名称空间“System.Data”中不存在类型或命名空间名称“DataSetExtensions”（您是否缺少程序集引用？）

I know this is a common error but I have the correct reference to the System.Data.DataSetExtensions.dll added to the project and my project is a SQL CLR project built for .net 4.5 and I'm getting the error at the following line:I know this is a common error but I have the co