Add an indexer in an interface

/*
C#: The Complete Reference
by Herbert Schildt

Publisher: Osborne/McGraw-Hill (March 8, 2002)
ISBN: 0072134852
*/
// Add an indexer in an interface.

using System;

public interface ISeries {
// an interface property
int next {
get; // return the next number in series
set; // set next number
}

// an interface indexer
int this[int index] {
get; // return the specified number in series
}
}

// Implement ISeries.
class ByTwos : ISeries {
int val;

public ByTwos() {
val = 0;
}

// get or set value using a property
public int next {
get {
val += 2;
return val;
}
set {
val = value;
}
}

// get a value using an index
public int this[int index] {
get {
val = 0;
for(int i=0; i