I'm trying to return a list derived classes (RedThingSets and BlueThingSets) as a list of their base (ThingSets) class so that I can have a more generic function returning them (this is just an example to illustrate).

(This is not actual code)

public abstract class ThingSet 
{
    public List<Thing> Things;
}

public class RedThingSet : ThingSet {}

public class BlueThingSet : ThingSet {}


public void MahClass()
{

    public List<RedThingSet> RedThingsList = new List<RedThingSet>();
    public List<BlueThingSet> BlueThingsList = new List<BlueThingSet>();

    public List<ThingSet> GetThingSet( bool giveMeTheBlue )
    {
        if ( giveMeTheBlue )
        {
            return BlueThingsList;
        }
        else
        {
            return RedThingsList;
        }
    }
}

Is this possible?

I've tried using as like this: return BlueThingsList as List<ThingSet>;

And I've tried casting it directly like this: return (List<ThingSet>) BlueThingsList;

But neither are working,

Am I just approaching this entirely wrong?