在 PageFilter 中为 ViewData 赋值时出错

Question

我正在开发一个 PageFilter，我可以在每个页面上调用它来为我的 _Layout 页面获取一些数据：

public class GetEventFilter : IAsyncPageFilter
{
    private readonly DBContext _context;

    public GetEventFilter(DBContext context)
    {
        _context = context;
    }

    public async Task OnPageHandlerExecutionAsync(PageHandlerExecutingContext context, PageHandlerExecutionDelegate next)
    {

        if (context.HandlerInstance is PageModel result)
        {

            //result.Response.StatusCode = 400;
            context.Result = result.Page();

            DateTime dt = DateTime.Now;

            var thisEvent = await _context.Events.Include(x => x.AdminCompany).Where(x => x.Domain == context.HttpContext.Request.Host.Host && x.Status == 1 && x.ActiveFromDateTime <= dt && x.ActiveToDateTime > dt).OrderBy(x => x.ActiveFromDateTime).FirstOrDefaultAsync();

            if (thisEvent != null)
            {
                if (thisEvent.Logo != null)
                {
                    result.ViewData["Site_EventLogo"] = "/data/" + thisEvent.AdminCompany.Id + "/" + thisEvent.Id + "/media/" + thisEvent.Logo;
                }
                else
                {
                    result.ViewData["Site_EventLogo"] = "/data/" + thisEvent.AdminCompany.Id + "/_system/media/" + thisEvent.AdminCompany.Logo;
                }
                result.ViewData["Site_EventId"] = thisEvent.Id.ToString();
                result.ViewData["Site_EventName"] = thisEvent.Name;
                result.ViewData["Site_EventDomain"] = thisEvent.Domain;
                result.ViewData["Site_CompanyId"] = thisEvent.AdminCompany.Id;
                result.ViewData["Site_CompanyName"] = thisEvent.AdminCompany.Name;
                result.ViewData["Site_CompanyAddress"] = thisEvent.AdminCompany.Address;
                result.ViewData["Site_CompanyPostalCode"] = thisEvent.AdminCompany.PostalCode;
                result.ViewData["Site_CompanyCity"] = thisEvent.AdminCompany.City;
                result.ViewData["Site_CompanyPhone"] = thisEvent.AdminCompany.Phone;
                result.ViewData["Site_CompanyEmail"] = thisEvent.AdminCompany.Email;
                //await next.Invoke();
            }
            else
            {
                context.HttpContext.Response.Redirect("/error/404");
            }

        }

    }

    public async Task OnPageHandlerSelectionAsync(PageHandlerSelectedContext context) => await Task.CompletedTask;

}

我的问题是，当我为 ViewData 对象赋值时，我得到一个NullReferenceException: Object reference not set to an instance of an object.-error。

我该如何解决这个问题？我确实知道 NullReferenceException 是什么，只是不知道为什么我在这个例子中得到它。

Answer 1

在讨论了这个问题之后，第一步是弄清楚为什么PageResult总是返回null。找到了这个问题的答案here。解决第一个问题后，发现在调用await next.Invoke();时遇到如下错误：If an IAsyncPageFilter provides a result value by setting the Result property of PageHandlerExecutingContext to a non-null value, then it cannot call the next filter by invoking PageHandlerExecutionDelegate。

为解决此错误，contex 结果的分配：context.Result = result.Page(); 被删除，然后再次调用该任务，从而解决了整个问题。