Trying to make it so when two of the same products are added to cart it increments quantity instead

var existingProduct = _cartService.GetCartItems().SingleOrDefault(cartItem => cartItem.Equals(productToAdd));

if (existingProduct != null)
{
    // Product already exists in the cart, increase its quantity
    existingProduct.Quantity++;
    existingProduct.Name = "Test1";
}
else
{
    // Product does not exist in the cart, add it with quantity 1
    productToAdd.Quantity = 1;
    productToAdd.Name = "Test2";
    _cartService.AddToCart(productToAdd);
}

var existingProduct = _cartService.GetCartItems().SingleOrDefault(cartItem => cartItem.Equals(productToAdd));

if (existingProduct != null)
{
    // Product already exists in the cart, increase its quantity
    existingProduct.Quantity++;
    existingProduct.Name = "Test1";
}
else
{
    // Product does not exist in the cart, add it with quantity 1
    productToAdd.Quantity = 1;
    productToAdd.Name = "Test2";
    _cartService.AddToCart(productToAdd);
}

public record Product(int Id, string Name);

public class CartItem(Product product, int count)
{
    public Product Product { get; init; } = product;
    public int Count { get; set; } = count;
}

public class Cart
{
    private readonly List<CartItem> _items = [];

    public void Add(Product product)
    {
        var cartItem = _items.SingleOrDefault(x => x.Product.Id == product.Id);

        if (cartItem == null)
        {
            _items.Add(new CartItem(product, 1));
            return;
        }

        cartItem.Count++;
    }
}

public record Product(int Id, string Name);

public class CartItem(Product product, int count)
{
    public Product Product { get; init; } = product;
    public int Count { get; set; } = count;
}

public class Cart
{
    private readonly List<CartItem> _items = [];

    public void Add(Product product)
    {
        var cartItem = _items.SingleOrDefault(x => x.Product.Id == product.Id);

        if (cartItem == null)
        {
            _items.Add(new CartItem(product, 1));
            return;
        }

        cartItem.Count++;
    }
}

var computer = new Product(2, "Computer");
var monitor = new Product(1, "Monitor");

var cart = new Cart();

cart.Add(computer);
cart.Add(computer);
cart.Add(monitor);
cart.Add(monitor);

Console.WriteLine(cart);

var computer = new Product(2, "Computer");
var monitor = new Product(1, "Monitor");

var cart = new Cart();

cart.Add(computer);
cart.Add(computer);
cart.Add(monitor);
cart.Add(monitor);

Console.WriteLine(cart);

2x Computer
2x Monitor

2x Computer
2x Monitor