Equivalent of StringBuilder for byte arrays

Would MemoryStream work for you? The interface might not be quite as convenient, but it offers a simple way to append bytes, and when you are done you can get the contents as a byte[] by calling ToArray().

A more StringBuilder-like interface could probably be achieved by making an extension method.

Update
Extension methods could look like this:

public static class MemoryStreamExtensions{    public static void Append(this MemoryStream stream, byte value)    {        stream.Append(new[] { value });    }    public static void Append(this MemoryStream stream, byte[] values)    {        stream.Write(values, 0, values.Length);    }}

Usage:

MemoryStream stream = new MemoryStream();stream.Append(67);stream.Append(new byte[] { 68, 69 });byte[] data = stream.ToArray();  // gets an array with bytes 67, 68 and 69

The MemoryStream approach is good, but if you want to have StringBuilder-like behavior add a BinaryWriter. BinaryWriter provides all the Write overrides you could want.

MemoryStream stream = new MemoryStream();BinaryWriter writer = new BinaryWriter(stream);writer.Write((byte)67);writer.Write(new byte[] { 68, 69 });

Probably List<byte>:

var byteList = new List<byte>();byteList.Add(42);byteList.AddRange(new byte[] { 1, 2, 3 });