SelectList 的问题 - NullReferenceException

Question

在为 chinook 数据库创建曲目时，我设置了一个 SelectList 来选择专辑。按照 Microsoft 文档的说明，出于某种原因（我看不到，因为我仍在努力学习），它不喜欢我在剃刀页面上的视图。演练的代码如下。

实体：

public class Album
{        
    public int AlbumId { get; set; }
    public string Title { get; set; }
    public int ArtistId { get; set; }

    //related entities
    public virtual ICollection<Track> Tracks { get; set; }
    public virtual Artist Artist { get; set; }

}

public class Track
{
    public int TrackId { get; set; }
    public string Name { get; set; }
    public int AlbumId { get; set; }
    public string Composer { get; set; }
    public int Milliseconds { get; set; }
    public int Bytes { get; set; }
    public double UnitPrice { get; set; }
    public int MediaTypeId { get; set; }
    public int GenreId { get; set; }

    //related entities

    public virtual Album Album { get; set; }

}

为 SelectedList 创建模型：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.RazorPages;
using Microsoft.AspNetCore.Mvc.Rendering;
using UWS.Project;

namespace Project.Pages.Albums
{
     public class AlbumPageModel : PageModel
     { 
        public SelectList AlbumNameSL { get; set; }

        public void PopulateAlbumsDropDownList(Chinook db,
        object selectedAlbum = null)
        {
           var albumsQuery = from a in db.Albums
                                  orderby a.Title // Sort by name.
                                  select a;

           AlbumNameSL = new SelectList(albumsQuery,
                       "AlbumId", "Title", selectedAlbum);
        }
     }
 }

现在是我创建轨道的模型：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.RazorPages;
using Microsoft.AspNetCore.Mvc.Rendering;
using UWS.Project;

namespace Project.Pages.Albums
{
    public class TrackCreateModel : AlbumPageModel
    {
        private Chinook db;
        public TrackCreateModel(Chinook injectedContext)
        {
            db = injectedContext;
        }

        public IActionResult OnGet(int? id)
        {
            PopulateAlbumsDropDownList(db);
            return Page();
        }

        [BindProperty]
        public Track Track { get; set; }

        public async Task<IActionResult> OnPostAsync()
        {
            var emptyTrack = new Track();

            if (await TryUpdateModelAsync<Track>(
                 emptyTrack,
                "track",   // Prefix for form value.
                 s => s.AlbumId, s => s.Name, s => s.Composer, s => s.Milliseconds, s => s.Bytes, s 
=> s.MediaTypeId, s => s.GenreId, s => s.UnitPrice))
            {
                db.Tracks.Add(emptyTrack);
                await db.SaveChangesAsync();
                return RedirectToPage("./Index");
            }

            // Select AlbumId if TryUpdateModelAsync fails.
            PopulateAlbumsDropDownList(db, emptyTrack.AlbumId);
            return Page();
        }
    }
}

尝试在我看来实现 SelectList：

对此我不熟悉，我一辈子都看不到这个问题。我用 SelectedList 在这里查看了以前的问题，但没有一个与我的问题有关。任何帮助将不胜感激。

Answer 1

不要在AlbumPageModel的PopulateAlbumsDropDownList方法中输入selectedValue

目前您要么将其留空，要么提供一个 id，但这无论如何都行不通