It's not a method, its not a variable, its not a property, what is it?

Question

I ran across something I don't understand today. Consider the following snippet:

public class EventStreamCollection<TKey, TValue>
{
    private readonly ConcurrentDictionary<TKey, TValue> _dictionary = new ConcurrentDictionary<TKey, TValue>();
    private readonly Func<TKey, TValue> _factory;
    public EventStreamCollection(Func<TKey, TValue> factory)
    {
        _factory = factory;
    }

    public TValue this[TKey key] => _dictionary.GetOrAdd(key, _factory);
}

What is this line

public TValue this[TKey key] => _dictionary.GetOrAdd(key, _factory);

It has no name that I can see. If it did, I guess it would be a property? What is it and how does it work?

Daniel A. White · Accepted Answer · 2018-05-22T14:59:43.993

8

That is a read-only, indexer property.

Indexers use this as the name. It allows you to support square brackets on an instance of your your type.

Using the => syntax, it makes it read-only.

edited May 22 '18 at 14:59

answered May 22 '18 at 14:56

Daniel A. White

181,601
45
354
430

It allows you to support square brackets on an instance variable of your type. – Philip Smith May 22 '18 at 14:59
@PhilipSmith updated – Daniel A. White May 22 '18 at 15:00
And it's possible only with C#6 or higher – Rahul May 23 '18 at 08:26

It's not a method, its not a variable, its not a property, what is it?

1 Answers1