How to detect if a LINQ enumeration is materialized?

Question

Is there some way of detecting whether an enumerable built using LINQ (to Objects in this case) have been materialized or not? Other than trying to inspect the type of the underlying collection?

Specifically, since enumerable.ToArray() will build a new array even if the underlying collection already is an array I'm looking for a way of avoiding ToArray() being called twice on the same collection.

Answer 1

Checking "is IEnumerable" can do this, there isn't another way. You could, however, not use the "var" declaration for the return type - because it is "hiding" from you the type. If you declare an explicit IEnumerable, then the compiler will let you know if that is what is being returned.

Answer 2

The enumerable won't have an "underlying collection", it will be a collection. Try casting it to one and use the resulting reference:

var coll = enumerable as ICollection<T>;
if (coll != null) {
    // We have a collection!
}