What is the use of the ArraySegment<T> class?

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?

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

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.

> 1. Buffer partioning for IO classes - Use the same buffer for simultaneous > read and write operations and have a > single structure you can pass around > the describes your entire operation.
> 2. Set Functions - Mathematically speaking you can represent any > contiguous subsets using this new > structure. That basically means you > can create partitions of the array, > but you can't represent say all odds > and all evens. Note that the phone > teaser proposed by The1 could have > been elegantly solved using > ArraySegment partitioning and a tree > structure. The final numbers could > have been written out by traversing > the tree depth first. This would have > been an ideal scenario in terms of > memory and speed I believe.
> 3. Multithreading - You can now spawn multiple threads to operate over the > same data source while using segmented > arrays as the control gate. Loops > that use discrete calculations can now > be farmed out quite easily, something > that the latest C++ compilers are > starting to do as a code optimization > step.
> 4. UI Segmentation - Constrain your UI displays using segmented > structures. You can now store > structures representing pages of data > that can quickly be applied to the > display functions. Single contiguous > arrays can be used in order to display > discrete views, or even hierarchical > structures such as the nodes in a > TreeView by segmenting a linear data > store into node collection segments.

In this example, we look at how you can use the original array, the Offset and Count properties, and also how you can loop through the elements specified in the ArraySegment.

using System;

class Program
{
    static void Main()
    {
        // Create an ArraySegment from this array.
        int[] array = { 10, 20, 30 };
        ArraySegment<int> segment = new ArraySegment<int>(array, 1, 2);

        // Write the array.
        Console.WriteLine("-- Array --");
        int[] original = segment.Array;
        foreach (int value in original)
        {
            Console.WriteLine(value);
        }

        // Write the offset.
        Console.WriteLine("-- Offset --");
        Console.WriteLine(segment.Offset);

        // Write the count.
        Console.WriteLine("-- Count --");
        Console.WriteLine(segment.Count);

        // Write the elements in the range specified in the ArraySegment.
        Console.WriteLine("-- Range --");
        for (int i = segment.Offset; i < segment.Count+segment.Offset; i++)
        {
            Console.WriteLine(segment.Array[i]);
        }
    }
}

ArraySegment Structure - what were they thinking?

What's about a wrapper class? Just to avoid copy data to temporal buffers.

public class SubArray<T> {
        private ArraySegment<T> segment;

        public SubArray(T[] array, int offset, int count) {
            segment = new ArraySegment<T>(array, offset, count);
        }
        public int Count {
            get { return segment.Count; }
        }

        public T this[int index] {
            get {
               return segment.Array[segment.Offset + index];
            }
        }

        public T[] ToArray() {
            T[] temp = new T[segment.Count];
            Array.Copy(segment.Array, segment.Offset, temp, 0, segment.Count);
            return temp;
        }

        public IEnumerator<T> GetEnumerator() {
            for (int i = segment.Offset; i < segment.Offset + segment.Count; i++) {
                yield return segment.Array[i];
            }
        }
    } //end of the class

Example:

byte[] pp = new byte[] { 1, 2, 3, 4 };
SubArray<byte> sa = new SubArray<byte>(pp, 2, 2);

Console.WriteLine(sa[0]);
Console.WriteLine(sa[1]);
//Console.WriteLine(b[2]); exception

Console.WriteLine();
foreach (byte b in sa) {
	Console.WriteLine(b);
}
        

Ouput:

3
4

3
4

The ArraySegment is MUCH more useful than you might think. Try running the following unit test and prepare to be amazed!

    [TestMethod]
    public void ArraySegmentMagic()
    {
        var arr = new[] {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};

        var arrSegs = new ArraySegment<int>[3];
        arrSegs[0] = new ArraySegment<int>(arr, 0, 3);
        arrSegs[1] = new ArraySegment<int>(arr, 3, 3);
        arrSegs[2] = new ArraySegment<int>(arr, 6, 3);
        for (var i = 0; i < 3; i++)
        {
            var seg = arrSegs[i] as IList<int>;
            Console.Write(seg.GetType().Name.Substring(0, 12) + i);
            Console.Write(" {");
            for (var j = 0; j < seg.Count; j++)
            {
                Console.Write("{0},", seg[j]);
            }
            Console.WriteLine("}");
        }
    }

You see, all you have to do is cast an ArraySegment to IList and it will do all of the things you probably expected it to do in the first place. Notice that the type is still ArraySegment, even though it is behaving like a normal list.

OUTPUT:

ArraySegment0 {0,1,2,}
ArraySegment1 {3,4,5,}
ArraySegment2 {6,7,8,}

In simple words: it keeps reference to an array, allowing you to have multiple references to a single array variable, each one with a different range.

In fact it helps you to use and pass sections of an array in a more structured way, instead of having multiple variables, for holding start index and length. Also it provides collection interfaces to work more easily with array sections.

For example the following two code examples do the same thing, one with ArraySegment and one without:

        byte[] arr1 = new byte[] { 1, 2, 3, 4, 5, 6 };
        ArraySegment<byte> seg1 = new ArraySegment<byte>(arr1, 2, 2);
        MessageBox.Show((seg1 as IList<byte>)[0].ToString());

and,

        byte[] arr1 = new byte[] { 1, 2, 3, 4, 5, 6 };
        int offset = 2;
        int length = 2;
        byte[] arr2 = arr1;
        MessageBox.Show(arr2[offset + 0].ToString());

Obviously first code snippet is more preferred, specially when you want to pass array segments to a function.