Why i can not use int ItemIndex = ItemIndexOfID in C#.Thanks

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

namespace Shopping
{
    public class Cart
    {
        private int _IDproduct;
        private string _nameProduct;
        private int _Quantity;
        private string _ImageUrl;
        private double _Price;
        private double _Total;




       public Cart(){}
       public  Cart(int IDProduct, string NameProduct, int Quantity, string ImageURL, double Price){

           _IDproduct = IDProduct;
           _nameProduct = NameProduct;
           _Quantity = Quantity;
           _ImageUrl = ImageURL; ;
           _Price = Price;
           _Total = Price*Quantity;
                }
       public int IDProduct {
           get { return _IDproduct; }
           set { _IDproduct = value; }
       }

       public string NameProduct
       {
           get { return _nameProduct; }
           set { _nameProduct = value; }
       }

       public int Quantity
       {
           get { return _Quantity; }
           set { _Quantity = value; }
       }

       public string ImageURL
       {
           get { return _ImageUrl; }
           set { _ImageUrl = value; }
       }

       public double Price
       {
           get { return _Price; }
           set { _Price = value; }
       }

       public double Total
       {
           get { return _Price*_Quantity; }

       }
             }

        public  class CartItem{

            private DateTime _dateCreate;
            private DateTime _dateUpdate;
            private List<Cart> _Item;
            public CartItem() {

             if(this._Item==null){

                     this._Item = new List<Cart>();
                     this._dateCreate = DateTime.Now;
             }
                        }

            public List<Cart> Item {
                get { return _Item; }
                set { _Item = value; }
            }

            public void Insert(int IDProduct, int Quantity, string ImageURL, string NameProuct, double Price) {

   int ItemIndex = ItemIndexOfID(ProductID);Error at ItemIndexOfID?
            
}


        }



}