Wrap 'open DataReader' exception with something that says to enable MARS

[kirthik]

DECISION: We will wrap the exception in something that says to use MARS.

If MARS is not enabled in connection string, browser shows the following error. Error message should suggest user to enable MARS to get past the error

An unhandled exception occurred while processing the request. InvalidOperationException: There is already an open DataReader associated with this Command which must be closed first. Remotion.Linq.Clauses.ResultOperatorBase.InvokeExecuteMethod(MethodInfo method, Object input) DataStoreException: An error occured while running a data store operation. See InnerException for details. Microsoft.Data.Entity.Query.EntityQueryExecutor.ExecuteCollection[T](QueryModel queryModel)

[kirthik]

@anpete mentioned that enabling MARS is a requirement now for project k apps (that need DB) so a lot of users will hit this issue.Guiding users in the right direction I think will be very useful here.

[rowanmiller]

@anpete - is required for all queries now, or just certain scenarios?

[anpete]

@rowanmiller Should just be certain queries.

@kirthik Were you running an explicit query? If so, what was it?

[kirthik]

@anpete - I thought you said " It is currently a limitation that we always require MARS". Did I misunderstand?

My scenario - I published music store to azure and created sql azure database. Connection string that is set does have not MARS by default. So on checkout of album in music store app, I saw this error in browser

[anpete]

@kirthik Sorry, I should have said "often require MARS". Are you able to post the stack trace?

[kirthik]

InvalidOperationException: There is already an open DataReader associated with this Command which must be closed first. Remotion.Linq.Clauses.ResultOperatorBase.InvokeExecuteMethod(MethodInfo method, Object input) Remotion.Linq.Clauses.ResultOperatorBase.InvokeGenericExecuteMethod[TInput,TResult](IStreamedData input, Func2 genericExecuteCaller)
Remotion.Linq.Clauses.ResultOperators.ValueFromSequenceResultOperatorBase.ExecuteInMemory(IStreamedData input)
Microsoft.Data.Entity.Query.ResultOperatorHandler.ExecuteResultOperator[TSource,TResult](IEnumerable1 source, ResultOperatorBase resultOperator, StreamedSequenceInfo streamedSequenceInfo) lambda_method(Closure , QueryContext , QuerySourceScope ) Microsoft.Data.Entity.Query.EntityQueryModelVisitor.<>c__DisplayClass01.b__1(QueryContext qc)
Microsoft.Data.Entity.Relational.RelationalDataStore.Query[TResult](QueryModel queryModel, StateManager stateManager)
Microsoft.Data.Entity.Query.EntityQueryExecutor.ExecuteCollection[T](QueryModel queryModel)
DataStoreException: An error occured while running a data store operation. See InnerException for details.
Microsoft.Data.Entity.Query.EntityQueryExecutor.ExecuteCollection[T](QueryModel queryModel)
Microsoft.Data.Entity.Query.EntityQueryExecutor.ExecuteSingle[T](QueryModel queryModel, Boolean _)
Remotion.Linq.Clauses.StreamedData.StreamedSingleValueInfo.ExecuteSingleQueryModel[T](QueryModel queryModel, IQueryExecutor executor)
Remotion.Linq.Clauses.StreamedData.StreamedSingleValueInfo.ExecuteQueryModel(QueryModel queryModel, IQueryExecutor executor)
Remotion.Linq.QueryModel.Execute(IQueryExecutor executor)
Remotion.Linq.QueryProviderBase.Execute(Expression expression)
Remotion.Linq.QueryProviderBase.System.Linq.IQueryProvider.Execute[TResult](Expression expression)
System.Linq.Queryable.Single[TSource](IQueryable1 source, Expression1 predicate)
MusicStore.Models.ShoppingCart.GetTotal() in ShoppingCart.cs
117. var album = _db.Albums.Single(a => a.AlbumId == item.AlbumId);
MusicStore.Controllers.ShoppingCartController.Index() in ShoppingCartController.cs
25. var viewModel = new ShoppingCartViewModel
--- End of stack trace from previous location where exception was thrown ---
Microsoft.AspNet.Mvc.ReflectedActionExecutor.d__2.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
Microsoft.AspNet.Mvc.ReflectedActionExecutor.d__1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
Microsoft.AspNet.Mvc.ReflectedActionInvoker.d__1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
Microsoft.AspNet.Mvc.ReflectedActionInvoker.d__1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
Microsoft.AspNet.Mvc.ReflectedActionInvoker.d__1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd(Task task)
Microsoft.AspNet.Mvc.MvcRouteHandler.d__1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
Microsoft.AspNet.Routing.Template.TemplateRoute.d__1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
Microsoft.AspNet.Routing.RouteCollection.d__1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
Microsoft.AspNet.Builder.RouterMiddleware.d__1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
Microsoft.AspNet.Security.Infrastructure.AuthenticationMiddleware1.<Invoke>d__1.MoveNext() --- End of stack trace from previous location where exception was thrown --- System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) Microsoft.AspNet.RequestContainer.ContainerMiddleware.<Invoke>d__1.MoveNext() --- End of stack trace from previous location where exception was thrown --- System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) Microsoft.AspNet.Diagnostics.ErrorPageMiddleware.<Invoke>d__1.MoveNext()

[anpete]

@kirthik, @rowanmiller This is because we don't have the ability to load nav props yet:

// TODO Collapse to a single query once EF supports querying related data
decimal total = 0;
foreach (var item in _db.CartItems.Where(c => c.CartId == ShoppingCartId))
{
    var album = _db.Albums.Single(a => a.AlbumId == item.AlbumId);
    total += item.Count * album.Price;
}

[rubenalves]

Is the ability to load nav props planned for the beta or alphaX ?

[rowanmiller]

It's down as a 'nice to have' for beta - #199.

Any connection strings that we scaffold will have MARS enabled and we can also look at wrapping the exception from SqlClient in another exception that mentions enabling MARS.

[rowanmiller]

I just also wanted to note that the behavior of requiring MARS for this pattern of querying is that same as it has been in past versions of EF too (though the pattern is more common until we support eager loading - a.k.a. Include)

[kirthik]

It might be helpful for users to understand when MARS is required. So adding notes/blog to explain the scenarios that need MARS will be useful I think.

[rowanmiller]

Given the SQL exception isn't very helpful, one thing we could do is wrap the exception with something that explains what MARS is, why it's need it, and how to enable it.

The downside is that this would be messy/intrusive to implement. Within our code, the exception occurs in Microsoft.Data.Entity.Relational.Query.QueryMethodProvider+Enumerable<T>+Enumerator.MoveNext(). We'd probably want to do this at the SQL Server layer (rather than relational) so we'd have to have SQL specific implementations of these components. I haven't dug into it much further, but it seems like it would be kinda messy. @anpete thoughts?

[rowanmiller]

Triage: We should do this on the DatabaseErrorPage