C# Declaring Type Definition Cannot create an instance of the abstract class or interface

Question

I created a class Cartline. Then I created a collection called ShoppingCart. When I try to declare ShoppingCart, I receive error. Does anyone know to fix this?

public class CartLine
{
    public int CartLineId { get; set; }
    public Product Product { get; set; }
    public int Quantity { get; set; }
}


using ShoppingCart = System.Collections.Generic.IEnumerable<ElectronicsStore.Models.CartLine>;

ShoppingCart shoppingcart = new ShoppingCart();

Cannot create an instance of the abstract class or interface 'IEnumerable<CartLine>'    ElectronicsStore

Answer 1

The easiest solution is to creating a new class called ShoppingCart that has a property which is a List of CartLine entities:

public class CartLine
{
    public int CartLineId { get; set; }
    public Product Product { get; set; }
    public int Quantity { get; set; }
}

public class ShoppingCart
{
    public IList<CartLine> CartLines {get;set;}
}


ShoppingCart shoppingcart = new ShoppingCart();

Answer 2

The error is self explanatory. Two options - you can create an alias using a concrete type (eg List<CartLine> ) but, instead, I'd recommend you define a class that inherits from List<CartLine> (or whatever collection best suits your needs):

public class ShoppingCart : List<CartLine>
{
    // implement constructors you want available
    public ShoppingCart(){}

    public ShoppingCart( IEnumerable<CartLine> collection ) : base( collection ) {}

    public ShoppingCart( int capacity ) : base( capacity ) {}

    // the benefit here is you can add useful properties
    // if CartLine had a price you could add a Total property, for example:
    public decimal Total => this.Sum( cl => cl.Quantity * cl.Price );
}

Then you may use as requested:

var cart = new ShoppingCart();
cart.Add( new CartLine() { ... } );
var cartTotal = cart.Total;
... etc ...

Collection initializer will also work:

var cart = new ShoppingCart() { new CartLine() { ... }, ... }

Or initialize with existing IEnumerable<CartLine> , eg an IQueryable<CartLine> using Entity Framework:

var cart = new ShoppingCart( dbContext.CartLines.Where( ... ) );