The type or namespace name Relation could not be found(are you missing a using directive or an assem

问题如下:

我先申明一个List <Relation> ,然后把他作为参数调用

List <Relation> relations = new List <Relation> ();
GetNameBydependence(relations);

然后把他作为参数调用GetNameBydependence方法,但在此方法中报错,(报错信息:The type or namespace name Relation could not be found(are you missing a using directive or an assembly reference?))

private static void GetNameBydependence(List <Relation> relations)
{
......
}

能否帮我解惑啊?
------解决方法--------------------------------------------------------
你要using Relation所在的命名安间
------解决方法--------------------------------------------------------
using Relation
------解决方法--------------------------------------------------------
引入Relation空间！