EFコア：クエリの結果を反復処理中にデータベースで例外が発生しました

Question

AzureでホストされているWebアプリケーションからこのエラーが発生しています。それは数日はOKを実行し、その後突然動作を停止します。

おそらく接続に問題はありますか？どのオブジェクト参照にインスタンスがないのですか？

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 

私のリポジトリまたはエンティティコードに特別なものはありません。

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 
} 

これは私のエンティティです。

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; } 
} 

これはおそらくこれに関連していますか？ https://github.com/aspnet/EntityFrameworkCore/issues/8026

私は自分のリポジトリメソッドを同期させ、コントローラで非同期にすることができますか？

Answer 1

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

Object reference not set to an instance of an object

まず、あなたはremote debug your web appしようとthis.DbContextを確認することができます。

第2に、要求ごとにコンテキストインスタンスを作成/使用することができます。this artciel shows some general guidelines when deciding on the lifetime of the contextをチェックすることができます。