title: multiply very big integer in .NET
body: <p>Assume that you have big integers like 1253999939979969998746 and 9999999999. Is there anyway to multiply these big integers in C#? </p>
  
  <p>Note: I've tried <code>System.Numerics.BigInteger</code> class constructor compiler says that <em>integer is too long</em>.</p>
  
  <p>What's your suggestion?</p>
  
  <p>p.s warn me if this question out of topics. </p>