C# 0 The Complete Reference

Indexers Do Not Require an Underlying Array

It is important to understand that there is no requirement that an indexer actually operate 

on an array. It simply must provide functionality that appears “array-like” to the user of the 

indexer. For example, the following program has an indexer that acts like a read-only array 

that contains the powers of 2 from 0 to 15. Notice, however, that no actual array exists. 

Instead, the indexer simply computes the proper value for a given index.

// Indexers don't have to operate on actual arrays.

using System;

class PwrOfTwo {

  /* Access a logical array that contains

     the powers of 2 from 0 to 15. */

  public int this[int index] {

    // Compute and return power of 2.

    get {

      if((index >= 0) && (index < 16)) return pwr(index);

      else return -1;

    }

    // There is no set accessor.

  }

  int pwr(int p) {

    int result = 1;

    for(int i=0; i < p; i++)

      result *= 2;

    return result;

  }

}

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class UsePwrOfTwo {

  static void Main() {

    PwrOfTwo pwr = new PwrOfTwo();

    Console.Write("First 8 powers of 2: ");

    for(int i=0; i < 8; i++)

      Console.Write(pwr[i] + " ");

    Console.WriteLine();

    Console.Write("Here are some errors: ");

    Console.Write(pwr[-1] + " " + pwr[17]);

    Console.WriteLine();

  }

}

The output from the program is shown here:

First 8 powers of 2: 1 2 4 8 16 32 64 128

Here are some errors: -1 -1

Notice that the indexer for