Question

我有这段代码，但是应用程序进入中断模式，并告诉我有一个未处理的异常：每当用户输入除A，B，C，DI之外的其他任何值，该值都不能为null，但是我的代码没有似乎有效。

 private async void Confirm_Clicked(object sender, EventArgs e)
    {
        if (txtUserEntry.Text != null)
        {
            if (txtUserEntry.Text == "A")
            { 
                await DisplayAlert("Wrong answer", "Sorry", "Next question");
                await Navigation.PushModalAsync(new Quiz_Page2());
            }
            else if (txtUserEntry.Text == "B")
            {
                await DisplayAlert("Wrong answer", "Sorry", "Next question");
                await Navigation.PushModalAsync(new Quiz_Page2());
            }
            else if (txtUserEntry.Text == "C")
            {
                await DisplayAlert("Wrong answer", "Sorry", "Next question");
                await Navigation.PushModalAsync(new Quiz_Page2());
            }
            else if (txtUserEntry.Text == "D")
            {
                scoreSettings.GlobalScore++;
                await DisplayAlert("Correct answer", "Well done", "Next question");
                await Navigation.PushModalAsync(new Quiz_Page2());
            }
            else if (txtUserEntry.Text != "A" || txtUserEntry.Text != "B" || txtUserEntry.Text != "C" || txtUserEntry.Text != "D")
            {
                await DisplayAlert("Please type either A,B,C,D", "", "");
            }

            else await DisplayAlert("Please type either A,B,C,D", "", "");
        }
    }
}

Answer 1

原因： 方法DisplayAlert具有三个参数：

//   title:
//     The title of the alert dialog.
//
//   message:
//     The body text of the alert dialog.
//
//   cancel:
//     Text to be displayed on the 'Cancel' button.

它们是不可空的。因此，当您将其设置为null时，它将崩溃。

解决方案：

给参数message和cancel赋一个值。如果您不想显示任何内容，可以将其设置为space

 await DisplayAlert("Please type either A,B,C,D", " ", " ");

在用户输入中仅接受某些字符

1 个答案: