linq - Entity Framework - System.IndexOutOfRangeException

Question

我正在使用查询 SQL Server 2008 数据库的 Entity Framework 4.1。不幸的是，我们经常遇到以下异常:

<ExceptionType>System.IndexOutOfRangeException, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</ExceptionType>
  <Message>Index was outside the bounds of the array.</Message>



at System.Data.SqlClient.SqlDataReader.ReadColumnHeader(Int32 i)
   at System.Data.SqlClient.SqlDataReader.IsDBNull(Int32 i)
   at System.Data.Common.Internal.Materialization.Shaper.ErrorHandlingValueReader`1.GetValue(DbDataReader reader, Int32 ordinal)
   at System.Data.Common.Internal.Materialization.Shaper.GetPropertyValueWithErrorHandling[TProperty](Int32 ordinal, String propertyName, String typeName)
   at lambda_method(Closure , Shaper )
   at System.Data.Common.Internal.Materialization.Shaper.HandleEntityAppendOnly[TEntity](Func`2 constructEntityDelegate, EntityKey entityKey, EntitySet entitySet)
   at lambda_method(Closure , Shaper )
   at System.Data.Common.Internal.Materialization.Coordinator`1.ReadNextElement(Shaper shaper)
   at System.Data.Common.Internal.Materialization.Shaper`1.SimpleEnumerator.MoveNext()
   at System.Linq.Enumerable.First[TSource](IEnumerable`1 source)
   at System.Linq.Queryable.First[TSource](IQueryable`1 source)
   at OnlineSelfService.Business.ContentServiceBusiness.GetPageContent(Int32 pageId)</StackTrace>

实际示例代码:

//Caller
  public EmployeeEntity GetEmployeeDetail(int employeeID)
    {
         IQueryable<Employee> result=null;
         if (myCaching.Contains("Employee"))
            {
                result = (IQueryable<Employee>)myCaching["Employee"];
            }
            else
            {
                result = dataAccess.GetEmployeeDetail();
                myCaching.AddToCache("Employee", result); //Expire in 2min
            }

            IQueryable<Employee> entityResult = from entity in result
                                                         where entity.employeeId == employeeID
                                                         select entity;
       if (entityResult.Count<Employee>() > 0)
                return entityResult.First<Employee>();
            return new EmployeeEntity();
   }

//DAL
public IQueryable<Employee> GetEmployeeDetail()
{
    DatabaseEntities ent = new DatabaseEntities(this._connectionString);
    IQueryable<Employee> result = from employee in ent.EmployeeEntity
                                           select employee;

    return result;
}

更新**使用缓存更新了我的代码。

我用谷歌搜索找到并回答，但找不到根本原因的明确答案。一些遇到过这个问题的人可以分享解决方案吗。

谢谢。

Answer 1

调用 .Count() 执行查询。我不认为 .First() 会再次执行它，但也许是，并且在这些调用之间发生了一些变化。您可以尝试将查询重写为:

(from entity in result
where entity.employeeId == employeeID
select entity).FirstOrDefault() ?? new EmployeeEntity();