Unsafe.cs

using System;
using System.Runtime.InteropServices;
using System.Text;

namespace Unsafe
{
    [StructLayout(LayoutKind.Explicit)]
    public struct Union
    {
        [FieldOffset(0)]
        public long Long;
        [FieldOffset(0)]
        public string String;
    }

    class MainClass
    {
        public static void Main(string[] args)
        {
            var text = "Go home, you're drunk!";
            var thing = new byte[1024];
            thing[16] = checked((byte)text.Length); // length prefix (255 char max due to my laziness)
            Encoding.Unicode.GetBytes(text, 0, text.Length, thing, 20); // the actual string
            var handle = GCHandle.Alloc(thing, GCHandleType.Pinned);
            var mu = new Union();
            mu.Long = handle.AddrOfPinnedObject().ToInt64();

            Console.WriteLine(mu.String.Length);
            Console.WriteLine(mu.String);

            // let's have some fun, just for good measures
            thing[16] = thing[17] = thing[18] = thing[19] = 0xFF;
            Console.WriteLine(mu.String.Length);

            handle.Free();
        }
    }
}