CreateDerivedCollection unnecessarily tries to cast IEnumerable<T> to IList<T>

[Aesir]

CreateDerivedCollection is an extension method on IEumerable but for some reason creates a superfluous variable

var thisAsColl = (IList<T>)This;

that is only used once later to cast it back to an IEnumerable

var origEnum = (IEnumerable<T>)thisAsColl;

This limits limits you to collection types that implement IList (and causes unexpected exceptions when you don't). No features on IList are used within the method.

[shiftkey]

Close this sucker