Question

I have 3 classes (A,B,C) and have to implement a storing method for all of the classes so I thought that of using a generic list like List<T> = new List<T>(); but it doesn’t allow me to use it.

I would like the method to be like this:

class Bascket
{
   List<T> list= new List<T>();

   public void addToBasket(T value)
   {
      list.Add(value);
   }
}

Answer 1

Assuming that A, B and C are items which you wish to store in the Basket object, you should create a base class of those items, and declare generic collection as collection of base class, i.e.

public interface IBasketItem
{ 
    /* put some common properties and methods here */
    public decimal Price { get; set; }
    public string Name { get; set; }
}
public class A : IBasketItem
{ /* A fields */ }
public class B : IBasketItem
{ /* B fields */ }
public class C : IBasketItem
{ /* C fields */ }

public class Basket
{
    private List<IBasketItem> _items = new List<IBasketItem>();

    public void Add(IBasketItem item)
    {
        _items.Add(item);
    }

    public IBasketItem Get(string name)
    {
        // find and return an item
    }
}

You can then use Basket class to store all your items.

Basket basket = new Basket();
A item1 = new A();
B item2 = new B();
C item3 = new C();
basket.Add(item1);
basket.Add(item2);
basket.Add(item3);

However, when retrieving items back, you should use common interface, or you should know of which type the object actually is. E.g:

IBasketItem myItem = basket.Get("cheese");
Console.WriteLine(myItem.Name);
// Take care, if you can't be 100% sure of which type returned item will be
// don't cast. If you cast to a wrong type, your application will crash.
A myOtherItem = (A)basket.Get("milk");
Console.WriteLine(myOtherItem.ExpiryDate);