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?

Answer 1

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.

Answer 2

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.