c# - "Invalid attempt to read when no data is present"在对查询结果启动 foreach 循环时

Question

我有这个问题:

 // _db derives from DbContext
 var toProcess = from jobItem in _db.Jobs
                 where jobItem.State == desiredState
                 select jobItem.ItemId;
 foreach (Guid itemId in toProcess ) //exception thrown on this line
 {
     // whatever
 }

大多数时候运行良好，但偶尔 foreach 会抛出:

System.InvalidOperationException: Invalid attempt to read when no data is present

具有以下堆栈跟踪:

System.Data.SqlClient.SqlDataReader.ReadColumnHeader(Int32 i)
System.Data.SqlClient.SqlDataReader.IsDBNull(Int32 i)
System.Data.Common.Internal.Materialization.Shaper.ErrorHandlingValueReader`1.GetValue(DbDataReader reader, Int32 ordinal)
System.Data.Common.Internal.Materialization.Shaper.GetColumnValueWithErrorHandling[TColumn](Int32 ordinal)
System.Data.Common.Internal.Materialization.Coordinator`1.ReadNextElement(Shaper shaper)
System.Data.Common.Internal.Materialization.Shaper`1.SimpleEnumerator.MoveNext()

这没有任何意义。我该如何解决这个问题？

Answer 1

根据您对我评论的回复:

There's some medium weight activity that is expected to be done within several seconds but I guess it can take longer. Do you think a timeout happens?

我想这是一个超时或类似的东西。您可以增加超时时间，或者您可以强制它求值并将其存储在内存中供以后处理。您可以通过在查询末尾添加 ToArray() 来执行此操作:

 var toProcess = (from jobItem in _db.Jobs
                 where jobItem.State == desiredState
                 select jobItem.ItemId).ToArray();

看看是否有帮助。