C# 0 The Complete Reference

PART I

C h a p t e r   1 2 :  

I n t e r f a c e s ,   S t r u c t u r e s ,   a n d   E n u m e r a t i o n s  

315

PART IPART I

  public int GetNext() {

    prev = val;

    val += 2;

    return val;

  }

  public void Reset() {

    val = start;

    prev = start - 2;

  }

  public void SetStart(int x) {

    start = x;

    val = start;

    prev = val - 2;

  }

  // A method not specified by ISeries.

  public int GetPrevious() {

    return prev;

  }

}

Notice that the addition of 

GetPrevious( )

 required a change to the implementations of the 

methods defined by 

ISeries

. However, since the interface to those methods stays the same, 

the change is seamless and does not break preexisting code. This is one of the advantages 

of interfaces.

As explained, any number of classes can implement an 

interface

. For example, here is a 

class called 

Primes

 that generates a series of prime numbers. Notice that its implementation 

of

ISeries

 is fundamentally different than the one provided by 

ByTwos

.

// Use ISeries to implement a series of prime numbers.

class Primes : ISeries {

  int start;

  int val;

  public Primes() {

    start = 2;

    val = 2;

  }

  public int GetNext() {

    int i, j;

    bool isprime;

    val++;

    for(i = val; i < 1000000; i++) {

      isprime = true;

      for(j = 2; j <= i/j; j++) {

        if((i%j)==0) {

          isprime = false;

          break;

        }

      }

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P a r t   I :  

T h e   C #   L a n g u a g e

      if(isprime) {

        val = i;

        break;

      }

    }

    return val;

  }

  public void Reset() {

    val = start;

  }

  public void SetStart(int x) {

    start = x;

    val = start;

  }

}

The key point is that even though