C# Academy · Lesson

Memory/ReadOnlyMemory emulation: zero-copy windows

Pass ArraySegment instead of arrays: create windows, implement Skip/Take/Slice, parse headers and payloads, and only copy at boundaries.

Memory/ReadOnlyMemory emulation: zero-copy windows is a free C# Academy lesson on CoddyKit — lesson 2 of 3. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the C# Academy learning path, one of 3 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Windows over arrays

Aim: Treat arrays as memory windows.

Use ArraySegment<T> to pass views
Build tiny Skip/Take/Slice helpers
Parse headers/payloads without copies
Only copy at I/O boundaries

Skip/Take/Slice helpers

Create tiny Slice, Skip, and Take helpers to compose windows clearly.

using System;

public static class Seg
{
  public static ArraySegment<T> Slice<T>(ArraySegment<T> s, int start, int count)
  {
    if (start < 0 || count < 0 || start + count > s.Count) throw new ArgumentOutOfRangeException();
    return new ArraySegment<T>(s.Array, s.Offset + start, count);
  }

  public static ArraySegment<T> Skip<T>(ArraySegment<T> s, int n)
  {
    if (n < 0 || n > s.Count) throw new ArgumentOutOfRangeException("n");
    return new ArraySegment<T>(s.Array, s.Offset + n, s.Count - n);
  }

  public static ArraySegment<T> Take<T>(ArraySegment<T> s, int n)
  {
    if (n < 0 || n > s.Count) throw new ArgumentOutOfRangeException("n");
    return new ArraySegment<T>(s.Array, s.Offset, n);
  }
}

public class Program
{
  public static void Main(string[] args)
  {
    int[] xs = new int[] { 10, 20, 30, 40, 50 };
    ArraySegment<int> view = new ArraySegment<int>(xs, 1, 3); // 20,30,40
    ArraySegment<int> head = Seg.Take<int>(view, 2);          // 20,30
    ArraySegment<int> tail = Seg.Skip<int>(view, 1);          // 30,40
    Console.WriteLine(head.Array[head.Offset] + "," + head.Array[head.Offset + 1]);
    Console.WriteLine(tail.Array[tail.Offset] + "," + tail.Array[tail.Offset + 1]);
  }
}

Window-to-window copy

Use Buffer.BlockCopy to move data between windows without creating new arrays.

using System;

public static class SegCopy
{
  public static int Copy(ArraySegment<byte> src, ArraySegment<byte> dst)
  {
    int n = src.Count < dst.Count ? src.Count : dst.Count;
    Buffer.BlockCopy(src.Array, src.Offset, dst.Array, dst.Offset, n);
    return n;
  }
}

public class Program
{
  public static void Main(string[] args)
  {
    byte[] a = new byte[] { 1, 2, 3, 4, 5, 6 };
    byte[] b = new byte[] { 0, 0, 0, 0, 0, 0 };

    ArraySegment<byte> winA = new ArraySegment<byte>(a, 2, 3); // 3,4,5
    ArraySegment<byte> winB = new ArraySegment<byte>(b, 1, 3); // target

    int copied = SegCopy.Copy(winA, winB);
    Console.WriteLine("Copied: " + copied);
    Console.WriteLine(string.Join(",", b)); // 0,3,4,5,0,0
  }
}

Header+payload parsing

Parse by offset and create a payload window. Convert to text only when needed.

using System;
using System.Text;

public static class Parser
{
  // Message layout: [len:2 bytes little-endian][type:1 byte][payload:len bytes]
  public static void Parse(ArraySegment<byte> msg)
  {
    if (msg.Count < 3) throw new ArgumentException("message too short");

    int o = msg.Offset;
    byte[] arr = msg.Array;

    int len = arr[o] | (arr[o + 1] << 8); // ushort little-endian
    byte typ = arr[o + 2];

    ArraySegment<byte> payload = new ArraySegment<byte>(arr, o + 3, len);
    Console.WriteLine("Type=" + typ + " Len=" + len);

    // Materialize only at the boundary (to string for display)
    string text = Encoding.ASCII.GetString(payload.Array, payload.Offset, payload.Count);
    Console.WriteLine("Payload: " + text);
  }
}

public class Program
{
  public static void Main(string[] args)
  {
    // Build: len=5 ("Hello"), type=42
    byte[] buf = new byte[3 + 5];
    buf[0] = 5; buf[1] = 0; buf[2] = 42;
    byte[] hello = Encoding.ASCII.GetBytes("Hello");
    Buffer.BlockCopy(hello, 0, buf, 3, 5);

    Parser.Parse(new ArraySegment<byte>(buf, 0, buf.Length));
  }
}

Read-only by convention

Read-only behavior is a convention in C# 6 with ArraySegment: never mutate inside functions that promise reading only.

using System;

public static class Checksums
{
  // Treat the segment as "read-only": never write to seg.Array here.
  public static int SumBytes(ArraySegment<byte> seg)
  {
    int end = seg.Offset + seg.Count;
    int total = 0;
    for (int i = seg.Offset; i < end; i++) total += seg.Array[i];
    return total & 0xFF;
  }
}

public class Program
{
  public static void Main(string[] args)
  {
    byte[] data = new byte[] { 10, 20, 30, 40, 50 };
    ArraySegment<byte> window = new ArraySegment<byte>(data, 1, 3); // 20,30,40
    int checksum = Checksums.SumBytes(window);
    Console.WriteLine("Checksum = " + checksum);
  }
}

Copy at boundaries

Copy only at the edge (sending to another layer, logging, or storing). Keep inner steps zero-copy.

using System;

public static class Materialize
{
  public static T[] ToArray<T>(ArraySegment<T> s)
  {
    T[] copy = new T[s.Count];
    Array.Copy(s.Array, s.Offset, copy, 0, s.Count);
    return copy;
  }
}

public class Program
{
  public static void Main(string[] args)
  {
    int[] data = new int[] { 5, 6, 7, 8, 9 };
    ArraySegment<int> middle = new ArraySegment<int>(data, 1, 3); // 6,7,8
    int[] copy = Materialize.ToArray<int>(middle); // copy at boundary
    Console.WriteLine(string.Join(",", copy));
  }
}

Why windows help

Quick check: Why pass ArraySegment to a parser instead of byte[]?

Recap

Recap: Use ArraySegment<T> as a Memory-like window. Compose with Skip/Take/Slice, parse by offsets, and copy only when you must.

Frequently asked questions

Is the “Memory/ReadOnlyMemory emulation: zero-copy windows” lesson free?

Yes — the full text of “Memory/ReadOnlyMemory emulation: zero-copy windows” is free to read here on the web, and the C# Academy course includes 3 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the C# Academy course, upgrade to CoddyKit PRO.

What will I learn in “Memory/ReadOnlyMemory emulation: zero-copy windows”?

Pass ArraySegment instead of arrays: create windows, implement Skip/Take/Slice, parse headers and payloads, and only copy at boundaries. You practise C# Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

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How long does the “Memory/ReadOnlyMemory emulation: zero-copy windows” lesson take?

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All lessons in this course

Span/ReadOnlySpan fundamentals (C# 6 emulation)
Memory/ReadOnlyMemory emulation: zero-copy windows
String-as-Span APIs (C# 6 emulation)

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