Nothing is known about the types at compile time.

object TransformObject(object oldObject, Type newType, Func<object, object> transform)
{
    if(obj.GetType().ImplementsGenericInterface(typeof(IList<>))
    && newType.ImplementsGenericInterface(typeof(IList<>)))
    {
        object newCollection = Activator.CreateInstance(newType);

        // This is where it gets tricky:
        // 1. How to iterate over the old collection?
        // 2. How to add each element in the new collection?
        object oldCollection = oldObject;
        foreach(var oldItem in oldCollection)
        {
            object newItem = transform(oldItem);
            newCollection.Add(newItem);
        }

        // 3. How to ensure that the order in the new collection is preserved?

        return newCollection;
    }

    return null;
}

Naturally this doesn't work.

What is known at run-time:

• oldCollection.GetType() implements type IList<AT>.


• newType implements type IList<BT>


• Function object transform(object oldObject) takes care of converting object of type AT into a new object of type BT. Or maps object into an object of different type. Mapping is not known compile-time.