错误：'值不能为空。参数名称：上下文' ASP.NET MVC

Question

我正在尝试从依赖注入中访问 _context，但是我收到了错误消息：

'值不能为空。参数名称：context'

我在AdminController.cs中有以下内容：

using Microsoft.AspNet.Identity;
using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Security.Cryptography;
using System.Web;
using System.Web.Mvc;
using Microsoft.AspNet.Identity.EntityFramework;
using System.Threading.Tasks;

namespace GuildCars.UI.Controllers
{
    public class AdminController : Controller
    {
        private readonly ApplicationDbContext _context;
        public AdminController(ApplicationDbContext context)
        {
            _context = context;
        }
        public AdminController() { }    
...

public ActionResult EditUser(string id)
        {
            var userMgr = new UserManager<ApplicationUser>(new UserStore<ApplicationUser>(_context)); //fail to get _context.
            var appUser = userMgr.FindById(id);
            //var appUser = userMgr.FindByEmail(email);

            var user = new UserEditViewModel
            {
                UserID = appUser.Id,
                FirstName = appUser.FirstName,
                LastName = appUser.LastName,
                Email = appUser.Email,
                Role = appUser.Role
            };


            return View(user);
        }  

我上面的依赖注入不起作用，但是如果我使用以下代码，它可以与 using 语句一起使用：

using (var ctx = new ApplicationDbContext())
                    {
                        ctx.Cars.Add(model.Car);

                        if (model.Car == null)
                            model.Car = new Car();

                        ctx.SaveChanges();
                    }

我检查了我的Startup.Auth.cs，确实有ApplicationDbContext.Create：

public void ConfigureAuth(IAppBuilder app)
        {
            // Configure the db context, user manager and signin manager to use a single instance per request
            app.CreatePerOwinContext(ApplicationDbContext.Create);
            app.CreatePerOwinContext<ApplicationUserManager>(ApplicationUserManager.Create);
            app.CreatePerOwinContext<ApplicationSignInManager>(ApplicationSignInManager.Create);
...

另外，我检查了 IdentityModels.cs 中是否有 Create() 方法，并且确实有。

public class ApplicationDbContext : IdentityDbContext<ApplicationUser>
    {
        public ApplicationDbContext()
            : base("DefaultConnection", throwIfV1Schema: false)
        {
        }

        public static ApplicationDbContext Create()
        {
            return new ApplicationDbContext();
        }

        public DbSet<Car> Cars { get; set; }
        public DbSet<Transaction> Transactions { get; set; }
        public DbSet<BodyStyle> BodyStyles { get; set; }
        public DbSet<ContactUs> ContactUs { get; set; }
        public DbSet<ExteriorColor> ExteriorColors { get; set; }
        public DbSet<InteriorColor> InteriorColors { get; set; }
        public DbSet<Make> Makes { get; set; }
        public DbSet<Model> Models { get; set; }
        public DbSet<Specials> Specials { get; set; }
        public DbSet<Transmission> Transmissions { get; set; }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            base.OnModelCreating(modelBuilder);

        }


    }

我对在 ASP.NET MVC 中使用依赖注入有点陌生，我的问题可能是什么？

Answer 1

Microsoft 在 ASP.NET Core 1（又名 ASP.NET MVC 6）中引入了 IServiceCollection。它是框架自带的依赖注入库。

在 MVC 6 之前，在 MVC 5 中我们可以使用外部依赖注入库，例如 unity、autofac 或 ninject。

Inline 是一个关于如何在 ASP.NET MVC 5 中使用统一 ioc 容器的示例

• 将 'Unity' Nuget 包安装到您的项目中。

• 将您的自定义类型添加到容器中。

• 为 IDependencyResolver 接口提供统一容器对象的实现，并将其分配给 HttpConfiguration 对象的 DependencyResolver。

   public void ConfigureAuth(IAppBuilder app)
   {
       var config = new HttpConfiguration();
  
       var unityContainer = new UnityContainer();
       unityContainer.RegisterType<ApplicationDbContext>();
       config.DependencyResolver = new UnityResolver(unityContainer);
  
       //More code
  
       app.UseWebApi(config);
   }
  
   public class UnityResolver : IDependencyResolver
   {
       protected IUnityContainer container;
  
       public UnityResolver(IUnityContainer container)
       {
           if (container == null)
           {
               throw new ArgumentNullException("container");
           }
           this.container = container;
       }
  
       public object GetService(Type serviceType)
       {
           try
           {
               return container.Resolve(serviceType);
           }
           catch (ResolutionFailedException)
           {
               return null;
           }
       }
  
       public IEnumerable<object> GetServices(Type serviceType)
       {
           try
           {
               return container.ResolveAll(serviceType);
           }
           catch (ResolutionFailedException)
           {
               return new List<object>();
           }
       }
  
       public IDependencyScope BeginScope()
       {
           var child = container.CreateChildContainer();
           return new UnityResolver(child);
       }
  
       public void Dispose()
       {
           Dispose(true);
       }
  
       protected virtual void Dispose(bool disposing)
       {
           container.Dispose();
       }
   }
  

使用 Unity 容器，您可以摆脱 app.CreatePerOwinContext 和 create 方法。