Use indexers to access elements in a class (or a struct) that encapsulates a collection. You can define multiple indexers, each with parameters of different types.

Indexers can be:

• virtual or abstract
• declared in interfaces
• read-only or read-write
• can have separate access restrictions for setters and getters
• single-dimension indexers with numeric parameters can participate in data binding

Keep in mind that:

• 'get' accessors should not have observable side effects
• when the JIT compiler inline property accessors, the performance of data members and properties is the same

Example: Implement an indexer:

class Tagline
{
    string[] words = "One ring to rule them all".Split();
 
    // Indexer definition
    public string this [int n]
    {
        get
        {
            if (n < 0 || n > words.length-1)
                throw new IndexOutOfRangeException("Index out of range.");
            else
                return words[n];
        }
 
        set
        {
            words[n] = value;
        }
    }
}
...
// Use the indexer.
Tagline s = new Tagline();
Console.WriteLine(s[3]); // displays the word "rule"

An indexer can take more than one parameter (a multidimensional indexer):

public string this [int x, int y]
{
    get { ... }
    set { ... }
}
 
public string this [int arg1, string arg2]
{
    get { ... }
    set { ... }
}

An indexer can use noninteger parameters to define maps and dictionaries:

public City this [string code]
{
    get { ... }
    set { ... }
}