public void DistinctBy_throws_ArgumentNullException()
{
IEnumerable <object> enumeration = new[] { "a", "b", "c" };
object action(object value) => value;
Assert.Throws <ArgumentNullException>(() => IEnumerableExtensions.DistinctBy((IEnumerable <object>)null, action).Iterate());
Assert.Throws <ArgumentNullException>(() => IEnumerableExtensions.DistinctBy(enumeration, (Func <object, object>)null).Iterate());
}