Question

Possible Duplicate:
why use IList or List?

I have the following:

public class CityViewModel
{
    public CityViewModel() {     
        Details = Enumerable.Range(1,10).Select(x => new City.Detail()).ToList(); 
    }
    public string Topic { get; set; }
    public City City { get; set; }
    public IList<City.Detail> Details { get; set; }
}

The suggestion as a way to create Details was given to me. What I am unsure about is why the following line is declared as an IList:

public IList<City.Detail> Details { get; set; }

Would it not be more correct to declare this as:

public List<City.Detail> Details { get; set; }

Can someone explain the difference to me.

Answer 1

Using an interface rather than the class is definitely the right way to go, both for information hiding and testability reasons. I would even go further, and suggest using ICollection<T> instead of IList<T> if you can: this would give you even more flexibility as far as choosing an alternative implementation goes, while letting your users perform mostly the same operations.

Moreover, I would probably not use an automatic assignable property for it: I’d return a read-only version, and provided separate methods for manipulating the list on my own class.

Here is what I would do:

public class CityViewModel {
    private readonly List<City.Detail> details;
    public CityViewModel() {
        // Here you assign a List to details
    }
    public ICollection<City.Detail> Details {
        get {
            return details.AsReadOnly();
        }
    }
}

This hides implementation of the list from users, does not let them modify the list behind your back, and does not even tell them that you’re using a List. If you would decide to use, say, a HashSet at some later date, you’d be able to swap it in without a fear of breaking something in the code that uses your class.