C＃铸造一个Li​​st< ObjBase>如表<&的OBJ GT;

为什么我不能投了列表< ObjBase> 为列表<的OBJ> ？为什么下面不工作：

Why can I not cast a List<ObjBase> as List<Obj>? Why does the following not work:

internal class ObjBase
   {
   }

internal class Obj : ObjBase
   {
   }   

internal class ObjManager
{
    internal List<Obj> returnStuff()
    {
       return getSomeStuff() as List<Obj>;
    }

    private List<ObjBase> getSomeStuff()
    {
       return new List<ObjBase>();
    }

}

相反，我必须这样做：

Instead I have to do this:

internal class ObjBase
   {
   }

internal class Obj : ObjBase
   {
   }

internal class ObjManager
{
    internal List<Obj> returnStuff()
    {
       List<ObjBase> returnedList = getSomeStuff();
       List<Obj> listToReturn = new List<Obj>(returnedList.Count);
       foreach (ObjBase currentBaseObject in returnedList)
       {
          listToReturn.Add(currentBaseObject as Obj);
       }
       return listToReturn;
    }

    private List<ObjBase> getSomeStuff()
    {
       return new List<ObjBase>();
    }
}

我得到在Visual Studio 2008中的以下错误（缩短为了提高可读性）：

I get the following error in Visual Studio 2008 (shortened for readability):

无法通过引用转换转换类型列表'到'清单'，装箱转换，取消装箱转换，包装转换，或者空类型转换

Cannot convert type 'List' to 'List' via a reference conversion, boxing conversion, unboxing conversion, wrapping conversion, or null type conversion

感谢。