LINQ selection by type of an object

Solution 1:

You can use the OfType Linq method for that:

var ofTypeA = collection.OfType<A>();

Regarding your unwillingness to loop throught the collection, you should keep in mind that Linq does not do magic tricks; I didn't check the implementation of OfType, but I would be surprised not to find a loop or iterator in there.

Solution 2:

For completeness, here is the source code of Enumerable.OfType<T>.

public static IEnumerable<TResult> OfType<TResult>(this IEnumerable source) {
    if (source == null) throw Error.ArgumentNull("source"); 
    return OfTypeIterator<TResult>(source); 
}

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

You can see that it lazily evaluates the source stream.

Solution 3:

You can use the OfType extension method for this

IEnumerable<A> filteredToA = list.OfType<A>();
IEnumerable<B> filteredToB = list.OfType<B>();