EF Core：迭代查询结果时数据库中发生异常

Question

我从Azure中托管的Web应用程序收到此错误。它可以正常运行几天，然后突然停止工作。

连接是否有问题？哪个对象引用没有实例？

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

我的存储库或实体代码没有什么特别的。

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
}

这是我的实体。

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; }
}  

可能与此有关吗？https://github.com/aspnet/EntityFrameworkCore/issues/8026

如果我将存储库方法转换为同步并在Controller上保持异步，是否有帮助？

Answer 1

正常运行几天后突然停止工作。

Object reference not set to an instance of an object

首先，您可以尝试远程调试您的 Web 应用程序并检查this.DbContext.

其次，您可以为每个请求创建/使用一个上下文实例，并且此 artciel 显示了在决定上下文生命周期时的一些一般准则，您可以检查它。