Dictionary key-value collection in C Sharp (C#)

Introduction

A dictionary stores pairs of keys and values so you can quickly find a value by its key.

When you want to look up a phone number by a person's name.

When you need to count how many times each word appears in a text.

When you want to store settings where each setting has a name and a value.

When you want to map product IDs to product details in a store.

When you want to cache data for fast access using unique keys.

Syntax

C Sharp (C#)

Dictionary<TKey, TValue> dictionaryName = new Dictionary<TKey, TValue>();

TKey is the type of the key, and TValue is the type of the value.

Keys must be unique; values can repeat.

Examples

This creates a dictionary where keys are names (strings) and values are ages (integers).

C Sharp (C#)

Dictionary<string, int> ages = new Dictionary<string, int>();

Adds a key "Alice" with value 30 to the dictionary.

C Sharp (C#)

ages.Add("Alice", 30);

Gets the age value for the key "Alice".

C Sharp (C#)

int age = ages["Alice"];

Checks if the key "Bob" exists in the dictionary.

C Sharp (C#)

bool hasBob = ages.ContainsKey("Bob");

Sample Program

This program creates a phone book dictionary, adds three contacts, and prints their numbers. It also lists all contacts.

C Sharp (C#)

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        Dictionary<string, string> phoneBook = new Dictionary<string, string>();
        phoneBook.Add("John", "555-1234");
        phoneBook.Add("Jane", "555-5678");
        phoneBook["Bob"] = "555-8765"; // Another way to add

        Console.WriteLine("John's number: " + phoneBook["John"]);

        if (phoneBook.ContainsKey("Jane"))
        {
            Console.WriteLine("Jane's number: " + phoneBook["Jane"]);
        }

        Console.WriteLine("All contacts:");
        foreach (var entry in phoneBook)
        {
            Console.WriteLine(entry.Key + ": " + entry.Value);
        }
    }
}

Important Notes

Trying to get a value with a key that does not exist will cause an error. Use ContainsKey to check first.

You can update a value by assigning a new value to an existing key.

Dictionaries are very fast for lookups compared to lists.

Summary

Dictionaries store data as key-value pairs for quick access.

Keys must be unique and are used to find values.

Use Add() to insert and indexer [] to get or set values.

Practice

1. What is the main purpose of a Dictionary in C#?

easy

A. To create graphical user interfaces

B. To store data in a sequential list

C. To perform mathematical calculations

D. To store data as key-value pairs for quick access

Solution

1. Step 1: Understand the Dictionary concept

   A Dictionary stores data as pairs where each key is linked to a value for fast lookup.

2. Step 2: Compare with other options

   Options B, C, and D describe lists, math, and UI, which are unrelated to Dictionary's purpose.

3. Final Answer:

   To store data as key-value pairs for quick access -> Option D

4. Quick Check:

   Dictionary = key-value pairs [OK]

Hint: Remember: Dictionary = keys + values for fast lookup [OK]

Common Mistakes:

• Confusing Dictionary with List
• Thinking Dictionary stores only values
• Assuming Dictionary is for UI or math

2. Which of the following is the correct way to add a key-value pair to a Dictionary<int, string> named dict?

easy

A. dict.Add(1, "apple");

B. dict.Insert(1, "apple");

C. dict["1"] = "apple";

D. dict.Put(1, "apple");

Solution

1. Step 1: Recall Dictionary methods

   The method to add a new key-value pair is Add(key, value).

2. Step 2: Check options

   Only dict.Add(1, "apple"); uses Add. dict[1] = "apple"; uses indexer which sets or updates but is not the method to add explicitly.

3. Final Answer:

   dict.Add(1, "apple"); -> Option A

4. Quick Check:

   Add() method adds key-value pair [OK]

Hint: Use Add() to insert new pairs, indexer to update [OK]

Common Mistakes:

• Using Insert or Put which don't exist
• Confusing Add() with indexer for adding
• Trying to add with wrong method name

3. What will be the output of this C# code?

var dict = new Dictionary<int, string>();
dict.Add(1, "one");
dict[2] = "two";
Console.WriteLine(dict[1] + ", " + dict[2]);

medium

A. two, one

B. 1, 2

C. one, two

D. Runtime error

Solution

1. Step 1: Understand dictionary additions

   Key 1 is added with value "one" using Add(). Key 2 is set to "two" using indexer.

2. Step 2: Check output of Console.WriteLine

   It prints values for keys 1 and 2 joined by comma: "one, two".

3. Final Answer:

   one, two -> Option C

4. Quick Check:

   dict[1] = one, dict[2] = two [OK]

Hint: Indexer sets or updates; Add inserts new pairs [OK]

Common Mistakes:

• Confusing keys with values in output
• Expecting keys printed instead of values
• Thinking indexer causes error if key missing

4. What is wrong with this code snippet?

var dict = new Dictionary<int, string>();
dict.Add(1, "apple");
dict.Add(1, "banana");

medium

A. It will compile and run without errors

B. It causes a runtime exception due to duplicate key

C. It overwrites the first value with the second

D. It causes a compile-time error

Solution

1. Step 1: Understand Add() behavior with duplicate keys

   Adding a key that already exists causes a runtime exception.

2. Step 2: Check code behavior

   Second Add with key 1 throws an ArgumentException at runtime.

3. Final Answer:

   It causes a runtime exception due to duplicate key -> Option B

4. Quick Check:

   Duplicate keys cause runtime error [OK]

Hint: Add() fails if key exists; use indexer to overwrite [OK]

Common Mistakes:

• Assuming Add overwrites existing key
• Expecting compile-time error instead of runtime
• Confusing Add with indexer behavior

5. Given a list of user IDs and names:

var users = new List<(int id, string name)> { (1, "Alice"), (2, "Bob"), (3, "Charlie") };

Which code correctly creates a Dictionary<int, string> mapping IDs to names using a dictionary comprehension style?

hard

A. var dict = users.ToDictionary(u => u.id, u => u.name);

B. var dict = new Dictionary<int, string>(); foreach(var u in users) dict.Add(u.name, u.id);

C. var dict = users.Select(u => new { u.id, u.name }).ToList();

D. var dict = new Dictionary<string, int>(); foreach(var u in users) dict[u.id] = u.name;

Solution

1. Step 1: Understand ToDictionary usage

   ToDictionary converts a list to a dictionary by specifying key and value selectors.

2. Step 2: Check each option

   var dict = users.ToDictionary(u => u.id, u => u.name); correctly maps id to name. var dict = new Dictionary<int, string>(); foreach(var u in users) dict.Add(u.name, u.id); swaps key and value and uses wrong types. var dict = users.Select(u => new { u.id, u.name }).ToList(); creates a list, not dictionary. var dict = new Dictionary<string, int>(); foreach(var u in users) dict[u.id] = u.name; has wrong dictionary types and indexer usage.

3. Final Answer:

   var dict = users.ToDictionary(u => u.id, u => u.name); -> Option A

4. Quick Check:

   ToDictionary creates dictionary from list [OK]

Hint: Use ToDictionary with key and value selectors [OK]

Common Mistakes:

• Swapping key and value in Add
• Using Select instead of ToDictionary
• Mismatching dictionary key-value types