Gets a robust view of any enumerable - this one will not break when you modify the underlying enumerable (e. g., modify the list). Originally written for lucaswerkmeister/wp_geohashing until I realized I didn't need it there.

gistfile1.cs

/// <summary>
/// Gets a robust enumerator for the IEnumerable. A robust enumerator will function even when the collection is modified while enumerating.
/// </summary>
/// <typeparam name="T">The type.</typeparam>
/// <param name="coll">The collection.</param>
/// <param name="lookBehind">If <code>true</code>, elements inserted before the current element will still be returned.</param>
/// <returns>An IEnumerable that will not throw an IllegalOperationException if the underlying collection is modified.</returns>
public static IEnumerable<T> AsRobustEnumerable<T>(this IEnumerable<T> coll, bool lookBehind = false) where T : class
{
	HashSet<T> alreadyReturned = new HashSet<T>();
	T latestElement = null;
	while (true)
	{
		bool hasSeenLatestElement = lookBehind;
		bool returnedElement = false;
		foreach (T t in coll)
		{
			hasSeenLatestElement |= latestElement == null || t.Equals(latestElement);
			if (!alreadyReturned.Contains(t) && hasSeenLatestElement)
			{
				latestElement = t;
				alreadyReturned.Add(t);
				yield return t;
				returnedElement = true;
				break;
			}
		}
		if (!returnedElement)
			yield break;
	}
}