LINQ: There is already an open DataReader associated with this Command which must be closed first

在Linq to SQL和Entity Framework，使用Linq查询数据的时候会出现错误：There is already an open DataReader associated with this Command which must be closed first 。

解决方案：

在数据库连接串上添加MultipleActiveResultSets=true。

Fixing the "There is already an open DataReader associated with this Command which must be closed first." exception in Entity Framework