SOMME MD5 D'UN FICHIER

public static string MD5(string file)
{
   if (System.IO.File.Exists (file))
   {
      System.IO.FileStream st=null;
      try
      {
         System.Security.Cryptography.MD5CryptoServiceProvider check;
         st= System.IO.File.Open (file,System.IO.FileMode.Open,System.IO.FileAccess.Read);
         check = new System.Security.Cryptography.MD5CryptoServiceProvider();
         byte[] somme = check.ComputeHash (st);
         string ret = "";
         foreach (byte a in somme)
         {
            if (a<16)
               ret += "0" + a.ToString ("X");
            else
               ret += a.ToString ("X");
         }
         return ret ;
      }
      catch 
      {
         throw;
      }
      finally
      {
         if ( st != null)
            st.Close();
      }
   }
   else
   {
      throw new System.IO.FileNotFoundException ("Fichier non trouvé.",file);
   }
}