add to cart btn throw selected item not move in shipping cart

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ASP.NET

why this problem occuring plz give ans

add to cart mate three class i made CartItem,ShoppingCart.cs,cat_detail.cs
i don't no where is my mistake

this code is my CartItem.cs code
and
cart tbl have column like:Cart_ID,id,u_nm,name,type,image,price,qty,subTotal

public class CartItem:IEquatable<CartItem>
{
    public int qty{get;set;}
    private int _cartid;
    public int Cart_ID
    {
        get { return _cartid; }
        set { _cartid = value;
        }
    }
    private cat_detail _book = null;
    public cat_detail cat
    {
        get {
            if (_book== null)
            {
               _book =new cat_detail(Cart_ID);
            }
            return _book;
        }
    }
    public string Name
    {
        get { return cat.name; }
    }
    public string Type
    {
        get { return cat.type; }

    }
    public decimal Price
    {
        get { return cat.price; }
    }
    public string Description
    {
        get { return cat.description; }
    }

    public decimal TotalPrice {
        get { return Price * qty; }
    }
    public CartItem(int id)
    {
        this.Cart_ID = id;
    }
    public bool Equals(CartItem id)
    {
        return id.Cart_ID ==this.Cart_ID;
    }
}