对象引用未设置为 Razor 中的对象实例

Question

我是 Razor 新手，在尝试遍历对象列表时遇到此错误。

这是我的观点：

@page
@model QuizModel
@{
    ViewData["Title"] = "Quiz Page";
}

<div class="text-center">
    <h1 class="display-4">Thanks for checking my first website out, @Model.Visitor.Name. Are you ready?</h1>
    <p>Let's see how much you know about me.</a>.</p>
</div>

<div class="text-center">
    <h1 class="display-4">Welcome</h1>
    <form method="post">
        @foreach (var question in @Model.QuestionList)
        {
            <label>@question.Query</label>
        }

        <button type="submit">Send</button>
    </form>
</div>

这是.cs

namespace myquiz.Pages
{
    public class QuizModel : PageModel
    {
        [ViewData]
        [BindProperty]
        public string Name { get; set; }
        [BindProperty]
        public Visitor Visitor { get; set; }
        public List<Question> QuestionList { get; set; }

        public void OnGet()
        {
            var quizService = new QuizService();
            ///QuestionList = new List<Question>();
            QuestionList = quizService.GetQuestions();
        }

        public void OnPost()
        {
            Name = Visitor.Name;
        }
    }
}

这里是服务

 public class QuizService 
    {
        public List<Question> GetQuestions()
        {
            return new List<Question>()
            {
                new Question()
                {
                    Id=1,
                    Query = "What's my favourite band?",
                    Option1 = "Beatles",
                    Option2 = "Rolling Stones",
                    Option3 = "Led Zeppelin",
                    Answer = "Led Zeppelin"
                },
                new Question()
                {
                    Id=2,
                    Query = "What's my favourite colour?",
                    Option1 = "Pink",
                    Option2 = "Yellow",
                    Option3 = "Maroon",
                    Answer = "Pink"
                },
            };
        }
    }

我尝试初始化评论中的列表，但它也没有工作:(

谢谢！

Answer 1

最好看看 quizService.GetQuestions() 方法的实现。但是你可以在方法之后检查变量。

QuestionList = quizService.GetQuestions();
if (QuestionList is null)
    QuestionList = new List<Question>();

最好在 GetQuestions() 方法中进行此检查。

Answer 2

您需要初始化 Visitor 和 QuestionList，因为您在视图中使用它们。 你可以改变

[BindProperty]
        public Visitor Visitor { get; set; }
        public List<Question> QuestionList { get; set; }

到

 [BindProperty]
        public Visitor Visitor { get; set; } = new Visitor();
 [BindProperty]
        public List<Question> QuestionList { get; set; } = new List<Question>();

这样就不需要在 OnGet 中初始化了。

更新：

服务 DI： 启动：

public void ConfigureServices(IServiceCollection services)
        {
            ...
            services.AddScoped<QuizService, QuizService>();

        }

页面模型：

public readonly QuizService quizService;
        public QuizModel(QuizService q){
            quizService = q;
        }

public void OnGet()
        {

            QuestionList = quizService.GetQuestions();
        }