如何在验证输入时解决错误？

Question

我正在尝试验证用户提供给用户历史记录的输入，并根据验证结果显示消息。我一直收到System.NullReferenceException：错误，尽管对象引用设置为对象的实例。

我正在处理剃须刀页面。这是C＃代码：

public class ConfirmModel : PageModel
{
    public CUser User { get; set; }

    private readonly IHostingEnvironment _env;
    private readonly CDataContext _context;
    private readonly IUserService _UserService;

    public ConfirmModel(IUserService UserService, CDataContext context, IHostingEnvironment IHostingEnvironment)
    {
        _env = IHostingEnvironment;
        _context = context;
        _UserService = UserService;

        User = UserService.GetUser();
    }

    public IList<CPreference> Pref { get; set; }
    public IList<CHistory> Hist { get; set; }
    public IList<COffers> Off { get; set; }
    public IList<CStudent> student{ get; set; }

    public ActionResult OnGet()
    {
        if (User.Role != Role.Admin && User.Role != Role.Student)
        {
            return RedirectToPage("/Denied");
        }
        Hist = _context.History.ToList();
        Off = _context.Offers.ToList();
        Pref = _context.Preference.ToList();
        student = _context.Student.ToList();
        checkHistory();

        return Page();
    }

    public bool NotInHistory(Courscategory cocat, CStudent stud)
    {
        bool var_ = false;
        foreach (var item in stud.courshistory)
        {
            if(item == cocat)
            {
                var_ = true;
            }
        }
        return var_;
    }

    public void checkHistory()
    {
        foreach(var item in student)
        {
            while(!item.is_accepted)
            {
                foreach(var hist_item in Pref)
                {
                    if(!NotInHistory(hist_item.Category_One, item) && !NotInHistory(hist_item.Category_two, item) && !NotInHistory(hist_item.Category_Three, item))
                    {
                        General.AddPreference = false;
                    }
                    else
                    {
                        General.AddPreference = true;
                    }
                }
            }
        }
    }

    public async Task<IActionResult> OnPostAsync()
    {
        return RedirectToPage("IndexPref");
    }
}

相应的HTML代码：

@page
@model ConfirmModel
@{
    ViewData["Title"] = "Confirm";
}

    @if(!General.AddPreference)
    {
        <div>
            <h4>Invalid go back</h4>
        </div>
    }

    @if(!General.AddPreference)
    {
        <div>
            <form enctype="multipart/form-data" id="orderForm" method="post">
                <div class="Custom-Form col-md-12">
                    <input type="submit" value="Hinzufügen" class="Custom-Button" />
                </div>
            </form>
        </div>
    }
</div>

General是可用于任何类访问的常规类。看起来像这样：

public static bool GAddPreference = true;

public static bool AddPreference
{
    get { return GAddPreference; }
    set { GAddPreference = value; }
}

我是C＃的新手，在某个地方我显然错了。我希望有人能帮助我。