Today I was doing some LINQ to objects. I was replacing a pair of nested foreach loops which built a string list and was looking for an elegant way to do it in LINQ. I ran into the problem that I needed to convert a list of lists to a single list. The following extension method works great for that. Works great for merging arrays of arrays without copying anything.

public static class ExtensionMethods
{
    public static IEnumerable<T> UnionAll<T>(this IEnumerable<IEnumerable<T>> eet)
    {
        foreach (var et in eet)
            foreach (var t in et)
                yield return t;
    }
}