C# 0 The Complete Reference

PhoneList

 to access the properties 

Name

 and 

Number

 for any type of telephone list. It also guarantees that only valid types are used to 

construct a 

PhoneList

 object. Notice that 

PhoneList

 throws a 

NotFoundException

 if a name 

or number is not found. This is a custom exception that is declared as shown here:

class NotFoundException : Exception {

  /* Implement all of the Exception constructors. Notice that

     the constructors simply execute the base class constructor.

     Because NotFoundException adds nothing to Exception,

     there is no need for any further actions. */

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  public NotFoundException() : base() { }

  public NotFoundException(string str) : base(str) { }

  public NotFoundException(string str, Exception inner) :

    base(str, inner) { }

  protected NotFoundException(

    System.Runtime.Serialization.SerializationInfo si,

    System.Runtime.Serialization.StreamingContext sc) :

      base(si, sc) { }

}

Although only the default constructor is used by this example, 

NotFoundException

implements all of the constructors defined by 

Exception

 for the sake of illustration. 

Notice that these constructors simply invoke the equivalent base class constructor 

defined by 

Exception

. Because 

NotFoundException

 adds nothing to 

Exception

, there 

is no reason for any further action.

The following program puts together all the pieces and demonstrates 

PhoneList

. Notice 

that a class called 

EmailFriend

 is also created. This class does not inherit 

PhoneNumber

.

Thus, it 

cannot

 be used to create a 

PhoneList

.

// A more practical demonstration of a base class constraint.

using System;

// A custom exception that is thrown if a name or number is not found.

class NotFoundException : Exception {

  /* Implement all of the Exception constructors. Notice that

     the constructors simply execute the base class constructor.

     Because NotFoundException adds nothing to Exception,

     there is no need for any further actions. */

  public NotFoundException() : base() { }

  public NotFoundException(string str) : base(str) { }

  public NotFoundException(string str, Exception inner) :

    base(str, inner) { }

  protected NotFoundException(

    System.Runtime.Serialization.SerializationInfo si,

    System.Runtime.Serialization.StreamingContext sc) :

      base(si, sc) { }

}

// A base class that stores a name and phone number.

class PhoneNumber {

  public PhoneNumber(string n, string num) {

    Name = n;

    Number = num;

  }

  public string Number { get; set; }

  public string Name { get; set; }

}

// A class of phone numbers for friends.

class Friend : PhoneNumber {

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  public Friend(string n, string num, bool wk) :

    base(n, num)

  {

    IsWorkNumber = wk;

  }

  public bool IsWorkNumber { get; private set; }

  // ...

}

// A class of phone numbers for suppliers.

class Supplier : PhoneNumber {

  public Supplier(string n, string num) :

    base(n, num) { }

  // ...

}

// Notice that this class does not inherit PhoneNumber.

class EmailFriend {

  // ...

}

// PhoneList can manage any type of phone list

// as long as it is derived from PhoneNumber.

class PhoneList where T : PhoneNumber {

  T[] phList;

  int end;

  public PhoneList() {

    phList = new T[10];

    end = 0;

  }

  // Add an entry to the list.

  public bool Add(T newEntry) {

    if(end == 10) return false;

    phList[end] = newEntry;

    end++;

    return true;

  }

  // Given a name, find and return the phone info.

  public T FindByName(string name) {

    for(int i=0; i

      // Name can be used because it is a member of

      // PhoneNumber, which is the base class constraint.

      if(phList[i].Name == name)

        return phList[i];

    }

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