What is the best or most interesting use of Extension Methods you've seen? [closed]

Answer 1

Extension method on int to decode a bitmask specifying days (with first day of week being Monday in this case) to an enumeration of DayOfWeek enums:

public static IEnumerable Days(this int dayMask)
{
    if ((dayMask & 1) > 0) yield return DayOfWeek.Monday;
    if ((dayMask & 2) > 0) yield return DayOfWeek.Tuesday;
    if ((dayMask & 4) > 0) yield return DayOfWeek.Wednesday;
    if ((dayMask & 8) > 0) yield return DayOfWeek.Thursday;
    if ((dayMask & 16) > 0) yield return DayOfWeek.Friday;
    if ((dayMask & 32) > 0) yield return DayOfWeek.Saturday;
    if ((dayMask & 64) > 0) yield return DayOfWeek.Sunday;
}