Convert List<DerivedClass> to List<BaseClass>

Question

The way to make this work is to iterate over the list and cast the elements. This can be done using ConvertAll:

List<A> listOfA = new List<C>().ConvertAll(x => (A)x);

You could also use Linq:

List<A> listOfA = new List<C>().Cast<A>().ToList();

First of all, stop using impossible-to-understand class names like A, B, C. Use Animal, Mammal, Giraffe, or Food, Fruit, Orange or something where the relationships are clear.

Your question then is "why can I not assign a list of giraffes to a variable of type list of animal, since I can assign a giraffe to a variable of type animal?"

The answer is: suppose you could. What could then go wrong?

Well, you can add a Tiger to a list of animals. Suppose we allow you to put a list of giraffes in a variable that holds a list of animals. Then you try to add a tiger to that list. What happens? Do you want the list of giraffes to contain a tiger? Do you want a crash? or do you want the compiler to protect you from the crash by making the assignment illegal in the first place?

We choose the latter.

This kind of conversion is called a "covariant" conversion. In C# 4 we will allow you to make covariant conversions on interfaces and delegates when the conversion is known to be always safe. See my blog articles on covariance and contravariance for details. (There will be a fresh one on this topic on both Monday and Thursday of this week.)

To quote the great explanation of Eric

What happens? Do you want the list of giraffes to contain a tiger? Do you want a crash? or do you want the compiler to protect you from the crash by making the assignment illegal in the first place? We choose the latter.

But what if you want to choose for a runtime crash instead of a compile error? You would normally use Cast<> or ConvertAll<> but then you will have 2 problems: It will create a copy of the list. If you add or remove something in the new list, this won't be reflected in the original list. And secondly, there is a big performance and memory penalty since it creates a new list with the existing objects.

I had the same problem and therefore I created a wrapper class that can cast a generic list without creating an entirely new list.

In the original question you could then use:

class Test
{
    static void Main(string[] args)
    {
        A a = new C(); // OK
        IList<A> listOfA = new List<C>().CastList<C,A>(); // now ok!
    }
}

and here the wrapper class (+ an extention method CastList for easy use)

public class CastedList<TTo, TFrom> : IList<TTo>
{
    public IList<TFrom> BaseList;

    public CastedList(IList<TFrom> baseList)
    {
        BaseList = baseList;
    }

    // IEnumerable
    IEnumerator IEnumerable.GetEnumerator() { return BaseList.GetEnumerator(); }

    // IEnumerable<>
    public IEnumerator<TTo> GetEnumerator() { return new CastedEnumerator<TTo, TFrom>(BaseList.GetEnumerator()); }

    // ICollection
    public int Count { get { return BaseList.Count; } }
    public bool IsReadOnly { get { return BaseList.IsReadOnly; } }
    public void Add(TTo item) { BaseList.Add((TFrom)(object)item); }
    public void Clear() { BaseList.Clear(); }
    public bool Contains(TTo item) { return BaseList.Contains((TFrom)(object)item); }
    public void CopyTo(TTo[] array, int arrayIndex) { BaseList.CopyTo((TFrom[])(object)array, arrayIndex); }
    public bool Remove(TTo item) { return BaseList.Remove((TFrom)(object)item); }

    // IList
    public TTo this[int index]
    {
        get { return (TTo)(object)BaseList[index]; }
        set { BaseList[index] = (TFrom)(object)value; }
    }

    public int IndexOf(TTo item) { return BaseList.IndexOf((TFrom)(object)item); }
    public void Insert(int index, TTo item) { BaseList.Insert(index, (TFrom)(object)item); }
    public void RemoveAt(int index) { BaseList.RemoveAt(index); }
}

public class CastedEnumerator<TTo, TFrom> : IEnumerator<TTo>
{
    public IEnumerator<TFrom> BaseEnumerator;

    public CastedEnumerator(IEnumerator<TFrom> baseEnumerator)
    {
        BaseEnumerator = baseEnumerator;
    }

    // IDisposable
    public void Dispose() { BaseEnumerator.Dispose(); }

    // IEnumerator
    object IEnumerator.Current { get { return BaseEnumerator.Current; } }
    public bool MoveNext() { return BaseEnumerator.MoveNext(); }
    public void Reset() { BaseEnumerator.Reset(); }

    // IEnumerator<>
    public TTo Current { get { return (TTo)(object)BaseEnumerator.Current; } }
}

public static class ListExtensions
{
    public static IList<TTo> CastList<TFrom, TTo>(this IList<TFrom> list)
    {
        return new CastedList<TTo, TFrom>(list);
    }
}

If you use IEnumerable instead, it will work (at least in C# 4.0, I have not tried previous versions). This is just a cast, of course, it will still be a list.

Instead of -

List<A> listOfA = new List<C>(); // compiler Error

In the original code of the question, use -

IEnumerable<A> listOfA = new List<C>(); // compiler error - no more! :)

As far as why it doesn't work, it might be helpful to understand covariance and contravariance.

Just to show why this shouldn't work, here is a change to the code you provided:

void DoesThisWork()
{
     List<C> DerivedList = new List<C>();
     List<A> BaseList = DerivedList;
     BaseList.Add(new B());

     C FirstItem = DerivedList.First();
}

Should this work? The First item in the list is of Type "B", but the type of the DerivedList item is C.

Now, assume that we really just want to make a generic function that operates on a list of some type which implements A, but we don't care what type that is:

void ThisWorks<T>(List<T> GenericList) where T:A
{

}

void Test()
{
     ThisWorks(new List<B>());
     ThisWorks(new List<C>());
}

You can only cast to readonly lists. For example:

IEnumerable<A> enumOfA = new List<C>();//This works
IReadOnlyCollection<A> ro_colOfA = new List<C>();//This works
IReadOnlyList<A> ro_listOfA = new List<C>();//This works

And you cannot do it for lists that support saving elements. The reason why is:

List<string> listString=new List<string>();
List<object> listObject=(List<object>)listString;//Assume that this is possible
listObject.Add(new object());

What now? Remember that listObject and listString are the same list actually, so listString now have object element - it shouldn't be possible and it's not.

I personally like to create libs with extensions to the classes

public static List<TTo> Cast<TFrom, TTo>(List<TFrom> fromlist)
  where TFrom : class 
  where TTo : class
{
  return fromlist.ConvertAll(x => x as TTo);
}