How to delete last character in a string in C#?

Building a string for post request in the following way,

  var itemsToAdd = sl.SelProds.ToList();
  if (sl.SelProds.Count() != 0)
  {
      foreach (var item in itemsToAdd)
      {
        paramstr = paramstr + string.Format("productID={0}&", item.prodID.ToString());
      }
  }

after I get resulting paramstr, I need to delete last character & in it

How to delete last character in a string using C#?