在应用程序运行期间发生DropDownList错误

我在DropDownList上遇到了问题。当我通过免费的主机运行应用程序时，我遇到了以下错误信息：

错误：已经有一个与此命令相关联的打开的DataReader，必须先关闭它。

错误来源信息如下：

第51行：@Html.DropDownList("ScreeningId", null, htmlAttributes: new { @class = "form-control" })

我额外添加了“View”和“Controller”文件：

public class ReservationsController : Controller

{

private AppDbContext db = new AppDbContext();

// GET: Reservations

public ActionResult Index()

{

return View(db.Reservations.ToList());

}

// GET: Reservations/Create

public ActionResult Create()

{

ViewBag.ScreeningId = new SelectList(db.Screenings, "Id", "Description");

HashSet seats = new HashSet(db.Reservations.Select(x => x.SeatNumber));

ViewBag.Seats = seats;

return View();

}

// POST: Reservations/Create

[HttpPost]

[ValidateAntiForgeryToken]

public ActionResult Create([Bind(Include = "Id,FirstName,SecondName,Phone,SeatNumber,ScreeningId")] Reservation reservation)

{

ViewBag.ScreeningId = new SelectList(db.Screenings, "Id", "Description");

if (ModelState.IsValid)

{

// 检查座位是否已经被预订

if (db.Reservations.Select(x => x.SeatNumber).Contains(reservation.SeatNumber))

{

return View(reservation);

}

else

{

db.Reservations.Add(reservation);

db.SaveChanges();

}

if (Session["Login"] != null)

{

return RedirectToAction("Index");

}

return RedirectToAction("Success");

}

return View(reservation);

}

protected override void Dispose(bool disposing)

{

if (disposing)

{

db.Dispose();

}

base.Dispose(disposing);

}

}

部分View文件内容如下：

@Html.LabelFor(model => model.Screening, "Seans", htmlAttributes: new { @class = "control-label col-md-2" })

@Html.DropDownList("ScreeningId", null, htmlAttributes: new { @class = "form-control" })

@Html.ValidationMessageFor(model => model.Screening, "", new { @class = "text-danger" })

[InvalidOperationException: 已经有一个与此命令相关联的打开的DataReader，必须先关闭它。]

堆栈跟踪：

System.Data.SqlClient.SqlInternalConnectionTds.ValidateConnectionForExecute(SqlCommand command) +1545589

System.Data.SqlClient.SqlConnection.ValidateConnectionForExecute(String method, SqlCommand command) +89

System.Data.SqlClient.SqlCommand.ValidateCommand(String method, Boolean async) +268

System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method, TaskCompletionSource`1 completion, Int32 timeout, Task& task, Boolean& usedCache, Boolean asyncWrite, Boolean inRetry) +118

System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method) +64

System.Data.SqlClient.SqlCommand.ExecuteReader(CommandBehavior behavior, String method) +161

System.Data.SqlClient.SqlCommand.ExecuteDbDataReader(CommandBehavior behavior) +41

System.Data.Common.DbCommand.ExecuteReader(CommandBehavior behavior) +12

System.Data.Entity.Infrastructure.Interception.DbCommandDispatcher.b__c(DbCommand t, DbCommandInterceptionContext`1 c) +9

System.Data.Entity.Infrastructure.Interception.InternalDispatcher`1.Dispatch(TTarget target, Func`3 operation, TInterceptionContext interceptionContext, Action`3 executing, Action`3 executed) +72

System.Data.Entity.Infrastructure.Interception.DbCommandDispatcher.Reader(DbCommand command, DbCommandInterceptionContext interceptionContext) +356

System.Data.Entity.Internal.InterceptableDbCommand.ExecuteDbDataReader(CommandBehavior behavior) +166

System.Data.Common.DbCommand.ExecuteReader(CommandBehavior behavior) +12

System.Data.Entity.Core.EntityClient.Internal.EntityCommandDefinition.ExecuteStoreCommands(EntityCommand entityCommand, CommandBehavior behavior) +37

[EntityCommandExecutionException: 在执行命令定义时发生错误。有关详细信息，请参阅内部异常。]

System.Data.Entity.Core.EntityClient.Internal.EntityCommandDefinition.ExecuteStoreCommands(EntityCommand entityCommand, CommandBehavior behavior) +112

System.Data.Entity.Core.Objects.Internal.ObjectQueryExecutionPlan.Execute(ObjectContext context, ObjectParameterCollection parameterValues) +744

System.Data.Entity.Core.Objects.<>c__DisplayClass7.b__6() +97

System.Data.Entity.Core.Objects.ObjectContext.ExecuteInTransaction(Func`1 func, IDbExecutionStrategy executionStrategy, Boolean startLocalTransaction, Boolean releaseConnectionOnSuccess) +288

System.Data.Entity.Core.Objects.<>c__DisplayClass7.b__5() +154

System.Data.Entity.SqlServer.DefaultSqlExecutionStrategy.Execute(Func`1 operation) +189

System.Data.Entity.Core.Objects.ObjectQuery`1.GetResults(Nullable`1 forMergeOption) +279

System.Data.Entity.Core.Objects.ObjectQuery`1.Execute(MergeOption mergeOption) +31

System.Data.Entity.Core.Objects.DataClasses.EntityReference`1.Load(MergeOption mergeOption) +139

System.Data.Entity.Core.Objects.DataClasses.RelatedEnd.DeferredLoad() +333

System.Data.Entity.Core.Objects.Internal.LazyLoadBehavior.LoadProperty(TItem propertyValue, String relationshipName, String targetRoleName, Boolean mustBeNull, Object wrapperObject) +87

System.Data.Entity.Core.Objects.Internal.<>c__DisplayClass7`2.b__2(TProxy proxy, TItem item) +106

System.Data.Entity.DynamicProxies.Screening_333144174B6E44255ACF2CFB15F4499A880DB4ECDBA71E01178031C116125BB8.get_Film() +23

CinemaTicketReservation.Models.Screening.get_Description() +34

[TargetInvocationException: 对象“System.Data.Entity.DynamicProxies.Screening_333144174B6E44255ACF2CFB15F4499A880DB4ECDBA71E01178031C116125BB8”的属性访问器“Description”引发了以下异常：“在执行命令定义时发生错误。有关详细信息，请参阅内部异常。”]

System.ComponentModel.ReflectPropertyDescriptor.GetValue(Object component) +378

System.Web.UI.DataBinder.GetPropertyValue(Object container, String propName) +87

System.Web.UI.DataBinder.Eval(Object container, String[] expressionParts) +86

System.Web.UI.DataBinder.Eval(Object container, String expression) +95

System.Web.Mvc.MultiSelectList.Eval(Object container, String expression) +35

System.Web.Mvc.<>c__DisplayClass2.b__1(Object item) +86

System.Linq.WhereSelectEnumerableIterator`2.MoveNext() +164

System.Collections.Generic.List`1..ctor(IEnumerable`1 collection) +387

System.Linq.Enumerable.ToList(IEnumerable`1 source) +58

System.Web.Mvc.MultiSelectList.GetListItemsWithValueField() +145

System.Web.Mvc.MultiSelectList.GetListItems() +44

System.Web.Mvc.MultiSelectList.GetEnumerator() +8

System.Collections.Generic.List`1..ctor(IEnumerable`1 collection) +375

System.Linq.Enumerable.ToList(IEnumerable`1 source) +58

CinemaTicketReservation.Controllers.ReservationsController.Create() +248

lambda_method(Closure , ControllerBase , Object[] ) +62

System.Web.Mvc.ActionMethodDispatcher.Execute(ControllerBase controller, Object[] parameters) +14

System.Web.Mvc.ReflectedActionDescriptor.Execute(ControllerContext controllerContext, IDictionary`2 parameters) +157

System.Web.Mvc.ControllerActionInvoker.InvokeActionMethod(ControllerContext controllerContext, ActionDescriptor actionDescriptor, IDictionary`2 parameters) +27

System.Web.Mvc.Async.AsyncControllerActionInvoker.b__39(IAsyncResult asyncResult, ActionInvocation innerInvokeState) +22

System.Web.Mvc.Async.WrappedAsyncResult`2.CallEndDelegate(IAsyncResult asyncResult) +29

System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +49

System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeActionMethod(IAsyncResult asyncResult) +32

System.Web.Mvc.Async.AsyncInvocationWithFilters.b__3d() +50

System.Web.Mvc.Async.<>c__DisplayClass46.b__3f() +228

System.Web.Mvc.Async.<>c__DisplayClass33.b__32(IAsyncResult asyncResult) +10

System.Web.Mvc.Async.WrappedAsyncResult`1.CallEndDelegate(IAsyncResult asyncResult) +10

System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +49

System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeActionMethodWithFilters(IAsyncResult asyncResult) +34

System.Web.Mvc.Async.<>c__DisplayClass2b.b__1c() +26

System.Web.Mvc.Async.<>c__DisplayClass21.b__1e(IAsyncResult asyncResult) +100

System.Web.Mvc.Async.WrappedAsyncResult`1.CallEndDelegate(IAsyncResult asyncResult) +10

System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +49

System.Web.Mvc.Controller.b__1d(IAsyncResult asyncResult, ExecuteCoreState innerState) +13

System.Web.Mvc.Async.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult) +29

System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +49

System.Web.Mvc.Controller.EndExecuteCore(IAsyncResult asyncResult) +36

System.Web.Mvc.Controller.b__15(IAsyncResult asyncResult, Controller controller) +12

System.Web.Mvc.Async.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult) +29

System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +49

System.Web.Mvc.Controller.EndExecute(IAsyncResult asyncResult) +26

System.Web.Mvc.Controller.System.Web.Mvc.Async.IAsyncController.EndExecute(IAsyncResult asyncResult) +10

System.Web.Mvc.MvcHandler.b__5(IAsyncResult asyncResult, ProcessRequestState innerState) +21

System.Web.Mvc.Async.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult) +29

System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +49

System.Web.Mvc.MvcHandler.EndProcessRequest(IAsyncResult asyncResult) +28

System.Web.Mvc.MvcHandler.System.Web.IHttpAsyncHandler.EndProcessRequest(IAsyncResult result) +9

System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +9987157

System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +155

我在一些主题中看到，这里需要使用MARS（MultipleActiveResultSets=true）。我已经添加了它，但仍然无法工作。您有什么想法吗？

这个主题参考了：

dropdownlist looks OK but not working [duplicate]