What does "this[0]" mean in C#?

Question

I was going through some library code and saw a method like:

public CollapsingRecordNodeItemList List
{
    get { return this[0] as CollapsingRecordNodeItemList; }
}

The class that contains this method is not a list or something iterable, so what exactly does this[0] mean?

the class has an [indexer](http://msdn.microsoft.com/en-us/library/vstudio/6x16t2tx.aspx) — Paolo Falabella, May 20 '13 at 13:07
@Nick I just saw that one of the parent classes has a line like `public GoObject this[string name] { get; set; }` maybe its related to this ? — Cemre Mengü, May 20 '13 at 13:10
@Cemre see if there is a `this[int index]` or `this[long index]` or something like that... you can't pass an integer (`0`) in as `string name`. — Marc Gravell, May 20 '13 at 13:12
@Cemre, thats just another indexer. The parent class is indexed with the `string` type. — Jodrell, May 20 '13 at 13:12
Marc is correct.. it is the same idea - it is an indexer but it is not the one your line is referring.. — G.Y, May 20 '13 at 13:14
Yeah as you said there was another one: `public virtual GoObject this[int index] { get; set; }`. Thanks for the comments! — Cemre Mengü, May 20 '13 at 13:26
Just an additional piece of information that might help future readers. Indexers were first introduced in the abstract class Collectionbase. This feature was later also made a part of List. — developer747, May 20 '13 at 18:38

score 96 · Accepted Answer · answered May 20 '13 at 13:07

Look for an indexer in the class.

C# lets you define indexers to allow this sort of access.

Here is an example from the official guide for "SampleCollection".

public T this[int i]
    {
        get
        {
            // This indexer is very simple, and just returns or sets 
            // the corresponding element from the internal array. 
            return arr[i];
        }
        set
        {
            arr[i] = value;
        }
    }

Here is the definition from the official language specification:

An indexer is a member that enables objects to be indexed in the same way as an array. An indexer is declared like a property except that the name of the member is this followed by a parameter list written between the delimiters [ and ]. The parameters are available in the accessor(s) of the indexer. Similar to properties, indexers can be read-write, read-only, and write-only, and the accessor(s) of an indexer can be virtual.

One can find the full and complete definition in section 10.9 Indexers of the specification.

I discovered a property in one of the parent classes but the index is `string` instead of `int` : `public GoObject this[string name] { get; set; }` . I guess there is another one somewhere else. — Cemre Mengü, May 20 '13 at 13:12
@Cemre The language specification states that (section 1.6.7.3) "Indexers can be overloaded, meaning that a class can declare multiple indexers as long as the number or types of their parameters differ." :) This might be the case here — Benjamin Gruenbaum, May 20 '13 at 13:13
Thanks for the comment. Just found the other one which was: `public virtual GoObject this[int index] { get; set; }` :) — Cemre Mengü, May 20 '13 at 13:23

score 10 · Answer 2 · answered May 20 '13 at 13:07

It means that the declaring type (or a base-class of that) has an "indexer" which presumably takes an int (or similar) and returns... something (perhaps object ?). The code calls the indexer's get accessor, passing 0 as the index - and then treats the returned value as a CollapsingRecordNodeItemList (or null the returned value isn't compatible with that).

For example:

public object this[int index] {
    get { return someOtherList[index]; }
}

Easiest thing to do is the step into it, though. That will tell you exactly where it is going.

score 2 · Answer 3 · edited May 20 '13 at 13:22

2

Assuming the class itself inherits from some form if IList/IList<T>, it's just returning (and casting) the first element in the collection.

public class BarCollection : System.Collections.CollectionBase
{
    public Bar FirstItem
    {
        get { return this[0] as Bar; }
    }

    #region Coming From CollectionBase
    public Object this[ int index ]  {
        get { return this.InnerList[index]; }
        set { this.InnerList[index] = value; }
    }
    #endregion
}

edited May 20 '13 at 13:22

user

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answered May 20 '13 at 13:07

Brad Christie

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4

`IEnumerable` does not define an indexer, I suspect you mean `IList`. – Jodrell May 20 '13 at 13:10
2

Indeed, still waking up and working on coffee. ;-) Good catch, thank you. – Brad Christie May 20 '13 at 13:11

score 1 · Answer 4 · answered May 20 '13 at 13:07

1

It means to invoke the item property's get method on this class. It's called the class's Indexer

Indexers allow instances of a class or struct to be indexed just like arrays. Indexers resemble properties except that their accessors take parameters.

answered May 20 '13 at 13:07

Damien_The_Unbeliever

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What does "this[0]" mean in C#?

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