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[英]Getting NullReferenceException and object reference not set to an instance of an object when checking if session is null with ASP.NET 5 and MVC 6
[英]NullReferenceException: Object reference not set to an instance of an object error in ASP.NET MVC
我是 ASP.NET MVC 的新手,我得到了
“NullReferenceException:Object 引用未設置為對象的實例”
嘗試顯示數據庫中的數據時出現異常。 我有這樣連接的Movies
和Reviews
表:
public class Movie
{
public int MovieId { get; set; }
public string Title { get; set; }
[DataType(DataType.Date)]
public DateTime ReleaseDate { get; set; }
public string Genre { get; set; }
public decimal Price { get; set; }
public virtual ICollection<Review> Reviews { get; set; }
}
public class Review
{
#region Review Properties
[Key]
public int ReviewId { get; set; }
[Required(ErrorMessage = "Comment can't be empty.")]
public string Comment { get; set; }
[DataType(DataType.Date)]
[DisplayFormat(DataFormatString = "{0:yyyy-MM-dd}")]
public DateTime Date { get; set; }
#endregion
#region Dependencies
[ForeignKey("Id")]
public int MovieId { get; set; }
public Movie Movie { get; set; }
#endregion
}
上下文文件如下所示:
using Microsoft.EntityFrameworkCore;
using MvcMovie.Models;
namespace MvcMovie.Data
{
public class MvcMovieContext : DbContext
{
public MvcMovieContext()
{
}
public MvcMovieContext(DbContextOptions<MvcMovieContext> options)
: base(options)
{
}
public DbSet<Movie> Movies { get; set; }
public DbSet<Review> Reviews { get; set; }
}
}
我需要在詳細信息視圖中顯示每部電影的評論,因此我創建了一個可以正常工作的評論添加部分(評論已添加到數據庫中)。 現在,當我嘗試顯示當前電影的評論時,我在 View 文件中使用了以下代碼:
@foreach (Review review in Model.Reviews)
{
<div class="container">
<div class="row">
<div class="col-md-2"></div>
<div class="col-md-8">
<div>
<p>@review.Comment</p>
<small class="pull-right">@review.Date</small>
</div>
</div>
</div>
</div>
<hr />
}
另外我應該提到,視圖使用這個 model: @model MvcMovie.Models.Movie
和Reviews controller
看起來像這樣:
public class ReviewsController : Controller
{
private readonly MvcMovieContext _context = new MvcMovieContext();
public ReviewsController(MvcMovieContext context)
{
_context = context;
}
public ActionResult New(Review review)
{
if (ModelState.IsValid)
{
try
{
review.Date = DateTime.Now;
_context.Reviews.Add(review);
_context.SaveChanges();
TempData["message"] = "Your review was added.";
}
catch (Exception)
{
}
}
return Redirect("/Movies/Details/" + review.MovieId.ToString());
}
}
Movies controller(僅將數據傳遞到視圖的詳細信息部分)如下所示:
namespace MvcMovie.Controllers
{
public class MoviesController : Controller
{
private readonly MvcMovieContext _context;
public MoviesController(MvcMovieContext context)
{
_context = context;
}
public async Task<IActionResult> Details(int? id)
{
if (id == null)
{
return NotFound();
}
var movie = await _context.Movies
.FirstOrDefaultAsync(m => m.MovieId == id);
if (movie == null)
{
return NotFound();
}
return View(movie);
}
}
}
異常出現在@foreach (Review review in Model.Reviews)
行上,所以我猜Model.Reviews
不知何故NULL
不知道為什么會發生這種情況,但表不知道為什么會發生這種情況,但表 I 對此有何建議?
嘗試修復您的詳細信息操作:
var movie = await _context.Movies.Include(r=>r.Reviews)
.FirstOrDefaultAsync(m => m.MovieId == id);
.....
並為防止將來出現此錯誤,請修復視圖代碼
@if ( Model.Reviews!=null)
{
@foreach (Review review in Model.Reviews)
{
....
}
}
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