generics - C# Casting a List<ObjBase> as List<Obj>

Question

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>();
    }

}

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>();
    }
}

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

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

Thanks.

Answer 1

You can use Cast and ToList extension methods from System.Linq to have this in one line.

Instead of

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

do this:

internal List<Obj> returnStuff()
{
   return getSomeStuff().Cast<Obj>().ToList();
}