LINQ-Expression fails but foreach loop works Entity-Framework

Question

My application is an ASP.NET Core 1.0 Web API.

I have the following LINQ-Expression in my code:

int number = 0;
var orders = await this.DataRepo.Where(data => data.number == number).ToListAsync();

If i try to run the code it fails and gives me the following error message:

Microsoft.EntityFrameworkCore.Query.Internal.SqlServerQueryCompilationContextFactory:Error: An exception occurred in the database while iterating the results of a query. System.Data.SqlClient.SqlException: Timeout expired. The timeout period elapsed prior to completion of the operation or the server is not responding. ---> System.ComponentModel.Win32Exception: Der Wartevorgang wurde abgebrochen --- End of inner exception stack trace --- at System.Data.SqlClient.SqlCommand.<>c.b__107_0(Task`1 result) at System.Threading.Tasks.ContinuationResultTaskFromResultTask`2.InnerInvoke() at System.Threading.Tasks.Task.Execute() --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.EntityFrameworkCore.Storage.Internal.RelationalCommand.d__20.MoveNext() --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.EntityFrameworkCore.Query.Internal.AsyncQueryingEnumerable.AsyncEnumerator.d__8.MoveNext() --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.EntityFrameworkCore.Query.Internal.AsyncLinqOperatorProvider.SelectAsyncEnumerable`2.SelectAsyncEnumerator.d__4.MoveNext() --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.EntityFrameworkCore.Query.Internal.AsyncLinqOperatorProvider.SelectAsyncEnumerable`2.SelectAsyncEnumerator.d__4.MoveNext() --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.EntityFrameworkCore.Query.Internal.AsyncLinqOperatorProvider.ExceptionInterceptor`1.EnumeratorExceptionInterceptor.d__5.MoveNext() ClientConnectionId:64923a0e-cf94-487c-be83-a43b719d8c45 Error Number:-2,State:0,Class:11

System.Data.SqlClient.SqlException: Timeout expired. The timeout period elapsed prior to completion of the operation or the server is not responding. ---> System.ComponentModel.Win32Exception: Der Wartevorgang wurde abgebrochen --- End of inner exception stack trace --- at System.Data.SqlClient.SqlCommand.<>c.b__107_0(Task`1 result) at System.Threading.Tasks.ContinuationResultTaskFromResultTask`2.InnerInvoke() at System.Threading.Tasks.Task.Execute() --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.EntityFrameworkCore.Storage.Internal.RelationalCommand.d__20.MoveNext() --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.EntityFrameworkCore.Query.Internal.AsyncQueryingEnumerable.AsyncEnumerator.d__8.MoveNext() --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.EntityFrameworkCore.Query.Internal.AsyncLinqOperatorProvider.SelectAsyncEnumerable`2.SelectAsyncEnumerator.d__4.MoveNext() --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.EntityFrameworkCore.Query.Internal.AsyncLinqOperatorProvider.SelectAsyncEnumerable`2.SelectAsyncEnumerator.d__4.MoveNext() --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.EntityFrameworkCore.Query.Internal.AsyncLinqOperatorProvider.ExceptionInterceptor`1.EnumeratorExceptionInterceptor.d__5.MoveNext() ClientConnectionId:64923a0e-cf94-487c-be83-a43b719d8c45 Error Number:-2,State:0,Class:11 Ausnahme ausgelöst: "System.Data.SqlClient.SqlException" in System.Private.CoreLib.ni.dll

however, if I change the LINQ-Expression to the following loop everything works fine:

var orders = new List<ExampleClass>();
int number = 0;

foreach (var data in DataRepo)
{
   if (data.number == number)
   {
      orders.Add(data);
   }
}

I havent seen anyone with the same problem..

Does anyone know why this is the case? Thank you very much

Answer 1

Since the timeout was reported by the SQL Client, I guess that it's not a problem with the Linq Expression but with the execution of the compiled SQL statement against the database. The exception gives two hints:

• The timeout period elapsed prior to completion of the operation
• The server is not responding

Microsoft.EntityFrameworkCore.Query.Internal.SqlServerQueryCompilationContextFactory:Error: An exception occurred in the database while iterating the results of a query. System.Data.SqlClient.SqlException: Timeout expired. The timeout period elapsed prior to completion of the operation or the server is not responding.

Regarding the fact that your second code loads the entire table into memory within the timeout period, we can exclude the second option.

The default timeout of the SQL Connection is 15 seconds. This should be appropriate to execute simple queries like the one your code will generate. When loading of the entire table is faster than filtering it by a single parameter in the where clause, there is most likely a problem with indexing:

• Missing index on 'number'
• The index is highly fragmented or its statistics are not up do date
• The index is not appropriate because thousands of rows have 'number' = 0
• The clustered index of the table is fragmented
• Other reasons

To focus the issue in the database, I would extract the SQL Query - either by tracing the SQL string in the debugger or using the SQL Server Profiler - execute it in the SQL Server Management Studio and analyze the Execution Plan for further information.

To exclude side effects of the asynchronous query execution, I would also doing some tests in the default synchronous context.