DefaultIfEmpty in LINQ

Question

Can somebody explain how DefaultIfEmpty() can be used in LINQ. I have ready some material but still need something solid to see what the use of it is.<

Answer 1

It basically returns a collection with a single element in case the source collection is empty.

var numbers = new int[] {1, 2, 3};
var aNumber = numbers.First();

returns 1

but

var numbers = new int[];
var aNumber = numbers.DefaultIfEmpty(12).Single();

returns 12 as the collection is empty