why do we need to create new instence of Icollection type property

Question

I have a couple of properties in my class which are of ICollection type. When I try to add any data in those properties without creating their new instance compiler shoots me an error saying I need to create instance of them. My question is, when I create a new instance of a class, then indirectly I create a new memory for the entire class and those properties (ICollection type) are inside class, so why do I need to create new instance for those properties.

public class User
{
    public string UserId { get; set; }
    public ICollection<Address> Addresses { get; set; }
    public ICollection<Contact> Contacts { get; set; }

}

public class Address
{
    public string AddressId { get; set; }

}

public class Contact
{
    public string ContactId { get; set; }
}

class Program
{
    static void Main(string[] args)
    {
        User user = new User();

        user.Contacts = new Collection<Contact>() {new Contact() {}};

        user.Addresses.Add(new Address()
        {
        });//Throws Error

        Console.ReadKey();
    }
}

score 4 · Accepted Answer · edited May 23 '17 at 10:26

Because ICollection is an interface and interfaces are reference types. That means by creating a variable, you did not create an instance. That only happens when you call new. You only created a reference to an instance.

By default, a reference is null which means it's not set to any particular instance. That's why when you call a method on a reference that was not set to an instance, you get a NullReferenceException.

score 1 · Answer 2 · answered Mar 21 '15 at 14:03

Agreed with everything that nvoigt said in their answer. All reference type properties (anything other than int, DateTime, string, etc.) defaults to a null when they are a property on a new object.

What you'll want to do in order to get around that NullReferenceException is to instantiate all of those properties when you create a new object.

public User() {
    this.Addresses = new List<Address>();
    this.Contacts = new List<Contact>();
}

With that in your constructor, you'll be able to add items to those collections without having to instantiate them outside your objects.

why do we need to create new instence of Icollection type property

2 Answers2