Can IEnumerable Collections Be Augmented Like Lists?

IEnumerable collections represent sequences of elements that may or may not be mutable. Unlike lists, IEnumerable collections do not inherently support item addition.

When attempting to add items to an IEnumerable collection, such as in the provided example, it is actually inefficient. The ToList() method converts the IEnumerable to a List, allowing item addition. However, the original IEnumerable remains unchanged, resulting in a collection with only one element.

Is there an alternative to items.Add(item)?

No, there is no direct equivalent to items.Add(item) in IEnumerable. However, you can create a new IEnumerable that includes both the original items and the new item. This technique relies on the Enumerable.Concat method:

items = items.Concat(new[] { "foo" });

This approach creates a non-mutable IEnumerable that incorporates all elements from the original collection and the added item. Additionally, it ensures that any future changes to the original collection are reflected in the new IEnumerable.