Question

I will start with code

class LocData 
{
    public virtual TypeA Copy () { ... }
    ...
}

class LocDataCollection<T> : List<T> where T: LocData
{
    public LocDataCollection<T> Copy() 
    {
        LocDataCollection<T> locDatas = new LocDataCollection<T>();
        foreach (T locData in this)
        {
            T locData2 = locData.Copy() as T;
            locDatas.Add(locData2);
        }
        return locDatas;
    }
  ...
}

and

class TypeA : LocData
{
  public new TypeA Copy () { ... }
  ...
}

class TypeACollection : LocDataCollection<TypeA>
{
}

test code:

TypeACollection typeAs = new TypeACollection();
...
TypeACollection typeAs2 = typeAs.Copy();

compile message:

“Cannot implicitly convert type ‘LocDataCollection’ to ‘TypeACollection’. An explicit conversion exists (are you missing a cast?)”

I have to change to

TypeACollection typeAs2 = typeAs.Copy() as TypeACollection;

compile pass but at run time error out. typeAs2 return as null even typeAs is not null.

Answer 1

typeAs.Copy() returns a LocDataCollection<TypeA>. But you cannot assign one of those to a TypeACollection because TypeACollection is more derived than LocDataCollection<TypeA>.

You could perhaps solve the problem by making your copy method receive a LocDataCollection<T>:

class LocDataCollection<T> : List<T> where T: LocData
{
    public void CopyTo(LocDataCollection<T> dest) 
    {
        ....
    }
}

and then at the call site:

TypeACollection typeAs = new TypeACollection();
TypeACollection typeAs2 = new TypeACollection();
typeAs.CopyTo(typeAs2);

Of course, this won’t copy any of the TypeACollection specific stuff, unless you added an override to that effect.

But I don’t really know enough about your problem to be confident that this is an appropriate solution.