MVC5/EF6:无法删除对象，因为在ObjectStateManager中找不到该对象

我有以下HttpPost Delete((方法，该方法在我的MVC5应用程序中运行。据我所知，与此控制器、视图甚至模型相关的内容均未更改。

    // POST: Admin/UserManagement/Delete/5
    [HttpPost, ActionName("DeleteConfirmed")]
    [ValidateAntiForgeryToken]
    public async Task<ActionResult> DeleteConfirmed(string id)
    {
        ApplicationUser applicationUser = db.Users.Find(id);
        if (applicationUser == null)
        {
            ModelState.AddModelError("", "Failed to find User ID for deletion.");
        }
        else
        {
            IdentityResult result = await UserManager.DeleteAsync(applicationUser);
            if (result.Succeeded)
            {
                await db.SaveChangesAsync();
                return RedirectToAction("Index", "UserManagement");
            }
            else
            {
                ModelState.AddModelError("", "Failed to Delete User.");
                var errors = string.Join(",", result.Errors);
                ModelState.AddModelError("", errors);
            }
        }
        return View(applicationUser);
    }

UserManager工件：

[CustomAuthorization(myRoles = "Admin")]
public class UserManagementController : Controller
{
    protected ApplicationDbContext db { get; set; }
    private ApplicationUserManager _userManager;
    public UserManagementController()
    {
        this.db = new ApplicationDbContext();
    }
    public UserManagementController(ApplicationUserManager userManager)
    {
        UserManager = userManager;
    }
    public ApplicationUserManager UserManager
{
        get
        {
            return _userManager ?? HttpContext.GetOwinContext().GetUserManager<ApplicationUserManager>();
        }
        private set
        {
            _userManager = value;
        }
    }

当我的代码到达IdentityResult result = await UserManager.DeleteAsync(applicationUser)时，它立即跳到下面的Dispose((方法，并且没有加载索引视图，而是给出：Server Error in '/' Application. The object cannot be deleted because it was not found in the ObjectStateManager.

    protected override void Dispose(bool disposing)
    {
        if (disposing)
        {
            db.Dispose();
            if (UserManager != null)
            {
                UserManager.Dispose();
                UserManager = null;
            }
        }
        base.Dispose(disposing);
    }

有人能告诉我哪里出了问题吗？我以前从未见过这种错误。以前，这个DeleteConfirmed((代码完全按照预期工作。

编辑：

启动.cs：

using Microsoft.Owin;
using Owin;
[assembly: OwinStartupAttribute(typeof(PROJECT.Startup))]
namespace PROJECT
{
    public partial class Startup
    {
        public void Configuration(IAppBuilder app)
        {
            ConfigureAuth(app);
        }
    }
}

EDIT2：

CustomAuthorization.cs(Helper(:

public class CustomAuthorization : AuthorizeAttribute
{
    public string myRoles { get; set; }
    public override void OnAuthorization(AuthorizationContext filterContext)
    {
        var userAuthInfo = HttpContext.Current.User;
        if (userAuthInfo != null)
        {
            if (!userAuthInfo.Identity.IsAuthenticated)
            {
                string returnUrl = filterContext.HttpContext.Request.RawUrl;
                filterContext.Result = new RedirectResult("/Account/Login?returnUrl=returnUrl");
                return;
            }
            string[] roles = myRoles.Split(',');

            var userAuth = false;
            foreach (string role in roles)
            {
                if (userAuthInfo.IsInRole(role))
                {
                    userAuth = true;
                    break;
                }
            }
            if (!userAuth)
            {
                var result = new RedirectResult("/Home?auth=0");
                filterContext.Result = result;
            }
        }
    }
}

EDIT3

启动。/App_Start/中的Auth.cs：

public partial class Startup
{
    // For more information on configuring authentication, please visit http://go.microsoft.com/fwlink/?LinkId=301864
    public void ConfigureAuth(IAppBuilder app)
    {
        // Configure the db context and user manager to use a single instance per request
        app.CreatePerOwinContext(ApplicationDbContext.Create);
        app.CreatePerOwinContext<ApplicationUserManager>(ApplicationUserManager.Create);
        // Enable the application to use a cookie to store information for the signed in user
        // and to use a cookie to temporarily store information about a user logging in with a third party login provider
        // Configure the sign in cookie
        app.UseCookieAuthentication(new CookieAuthenticationOptions
        {
            AuthenticationType = DefaultAuthenticationTypes.ApplicationCookie,
            LoginPath = new PathString("/Account/Login"),
            Provider = new CookieAuthenticationProvider
            {
                OnValidateIdentity = SecurityStampValidator.OnValidateIdentity<ApplicationUserManager, ApplicationUser>(
                    validateInterval: TimeSpan.FromMinutes(30),
                    regenerateIdentity: (manager, user) => user.GenerateUserIdentityAsync(manager))
            }
        });
        app.UseExternalSignInCookie(DefaultAuthenticationTypes.ExternalCookie);
    }
}

解决方案正如haim770所指出的，我在DeleteConfirmed()操作方法中错误地使用了2个DbContext引用。

将ApplicationUser applicationUser = db.Users.Find(id)更改为async，一切正常：ApplicationUser applicationUser = await UserManager.FindByIdAsync(id);

感谢所有花时间提供帮助的人！

public UserManagementController()
{
    this.db = context.Get<ApplicationDbContext>();
}