What does “this [0]” mean in C #?

Question

I looked through some library code and saw a method like:

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

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

Answer 1

Find the indexer in the class.

C # allows you to define indexers to allow this kind 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 :

Index is a member that allows objects to be indexed just like an array. An indexer is declared as a property, except that it is the name of the element, followed by a list of parameters written between the delimiters [and]. Parameters are available in the indexer accessories (s). Like properties, indexers can be read and written, read-only and write-only, and indexer accessors (and) can be virtual.

A complete and complete definition can be found in section 10.9 of the Indexers specification.

Answer 2

This means that the declaration type (or base class) has an “indexer” that supposedly takes int (or similar) and returns ... something (maybe an object ?). The code calls the get accessor indexer, passing 0 as an index - and then treats the return value as CollapsingRecordNodeItemList (or null return value is incompatible with this).

For example:

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

The easiest way to do this is step into it. This will tell you exactly where it is going.

Answer 3

Assuming the class itself is inherited from some form, if IList / IList<T> , it simply returns (and distinguishes) 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 } 

Answer 4

This means calling the item property get method in this class. He called the Indexer class

Indexers allow you to index instances of a class or structure as arrays. Indexers resemble properties, except that their accessors accept parameters.