Question

I need a List of items with sharable behavior.

Example:

Person class

has List<PhoneNumber>
where only one PhoneNumber is bool IsPrimary
same for other items
has List<EmailAddress>
has List<Address>

Imagine each item (PhoneNumber, EmailAddress, Address) share the same interface ICanBePrimary which contains the requirement of one property bool IsPrimary and when in a List<ICanBePrimary> only one item in the list can have a true value for IsPrimary.

Answer 1

The cleanest approach would be to hide a list behind a class that lets you enumerate over the content, and provides additional methods for identifying the primary item:

class AddressList : List<Address> {
    private int indexOfPrimaryAddress = 0;
    public Address PrimaryAddress {
        get {
            return this[indexOfPrimaryAddress];
        }
        set {
            indexOfPrimaryAddress = this.IndexOf(value);
        }
    }
    // Override more methods to make sure that the index does not become "hanging"
}

An even cleaner implementation would encapsulate the list inside your AddressList class, and expose only the methods that you want exposed:

class AddressList : IList<Address> {
    private int indexOfPrimaryAddress = 0;
    private readonly IList<Address> actualList = new List<Address>();
    // Implement the List<Address> by forwarding calls to actualList
}