C# List of lists Flattener

Flatten.cs

public static class ListExtension
{
    /// <summary>
    /// Creates a single flat list from a list of lists input. 
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="inListOfLists"></param>
    /// <returns>A single flatten list from a list of lists input</returns>
    public static List<T> Flatten<T>(this List<List<T>> inListOfLists)
    {
        List<T> outList = new List<T>();
        foreach (var list in inListOfLists)
        {
            outList.AddRange(list);
        }
        return outList;
    }
}