c# - What is the use of the ArraySegment<T> class?

Question

I just came across the ArraySegment<byte> type while subclassing the MessageEncoder class.

I now understand that it's a segment of a given array, takes an offset, is not enumerable, and does not have an indexer, but I still fail to understand its usage. Can someone please explain with an example?

Answer 1

ArraySegment<T> has become a lot more useful in .NET 4.5+ and .NET Core as it now implements:

• IList<T>
• ICollection<T>
• IEnumerable<T>
• IEnumerable
• IReadOnlyList<T>
• IReadOnlyCollection<T>

as opposed to the .NET 4 version which implemented no interfaces whatsoever.

The class is now able to take part in the wonderful world of LINQ so we can do the usual LINQ things like query the contents, reverse the contents without affecting the original array, get the first item, and so on:

var array = new byte[] { 5, 8, 9, 20, 70, 44, 2, 4 };
array.Dump();
var segment = new ArraySegment<byte>(array, 2, 3);
segment.Dump(); // output: 9, 20, 70
segment.Reverse().Dump(); // output 70, 20, 9
segment.Any(s => s == 99).Dump(); // output false
segment.First().Dump(); // output 9
array.Dump(); // no change