Having prob looping through an array of struct elements

Hi everyone, I am trying to loop through an array of structure elements and based on a condition of equality I want to modify one of the elements of the struct. Here is the code for it. public struct CartItem { public int product_id; public int[] product_option_id; public int qty; public int school_id; } public class Cart { CartItem[] cartItems; public void AddItem(CartItem NewCartItem) { foreach (CartItem ci in cartItems) { if (ci.Equals(NewCartItem)) ci.qty = NewCartItem.qty; } } } The error I am getting is during compile. It complains on line //ci.qty = NewCartItem.qty; The compiler says "The left hand side on an assigment must be a variable, property or indexer. Can someone please help me out here... Thanks, Mansoor.