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(); } 

Answer 2

I can only describe the "problem" from a Java view, but from what little I know this aspect is the same in both C# and Java:

A List<ObjBase> is not a List<Obj>, because it could contain an ObjBase object which is not a Obj object.

The other way around a List<Obj> can not be cast to a List<ObjBase> because the former guarantees to accept an Add() call with a ObjBase argument, which the latter will not accept!

So to summarize: even though a Obj is-a ObjBase a List<Obj> is not a List<ObjBase>.