Question

I have an object hierarchy

public class MyBase {}
public class MyDerived : MyBase {}

and have

List<MyBase> myList;

that is actually filled with instances of MyDerived

In order to access that list as a List<MyDerived>, I’m doing the following:

myList.Cast<MyDerived>().ToList()

I read the MSDN docs on Enumerable.Cast<T>, but it’s not clear to me whether the Cast<T> and ToList operations make a new copy of the objects in memory, or simply allow the compiler to access the existing objects as if they were a List<MyDerived>.

Answer 1

As its name implies, Cast<T>() just casts objects:

public static IEnumerable<TResult> Cast<TResult>(this IEnumerable source) { 
    IEnumerable<TResult> typedSource = source as IEnumerable<TResult>;
    if (typedSource != null) return typedSource; 
    if (source == null) throw Error.ArgumentNull("source");
    return CastIterator<TResult>(source);
}

static IEnumerable<TResult> CastIterator<TResult>(IEnumerable source) {
    foreach (object obj in source) yield return (TResult)obj; 
} 

In .Net, it is fundamentally impossible to copy an arbitrary object. It would make no sense for Cast<T>() to copy things.

Note that if T is a value type, Cast<T>() will copy the structs; value types are always copied. (except when passed as ref parameters)