Filter in getter on a collection

Question

public Schedule 
{
    public Schedule()   
    {    
        Payments = new List<Payment>();
    }

    public List<Payment> Payments 
    {
        get => Payments.Where(x => x.IsActive).ToList();
        set { };
    }
}

I want to return only Active payments from a collection whenever I get it. Is there a way to make this work? The problem is the code is setting the entire collection and also doing Adds to it throughout the application so I can't just use a private variable and call

set { _payments = value }

In other words, there are places where

Payments = new List<Payment>();
// elsewhere
Payments.Add(payment);

I want to make sure whenever the list is referenced anywhere that only the items in the list that are returned are IsActive.

Anybody know how to make this work?

Answer 1

If you want Payments to return a new collection, adding items to that new collection just isn't going to alter the collection your class owns.

This will work.

private List<Payment> _payments = new List<Payment>;

public IEnumerable<Payment> Payments => _payments.Where(x => x.IsActive);

public void AddPayment(Payment pmt) => _payments.Add(pmt);  

//  You can write ClearPayments(), RemovePayment(), etc.

You might instead want to write a subclass PaymentList : IList<T> that has an internal List<Payment>. It would have special behavior for GetEnumerator() and the indexer, and simply forward everything else to the private collection. That gets to be a lot of work though, when you start thinking about it.

It depends on what you really need here. If the rest of your code needs Schedule.Payments to return something that can be treated as a List<Payment>, something will have to change.