1]}";
            }
            catch
            {
                this.LabelMessage.Text = "Connection Error: Please Try Again.";
            }
        }

        private async void ButtonConfirmEmail_Clicked(object sender, EventArgs e)
        {
            try
            {
                string userInputToken = this.EntryConfirmationCode.Text.Trim();
                ServerConnector.QueryDB($"confirmEmail/{this.username}/{userInputToken}/-");
                string returnData = ServerConnector.ReceiveFromDB();

                if (returnData == "true/-")
                {
                    OnEmailConfirmed();
                    await this.Navigation.PopModalAsync(true);
                }
                else
                {
                    this.LabelMessage.Text = "Email could not be confirmed. Please try your code again.";
                }
            }
            catch
            {
                this.LabelMessage.Text = "Connection Error: Please try again.";
            }
        }

        private void ButtonFixEmail_Clicked(object sender, EventArgs e)
        {
            string newEmail = this.EntryChangeEmail.Text;
            ServerConnector.QueryDB($"changeEmail/{this.username}/{newEmail}/-");
            string result = ServerConnector.ReceiveFromDB();

            if (result == "true/-")
            {
                this.LabelMessage.Text = $"Enter the confirmation code sent to {newEmail}";
            }
            else
            {
                this.LabelMessage.Text = $"Email could not be changed: {result.Split('/')[1]}";
            }
        }

        private async void ButtonClose_Clicked(object sender, EventArgs e)
        {
            await this.Navigation.PopModalAsync(true);
        }

        protected virtual void OnEmailConfirmed()
        {
            EmailConfirmed?.Invoke(this, EventArgs.Empty);
        }
    }
}

在向事件订阅方法之前调用堆栈：

appFBLA2019.CreateAccountPage.ButtonCreateAccount_p中的0xC0在C：\ Users \ chung \ source \ repos \ appFBLA2019 \ appFBLA2019 \ appFBLA2019 \ CreateAccountPage.xaml.cs：30,21 C＃

在订阅事件方法之后调用堆栈：

appFBLA2019.CreateAccountPage.ButtonCreateAccount_Click中的0x1B8在C：\ Users \ chung \ source \ repos \ appFBLA2019 \ appFBLA2019 \ appFBLA2019 \ CreateAccountPage.xaml.cs：39,13 C＃

Answer 1

在进一步测试中，我注意到此问题在iOS和Android上均会发生，但仅在使用Xamarin Live Player运行应用程序时才会发生。我联系了Microsoft，他们指出Xamarin Live Player不幸地有局限性。直接部署到设备不会造成任何问题，并且代码运行正常。

为何在Xamarin.Forms中订阅事件（但仅在iOS上）时为什么会收到NullReferenceException？

1 个答案: