UserManager.Create:mscorlib.dll中发生类型'System.MissingMethodException'的异常，但未在用户代码中处理 - c#

我目前正在Visual Studio Express 2013 for Web中构建网站，并且使用默认的成员资格功能供用户注册。但是，当我在localhost上打开网站并尝试注册用户时，收到以下错误：

An exception of type 'System.MissingMethodException' occurred in mscorlib.dll 
but was not handled in user code

Additional information: Method not found: 'System.Threading.Tasks.Task`1<!!0>
System.Data.Entity.QueryableExtensions.FirstOrDefaultAsync(System.Linq.IQueryable`1<!!0>,
System.Linq.Expressions.Expression`1<System.Func`2<!!0,Boolean>>)'.

这是我的浏览器中的堆栈跟踪：

[MissingMethodException: Method not found: 'System.Threading.Tasks.Task`1<!!0> System.Data.Entity.QueryableExtensions.FirstOrDefaultAsync(System.Linq.IQueryable`1<!!0>, System.Linq.Expressions.Expression`1<System.Func`2<!!0,Boolean>>)'.]
   Microsoft.AspNet.Identity.EntityFramework.UserStore`6.GetUserAggregateAsync(Expression`1 filter) +0
   Microsoft.AspNet.Identity.EntityFramework.UserStore`6.FindByNameAsync(String userName) +1071
   Microsoft.AspNet.Identity.UserManager`2.FindByNameAsync(String userName) +177
   Microsoft.AspNet.Identity.<ValidateUserName>d__4.MoveNext() +803
   System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) +93
   System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) +52
   System.Runtime.CompilerServices.TaskAwaiter.GetResult() +21
   Microsoft.AspNet.Identity.<ValidateAsync>d__0.MoveNext() +468
   System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) +93
   System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) +52
   System.Runtime.CompilerServices.TaskAwaiter`1.GetResult() +24
   Microsoft.AspNet.Identity.CultureAwaiter`1.GetResult() +123
   Microsoft.AspNet.Identity.<CreateAsync>d__0.MoveNext() +795
   System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) +93
   System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) +52
   System.Runtime.CompilerServices.TaskAwaiter`1.GetResult() +24
   Microsoft.AspNet.Identity.CultureAwaiter`1.GetResult() +123
   Microsoft.AspNet.Identity.<CreateAsync>d__d.MoveNext() +970
   System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) +93
   System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) +52
   System.Runtime.CompilerServices.TaskAwaiter`1.GetResult() +24
   Microsoft.AspNet.Identity.AsyncHelper.RunSync(Func`1 func) +409
   Microsoft.AspNet.Identity.UserManagerExtensions.Create(UserManager`2 manager, TUser user, String password) +260
   Account_Register.CreateUser_Click(Object sender, EventArgs e) in c:\Users\Matthew\Documents\College\Senior\CSE 448\Capstone\Capstone\Account\Register.aspx.cs:14
   System.Web.UI.WebControls.Button.OnClick(EventArgs e) +9628026
   System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument) +103
   System.Web.UI.WebControls.Button.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) +10
   System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) +13
   System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) +35
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1724

引发异常的代码用星号标记：

protected void CreateUser_Click(object sender, EventArgs e)
    {
        var manager = new UserManager();
        var user = new ApplicationUser() { UserName = UserName.Text };

        *****IdentityResult result = manager.Create(user, Password.Text);
        if (result.Succeeded)
        {
            IdentityHelper.SignIn(manager, user, isPersistent: false);
            IdentityHelper.RedirectToReturnUrl(Request.QueryString["ReturnUrl"], Response);
        }
        else
        {
            ErrorMessage.Text = result.Errors.FirstOrDefault();
        }
    }

我已经搜索了一段时间，但似乎找不到解决方案。我尝试更新所有参考，但无济于事。有人遇到过这种情况么？

参考方案

得到它了！ FirstOrDefaultAsync扩展方法仅存在于.Net 4.5实体框架程序集中。在不同的项目中，我混合使用了.Net 4.0和.Net 4.5。当我将它们全部更改为.Net 4.5时，可以使用Async代码。

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