using System;

using System.IO;

public class Program

{

    public static void Main()

    {

        var x = new MemoryStream(); x.Write(new byte[] { 1, 2, 3 });

        var y = new MemoryStream(); y.Write(new byte[] { 4, 5, 6 }); 

        x.Position = x.Length;

        y.Position = 0;

        y.CopyTo(x);

        Console.WriteLine(BitConverter.ToString(x.ToArray()));

    }

}