Chris Pietschmann

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C#: Give your object a Default Indexer Property

I've seen/used objects within the .NET Framework that have default indexer properties (ie: SqlDataReader, System.Collections.Generic.List). Now how exactly do I give my own custom object type a default indexer property?

Well, it's actually rather simple. Heres an example of a Club object with a People collection of type Person that has a default indexer property set up:

public class Club
{
  /// <summary>
  /// Collection of Person objects in the Club
  /// </summary>
  private List<Person> _people = new List<Person>();
  public List<Person> People
  {
    get { return _people; }
    set { _people = value; }
  }

  /// <summary>
  /// The Default property of the Club object
  /// </summary>
  public Person this[int index]
  {
    get { return _people[index]; }
    set { _people[index] = value; }
  }
}

public class Person
{
  public Person(string name)
  {
    this._name = name;
  }

  private string _name;
  public string Name
  {
    get { return _name; }
    set { _name = value; }
  }
}

And, here's example code of using that object:

// Create instance of Club object
Club club = new Club();

// Add a couple Person objects to the Peopl collection of the Club
club.People.Add(new Person("Chris"));
club.People.Add(
new Person("Kate"));

// Change the name of the first Person in the People
// collection using the traditional method
club.People[0].Name = "Joe";

// Change the name of the first Person in the People
// collection using the Club objects Default property
club[0].Name = "John";

And, remember just like any other method/property the Default Indexer Property can be overloaded. For instance, you can define one that accepts an Index as type integer, and also have one that accepts an Index of type string that does a dictionary lookup.

Update 3/23/2007: I have updated this post and changed the title, since there seems to be some confusion as to what I'm describing is a Default Property or Indexer Property. I think I'll just refer to it as a Default Indexer Property. Sorry for the confusion.

For further reference: MSDN: Default Properties for your Components

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