EF Core: An exception occurred in the database while iterating the results of a query

Question

I'm getting this error from my web app hosted in Azure. It runs ok for a few days and then suddenly stops working.

Is there an issue with the connection perhaps? Which object reference does not have an instance?

YYYY-MM-DD hh:mm:ss [Error] An exception occurred in the database while iterating the results of a query.
System.NullReferenceException: Object reference not set to an instance of an object.    at Microsoft.EntityFrameworkCore.Query.Internal.AsyncQueryingEnumerable.AsyncEnumerator.<BufferAllAsync>d__12.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 System.Runtime.CompilerServices.TaskAwaiter.GetResult()    at Microsoft.EntityFrameworkCore.Query.RelationalQueryContext.<RegisterValueBufferCursorAsync>d__14.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 System.Runtime.CompilerServices.TaskAwaiter.GetResult()    at Microsoft.EntityFrameworkCore.Query.Internal.AsyncQueryingEnumerable.AsyncEnumerator.<BufferlessMoveNext>d__9.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 System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()  at Microsoft.EntityFrameworkCore.Query.Internal.AsyncQueryingEnumerable.AsyncEnumerator.<MoveNext>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.<MoveNext>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 System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()  at Microsoft.EntityFrameworkCore.Query.Internal.AsyncLinqOperatorProvider.<_FirstOrDefault>d__82`1.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 System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()  at Microsoft.EntityFrameworkCore.Query.Internal.TaskResultAsyncEnumerable`1.Enumerator.<MoveNext>d__3.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 System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()  at Microsoft.EntityFrameworkCore.Query.Internal.AsyncLinqOperatorProvider.SelectAsyncEnumerable`2.SelectAsyncEnumerator.<MoveNext>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 System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()  at Microsoft.EntityFrameworkCore.Query.Internal.AsyncLinqOperatorProvider.ExceptionInterceptor`1.EnumeratorExceptionInterceptor.<MoveNext>d__5.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 System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()  at MyProject.Data.SectionRepository.<GetByTypePathAndAlias>d__6.MoveNext() in /Users/webaccount/myproject/src/MyProject.Data/Repositories/SectionRepository.cs:line 34

There's nothing special about my repository or entity code.

public async Task<Section> GetByTypeId(string typeId) 
{
    var sections = from s in this.DbContext.Sections
        where s.TypeId == typeId
        select s;

    return await sections.FirstOrDefaultAsync(); // <-- LINE 34
}

This is my entity.

public class Section
{
        [Key]
        public int Id { get; set; }
        public DateTimeOffset CreatedDate { get; set; }
        public string TypeId { get; set; }
        public string Name { get; set; }
        public string Description { get; set; }
}  

Is it related to this perhaps? https://github.com/aspnet/EntityFrameworkCore/issues/8026

Will it help if I convert my repository methods to sync and staying async at the Controller?

Answer 1

It runs ok for a few days and then suddenly stops working.

Object reference not set to an instance of an object

Firstly, you can try to remote debug your web app and check the this.DbContext.

Secondly, you can create/use a context instance per request, and this artciel shows some general guidelines when deciding on the lifetime of the context, you can check it.