Override Index Return in C# Collection

Question

I am adding functionality to an existing .Net collection. In order to do this, I am overriding several functions. I have not been able to override the return value of the basic array return in the collection. For example, if I call emailMessage.To[i], it does not return the proper value, but if I call emailMessage.Item(i), it returns the correct value. Below is the code from my class. What do I need to override to correct the first error?

namespace EmailService
{
    public class MailAddressCollection : System.Net.Mail.MailAddressCollection
    {
        public MailAddressCollection() : base()
        {
        }

        public void Add(MailAddress Address)
        {
             base.Add(Address);
        }

        public MailAddress Item(int index)
        {
            return (MailAddress)(base.Items[index]);
        }

    }
}

possible duplicate of [How do I overload the square-bracket operator in C#?](http://stackoverflow.com/questions/287928/how-do-i-overload-the-square-bracket-operator-in-c) — , Jul 21 '10 at 19:02
See this question: [How do I overload the square-bracket operator in C#?](http://stackoverflow.com/questions/287928/how-do-i-overload-the-square-bracket-operator-in-c) — , Jul 21 '10 at 19:02
I think it's better to use composition instead of inheritance in this case to adhere to the [Liskov substitution principle](http://en.wikipedia.org/wiki/Liskov_substitution_principle) — Maksymilian Majer, Oct 07 '14 at 11:48

score 8 · Accepted Answer · answered Jul 21 '10 at 19:01

8

public MailAddress this[int index] 
{
   get { return ((MailAddress)(base.Items[index]); }
}

answered Jul 21 '10 at 19:01

Adam Lear

38,111
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@Brian: it can be hidden with the 'new' keyword. – Adam Lear Jul 21 '10 at 19:18
I needed the new keyword. "public new MailAddress this[int index]" – SchwartzE Jul 21 '10 at 19:23

score 8 · Answer 2 · answered Jul 21 '10 at 19:06

You are a little bit out of luck, because MailAddressCollection doesn't set the indexer as virtual.

You CAN use the new keyword:

public new MailAddress this[int index]
{
    get
    {
        return base[index];    
    }           
}

But, your indexer will only work if you have a reference to YOUR collection type. If you use polymorphism, your indexer will not get called.

score 0 · Answer 3 · answered Jul 21 '10 at 19:09

0

Or alternatively

    public MailAddress this[int index]
    {
        get { return base[index] as MailAddress; }
    }

answered Jul 21 '10 at 19:09

John Alexiou

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Override Index Return in C# Collection

3 Answers3