无法解析“ Data.ApplicationDbContext”类型的服务

Question

将数据库上下文注入控制器时遇到问题。

在我的startup.cs文件中，我这样定义上下文；

services.AddDbContext<ApplicationDbContext>(options =>                options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));

在我的控制器中，我试图注入上下文；

namespace AdminPortal.Controllers
{
    [Route("AdminUser")]
    [Authorize]
    public class AdminUserController : Controller
    {
        private ApplicationDbContext _context;

        public AdminUserController(ApplicationDbContext context)
        {
            _context = context;
        }

        // GET
        [Route("")]
        public IActionResult Index()
        {
            var users = _context.Users.ToList();

            return View(new UserListModel(users));
        }
    }
}

问题是，当我尝试查看路线时遇到以下错误。

InvalidOperationException: Unable to resolve service for type 'Data.ApplicationDbContext' while attempting to activate 'AdminPortal.Controllers.AdminUserController'.
Microsoft.Extensions.DependencyInjection.ActivatorUtilities.GetService(IServiceProvider sp, Type type, Type requiredBy, bool isDefaultParameterRequired)
lambda_method(Closure , IServiceProvider , object[] )
Microsoft.AspNetCore.Mvc.Controllers.ControllerActivatorProvider+<>c__DisplayClass4_0.<CreateActivator>b__0(ControllerContext controllerContext)
Microsoft.AspNetCore.Mvc.Controllers.ControllerFactoryProvider+<>c__DisplayClass5_0.<CreateControllerFactory>g__CreateController|0(ControllerContext controllerContext)
Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.Next(ref State next, ref Scope scope, ref object state, ref bool isCompleted)
Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.InvokeInnerFilterAsync()
Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.InvokeNextResourceFilter()
Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.Rethrow(ResourceExecutedContext context)
Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.Next(ref State next, ref Scope scope, ref object state, ref bool isCompleted)
Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.InvokeFilterPipelineAsync()
Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.InvokeAsync()
Microsoft.AspNetCore.Routing.EndpointMiddleware.Invoke(HttpContext httpContext)
Microsoft.AspNetCore.Routing.EndpointRoutingMiddleware.Invoke(HttpContext httpContext)
Microsoft.AspNetCore.Authentication.AuthenticationMiddleware.Invoke(HttpContext context)
Microsoft.AspNetCore.StaticFiles.StaticFileMiddleware.Invoke(HttpContext context)
Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore.MigrationsEndPointMiddleware.Invoke(HttpContext context)
Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore.DatabaseErrorPageMiddleware.Invoke(HttpContext httpContext)
Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore.DatabaseErrorPageMiddleware.Invoke(HttpContext httpContext)
Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)

最后是db上下文；

namespace AdminPortal.Data
{
    public class ApplicationDbContext : IdentityDbContext
    {
        public ApplicationDbContext(DbContextOptions<ApplicationDbContext> options)
            : base(options)
        {
        }       

    }
}

我正在使用身份，并且似乎可以使用上下文。