Question

Other than readability, what is the difference between the following linq queries and when and why would I use one over the other:

IEnumerable<T> items = listOfItems.Where(d => d is T).Cast<T>();

and

IEnumerable<T> items = listOfItems.OfType<T>();

Update:
Dang, sorry introduced some bugs when trying to simplify my problem

Answer 1

Let us compare three methods (pay attention to generic arguments):

1. listOfItems.Where(t => t is T) called on IEnumerable<X> will still return IEnumerable<X> just filtered to contain only elements of the type T.

2. listOfItems.OfType<T>() called on IEnumerable<X> will return IEnumerable<T> containing elements that can be casted to type T.

3. listOfItems.Cast<T>() called on IEnumerable<X> will return IEnumerable<T> containing elements casted to type T or throw an exception if any of the elements cannot be converted.

And listOfItems.Where(d => d is T).Cast<T>() is basically doing the same thing twice – Where filters all elements that are T but still leaving the type IEnumerable<X> and then Cast again tries to cast them to T but this time returning IEumerable<T>.