Question

public abstrct class Item
{
   public string Name {get;set;}
}

public class Music : Item
{
    public double Price {get;set;} 
}

public class Game : Item
{
  public string Image {get;set;}
}

public class Inventory
{

private IList<Item> _games;
private IList<Item> _musics;

public Inventory()
{
  _games = new List<Item>();
  _musics = new List<Item>();
}

public void Add<T>(T item) where T : Item
{
if(typeof(T) == typeof(Game))
{
    _game.add(item);
}
if(typeof(T) == typeof(Music))
{
    _muisc.add(item);
}


public List<T> GetCollection<T>() where T : Item
{
  return (List<T>) _muiscs;
}

class Porgram
{
  static void Main(string[] args)
{
   Inventory inventory = new Inventory();
   var music1 = new Music(){ Name ="aa", Price = 10};
   var Music2 = new Music() { Name ="bb", price = 20 };

inventory.add(music1);
inventory.add(music2);


List<Music> myMusics = inventory.GetCollection<Music>();


}

The code will compile but it will throw exception when try to Call Get Collection method.

I am not sure really why? I am guess i am using generic incorrect.

Answer 1

A List<Item> cannot be cast to a List<Music>. While Music is a subclass of Item, generic types do not follow the same inheritance pattern as their collection type. The simplest way to fix your code would be to replace the cast in your GetCollection method with a call to the Linq extension method cast, followed by ToList. That said, I think your entire class could be redesigned to handle this sort of inheritence better.

So, your GetCollection method looks like this:

public List<T> GetCollection<T>() where T : Item
{
    return _musics.Cast<T>().ToList();
}