C#（.NET 6）中的字典与哈希表：详细指南

Dictionaries and Hash Tables in C#

What is a dictionary and why is it needed?

Imagine you have a magic notebook. In a regular notebook, you can write down a friend's name and their phone number. When you need to call, you look up the name in the notebook and find the number. A dictionary in programming works exactly the same way!

A Dictionary is a collection that stores "key — value" pairs. The key is what we search by (for example, a friend's name), and the value is what we are looking for (the phone number).

The main advantage of a dictionary is very fast lookup. It doesn't matter if the dictionary stores 10 entries or 10,000 — searching by key happens almost instantly. This is possible thanks to a special data structure called a hash table.

How to create and use a dictionary

In C#, the dictionary is called Dictionary, where TKey is the key type and TValue is the value type.

// Creating an empty dictionary where the key is a string (name) and the value is a string (phone number)
Dictionary<string, string> phoneBook = new Dictionary<string, string>();

// Adding entries
phoneBook.Add("Anna", "+7-912-345-67-89");
phoneBook.Add("Peter", "+7-923-456-78-90");

// Getting a value by key
string petePhone = phoneBook["Peter"];
Console.WriteLine($"Peter's phone: {petePhone}");

// Checking if a key exists in the dictionary
if (phoneBook.ContainsKey("Anna"))
{
    Console.WriteLine("Anna is in the phonebook!");
}

// Changing a value
phoneBook["Anna"] = "+7-999-888-77-66";

// Deleting an entry
phoneBook.Remove("Peter");

Another way to create a dictionary

You can create a dictionary with initial data right away:

Dictionary<string, string> greetings = new Dictionary<string, string>()
{
    { "Russia", "Hello!" },
    { "France", "Bonjour!" },
    { "Italy", "Ciao!" },
    { "England", "Hello!" }
};

Console.WriteLine($"In England they say: {greetings["England"]}");

Iterating over all dictionary elements

To go through all entries in a dictionary, we use a foreach loop. Each element is a KeyValuePair that has Key and Value properties.

Dictionary<string, int> products = new Dictionary<string, int>()
{
    { "Bread", 45 },
    { "Milk", 80 },
    { "Apples", 120 }
};

Console.WriteLine("Our products:");
foreach (var item in products)
{
    Console.WriteLine($"{item.Key} costs {item.Value} rubles");
}

Program output:

Our products:
Bread costs 45 rubles
Milk costs 80 rubles
Apples costs 120 rubles

Dictionary with a list of values

Sometimes you need one key to have multiple values. For example, for each day of the week — a list of tasks. For this, you can use a dictionary where the value is a list (List).

// Dictionary where the key is a day of the week, the value is a list of tasks
Dictionary<string, List<string>> weeklyPlan = new Dictionary<string, List<string>>();

// Adding a list of tasks for Monday
weeklyPlan["Monday"] = new List<string>() { "Do homework", "Feed the cat" };

// Adding another task for Monday
weeklyPlan["Monday"].Add("Read a book");

// Adding Tuesday
weeklyPlan["Tuesday"] = new List<string>() { "Go to the pool" };

// Printing all tasks for Monday
Console.WriteLine("Tasks for Monday:");
foreach (string task in weeklyPlan["Monday"])
{
    Console.WriteLine($"- {task}");
}

Analysis of typical tasks

Task 1: Phonebook

Need to find a phone number by name. If the name is not found — report it.

Dictionary<string, string> phoneBook = new Dictionary<string, string>()
{
    { "Masha", "8-912-345-67-89" },
    { "Sasha", "8-923-456-78-90" },
    { "Dasha", "8-934-567-89-01" }
};

Console.Write("Enter a name to search for: ");
string name = Console.ReadLine();

if (phoneBook.ContainsKey(name))
{
    Console.WriteLine($"Phone number for {name}: {phoneBook[name]}");
}
else
{
    Console.WriteLine("This name is not in the phonebook");
}

Task 2: Deleting a contact

The user enters a name, and the program deletes that contact from the dictionary.

Dictionary<string, string> contacts = new Dictionary<string, string>()
{
    { "Ivan", "8-111-222-33-44" },
    { "Maria", "8-555-666-77-88" },
    { "Alexey", "8-999-000-11-22" }
};

Console.Write("Enter the contact name to delete: ");
string nameToDelete = Console.ReadLine();

if (contacts.Remove(nameToDelete))
{
    Console.WriteLine($"Contact '{nameToDelete}' deleted!");
}
else
{
    Console.WriteLine($"Contact '{nameToDelete}' does not exist");
}

// Showing remaining contacts
Console.WriteLine("Remaining contacts:");
foreach (var contact in contacts)
{
    Console.WriteLine($"{contact.Key}: {contact.Value}");
}

Task 3: Printing all dictionary keys

The program prints a list of all products (names only) from the dictionary.

Dictionary<string, int> store = new Dictionary<string, int>()
{
    { "Apples", 150 },
    { "Bananas", 120 },
    { "Oranges", 180 },
    { "Grapes", 250 }
};

Console.WriteLine("List of available products:");
foreach (string productName in store.Keys)  // Keys is a collection of all keys
{
    Console.WriteLine($"- {productName}");
}

Common beginner mistakes

Mistake 1: Accessing a non-existent key

Dictionary<string, int> ages = new Dictionary<string, int>();
ages["Vasya"] = 10;

// Error! The key "Petya" is not in the dictionary
// Console.WriteLine(ages["Petya"]);  // Will throw an exception!

// Correct: check first
if (ages.ContainsKey("Petya"))
{
    Console.WriteLine(ages["Petya"]);
}

Mistake 2: Adding