ASP.NET Core返回System.NullReferenceException

Question

当我想在Detail View.cs中显示有关书籍的详细信息时，我会收到此类消息 System.NullReferenceException：'对象引用未设置为对象的实例。' LibaryData.Models.LibaryAsset.Status.get返回null。

我重新填写了应用程序，似乎在我的模型名为LibaryAssets.cs的字段中调用Status有一个数据类型状态

[Required]
public Status  Status { get; set; }

到目前为止我的代码：

LibaryAsset.cs

    using System;
    using System.Collections.Generic;
    using System.Text;
    using System.ComponentModel.DataAnnotations;

    namespace LibaryData.Models
    {
        public abstract class LibaryAsset
        {
            public int Id { get; set; }

            [Required]
            public string Title { get; set; }

            [Required]
            public int Year { get; set; }

            [Required]
            public Status  Status { get; set; }

            [Required]
            public decimal Cost { get; set; }

            public int NumberOfCopies { get; set; }

            public virtual LibaryBranch Location { get; set; }


            public string ImageUrl { get; set; }

        }

    }


**CatalogController.cs**



using LibaryData;
using LibaryData.Models;
using LibaryManagmentSystems.Models.Catalog;
using Microsoft.AspNetCore.Mvc;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace LibaryManagmentSystems.Controllers
{
    public class CatalogController : Controller
    {
        private ILibaryAsset _assets;
        private ICheckout _checkouts;

        public CatalogController(ILibaryAsset assets, ICheckout checkout)
        {
            _assets = assets;
            _checkouts = checkout;
        }

        public IActionResult Index()
        {
            var assetModels = _assets.GetAll();

            var listingResult = assetModels
                .Select(result => new AssetIndexListingModel
                {
                    Id =result.Id,
                    ImageUrl = result.ImageUrl,
                    AuthorOrDirector = _assets.GetAuthorOrDirector(result.Id),
                    DeweyCallNumber = _assets.GetDeweyIndex(result.Id),
                    Title = result.Title,
                    Type = _assets.GetType(result.Id)
                });
            var model = new AssetIndexModel()
            {
                Assets = listingResult
            };
            return View(model);
        }

        public IActionResult Detail(int id)
        {
            var asset = _assets.GetById(id);
            var currentHolds = _checkouts.GetCurrentHolds(id)
                .Select(a => new AssetHoldModel
                {
                    HoldPlace = _checkouts.GetCurrentHoldPlaced(a.Id).ToString("d"),
                    PatronName = _checkouts.GetCurrentHoldPatronName(a.Id)



                });


            var model = new AssetDetailModel
            {
                AssetID = id,
                Title = asset.Title,
                Type = _assets.GetType(id),
                Year = asset.Year,
                Cost = asset.Cost,
                Status = asset.Status.Name,
                ImageURL = asset.ImageUrl,
                AuthorOrDirector = _assets.GetAuthorOrDirector(id),
                CurrentLocation = _assets.GetCurrentLocation(id).Name,
                DeweyCallNumber = _assets.GetDeweyIndex(id),
                CheckoutHistory = _checkouts.GetCheckOutHistory(id),
                ISBN = _assets.GetIsbn(id),
                LatestChechout = _checkouts.GetLatesCheckout(id),
                PatronName = _checkouts.GetCurrentCheckoutPatron(id),
                CurrentHolds = currentHolds

            };



            return View(model);


        }
    }
}

所以我在public IActionResult Detail(int id)

中得到了例外

 var model = new AssetDetailModel
            {
                AssetID = id,
                Title = asset.Title,
                Type = _assets.GetType(id),
                Year = asset.Year,
                Cost = asset.Cost,
                Status = asset.Status.Name,
                ImageURL = asset.ImageUrl,
                AuthorOrDirector = _assets.GetAuthorOrDirector(id),
                CurrentLocation = _assets.GetCurrentLocation(id).Name,
                DeweyCallNumber = _assets.GetDeweyIndex(id),
                CheckoutHistory = _checkouts.GetCheckOutHistory(id),
                ISBN = _assets.GetIsbn(id),
                LatestChechout = _checkouts.GetLatesCheckout(id),
                PatronName = _checkouts.GetCurrentCheckoutPatron(id),
                CurrentHolds = currentHolds

            };

有任何建议，评论？

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