public AESCBC(byte[] key, byte[16] inputIV)
{

   //blah blah

}

Contract.Requires(inputIV.Length == 16);

public AESCBC(byte[] key, byte[] inputIV)
{
   if(inputIV.Length != 16)
       throw new ArgumentException("inputIV should be byte[16]");

   //blah blah

}