What is the use of the ArraySegment<T> class?
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>IEnumerableIReadOnlyList<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
It is a puny little soldier struct that does nothing but keep a reference to an array and stores an index range. A little dangerous, beware that it does not make a copy of the array data and does not in any way make the array immutable or express the need for immutability. The more typical programming pattern is to just keep or pass the array and a length variable or parameter, like it is done in the .NET BeginRead() methods, String.SubString(), Encoding.GetString(), etc, etc.
It does not get much use inside the .NET Framework, except for what seems like one particular Microsoft programmer that worked on web sockets and WCF liking it. Which is probably the proper guidance, if you like it then use it. It did do a peek-a-boo in .NET 4.6, the added MemoryStream.TryGetBuffer() method uses it. Preferred over having two out arguments I assume.
In general, the more universal notion of slices is high on the wishlist of principal .NET engineers like Mads Torgersen and Stephen Toub. The latter kicked off the array[:] syntax proposal a while ago, you can see what they've been thinking about in this Roslyn page. I'd assume that getting CLR support is what this ultimately hinges on. This is actively being thought about for C# version 7 afaik, keep your eye on System.Slices.
Update: dead link, this shipped in version 7.2 as Span.
Update2: more support in C# version 8.0 with Range and Index types and a Slice() method.