Creating an Application Using C# Dictionary

Lesson 2

Welcome! Today, we will explore creating a simple address book application using a C# Dictionary. This task will help you understand how to manipulate a Dictionary in C#, focusing on adding, retrieving, and deleting entries. By the end of this lesson, you'll have a solid grasp of these fundamental operations.

Introducing Methods to Implement

In this task, we will implement three methods to manage our address book:

AddContact(string name, string phoneNumber): Adds a new contact. Returns false if the contact already exists; otherwise, it adds the contact and returns true. In this task, let's assume phone numbers do not change, so it's not allowed to overwrite an existing contact's number.
GetContact(string name): Retrieves the phone number for a given name. Returns null if the contact does not exist.
DeleteContact(string name): Deletes a contact with the given name. Returns true if the contact exists and is deleted, false otherwise.

Let's break down each method in detail in the next sections.

Step 1: Implementing AddContact

This method adds a new contact to the address book with the given name and phoneNumber. If the contact already exists, it returns false. Otherwise, it adds the contact and returns true.

Question: Why do you think we need to check if the contact already exists?

Answer: To avoid duplicating existing entries. Also, if a contact with the same name already exists, we shouldn't allow overwriting its phone number in this method, as it's only for creation.

Here is the method implementation:

C#
1using System;
2using System.Collections.Generic;
3
4public class AddressBook
5{
6    private Dictionary<string, string> contacts;
7
8    public AddressBook()
9    {
10        contacts = new Dictionary<string, string>();
11    }
12
13    public bool AddContact(string name, string phoneNumber)
14    {
15        if (contacts.ContainsKey(name))
16        {
17            return false;
18        }
19        contacts.Add(name, phoneNumber);
20        return true;
21    }
22
23    public IEnumerable<KeyValuePair<string, string>> GetAllContacts()
24    {
25        return contacts;
26    }
27}
28
29public class Program
30{
31    public static void Main()
32    {
33        AddressBook addressBook = new AddressBook();
34        Console.WriteLine(addressBook.AddContact("Alice", "123-456-7890")); // True
35        Console.WriteLine(addressBook.AddContact("Alice", "098-765-4321")); // False
36        foreach (var contact in addressBook.GetAllContacts())
37        {
38            Console.WriteLine($"{contact.Key}: {contact.Value}"); // Alice: 123-456-7890
39        }
40    }
41}

In this method:

We verify if the contact already exists using if (contacts.ContainsKey(name)).
If it exists, we return false.
If it doesn't exist, we add it to our Dictionary using contacts.Add(name, phoneNumber); and return true.

Step 2: Implementing GetContact

This method retrieves the phone number associated with a given name. If the contact does not exist, it returns null.

Question: What do we gain by returning null when a contact doesn't exist?

Answer: It provides a clear indicator that the contact is not in the address book, allowing us to handle such cases gracefully.

Here is the method implementation:

C#
1using System;
2using System.Collections.Generic;
3
4public class AddressBook
5{
6    private Dictionary<string, string> contacts;
7
8    public AddressBook()
9    {
10        contacts = new Dictionary<string, string>();
11    }
12
13    public bool AddContact(string name, string phoneNumber)
14    {
15        if (contacts.ContainsKey(name))
16        {
17            return false;
18        }
19        contacts.Add(name, phoneNumber);
20        return true;
21    }
22
23    public string? GetContact(string name) // Modify return type to allow null
24    {
25        contacts.TryGetValue(name, out string? phoneNumber); // Allow nullable type
26        return phoneNumber;
27    }
28}
29
30public class Program
31{
32    public static void Main()
33    {
34        AddressBook addressBook = new AddressBook();
35        addressBook.AddContact("Alice", "123-456-7890");
36        Console.WriteLine(addressBook.GetContact("Alice")); // 123-456-7890
37        Console.WriteLine(addressBook.GetContact("Bob")); // empty line, as it's null
38    }
39}

In this method:

We use contacts.TryGetValue(name, out string phoneNumber);, which tries to get the value for the key name. If the key exists, it returns the phone number; otherwise, it returns null.

Step 3: Implementing DeleteContact

This method deletes a contact with the given name. If the contact exists and is deleted, it returns true. If the contact does not exist, it returns false.

Question: What could be a real-world consequence of not checking if the contact exists before deletion?

Answer: One consequence can be assuming that a contact was removed when, in fact, it was never there, leading to inaccurate tracking of records.

Here is the method implementation:

C#
1using System;
2using System.Collections.Generic;
3
4public class AddressBook
5{
6    private Dictionary<string, string> contacts;
7
8    public AddressBook()
9    {
10        contacts = new Dictionary<string, string>();
11    }
12
13    public bool AddContact(string name, string phoneNumber)
14    {
15        if (contacts.ContainsKey(name))
16        {
17            return false;
18        }
19        contacts.Add(name, phoneNumber);
20        return true;
21    }
22
23    public string? GetContact(string name) // Modify return type to allow null
24    {
25        contacts.TryGetValue(name, out string? phoneNumber); // Allow nullable type
26        return phoneNumber;
27    }
28
29    public bool DeleteContact(string name)
30    {
31        return contacts.Remove(name);
32    }
33
34    public IReadOnlyDictionary<string, string> GetAllContacts() // Added method to retrieve contacts
35    {
36        return contacts;  // Return the entire dictionary as read-only
37    }
38}
39
40public class Program
41{
42    public static void Main()
43    {
44        AddressBook addressBook = new AddressBook();
45        addressBook.AddContact("Alice", "123-456-7890");
46        Console.WriteLine(addressBook.DeleteContact("Alice")); // True
47        Console.WriteLine(addressBook.DeleteContact("Bob")); // False
48
49        foreach (var contact in addressBook.GetAllContacts()) // Use GetAllContacts method
50        {
51            Console.WriteLine($"{contact.Key}: {contact.Value}"); // (Empty, as there are no contacts now)
52        }
53    }
54}

In this method:

We use return contacts.Remove(name);, which removes the entry if the key exists and returns true. If the key is not found, it returns false.

Summary

In this lesson, we explored creating a simple address book application using a C# Dictionary. We implemented three key methods: AddContact, GetContact, and DeleteContact to manage contacts effectively by focusing on adding, retrieving, and deleting entries. With the benefits of Dictionaries, such as efficient operations, flexible key usage, and automatic handling of unique keys, we've demonstrated how to construct a robust address book application. Great work! Now, let's proceed to the practice exercises to reinforce your understanding.

Enjoy this lesson? Now it's time to practice with Cosmo!

Practice is how you turn knowledge into actual skills.