Dictionary in C#

In this article, we will learn about the dictionaries in C#.

A dictionary is a collection of keys and values.

The Dictionary generic class provides a mapping from a set of keys to a set of values. Each addition to the dictionary consists of a value and its associated key. Retrieving a value by using its key is very fast, close to O(1), because the Dictionary class is implemented as a hash table.

Creating a Dictionary

We create a Dictionary C# using the following syntax:
Dictionary<TKey, TValue> dict = new Dictionary<TKey, TValue>();
<TKey> here denotes any datatype of the Key which we want to create a dictionary of.
Similiarly, <TValue> denotes any datatype of the Value

Properties of Dictionary

• Count: This property gets the number of keys actually contained in the dictionary.
• Comparer: Determines equality for the keys in the dictionary.
• Dict[TKey]: Gets or sets the value associated with the specified key.
• Keys: Gets a collection containing the keys in the Dictionary<TKey,TValue>.
• Values: Gets a collection containing the values in the Dictionary<TKey,TValue>.

Methods of Dictionary

• Add(TKey, TValue): Adds the specified key and value to the dictionary.
• ContainsKey(TKey): Determines whether the Dictionary<TKey,TValue> contains the specified key.
• ContainsValue(TValue): Determines whether the Dictionary<TKey,TValue> contains a specific value.
• Remove(TKey): Removes the value with the specified key from the Dictionary<TKey,TValue>.
• Clear(): Removes all keys and values from the Dictionary<TKey,TValue>.

Dictionary Operation Examples:

Different Dictionary operations are demonstrated in the example below:

using System;
using System.Collections.Generic;
namespace DictionaryDemo {
    class Example {
        static void Main(string[] args) {
            Dictionary < int, int > D = new Dictionary < int, int > ();
            D.Add(5, 1);          
            // D.Add(5, 2); will throw an error
            // because 5 has already been added
            
            D[5] = 2;       
            D[2] = 10;                          // We can use indexing property to add elements
            D[10] = 3;
            Console.WriteLine(D.Count);        // Prints 3 because 5 is only added once in the dictionary
            
            Console.WriteLine(D[5]);            // Prints 2, the last entered value for 5.
            
            Console.WriteLine(D.ContainKey(5));  // Checks if 5 is present
            
            D.Remove(5);                          // Removes 5
            
            Console.WriteLine(D.ContainsKey(5));  // Prints False this time
            
            D[2]++;
            Console.WriteLine(D[2]);            // Prints 11 as we performed an arithmetic operation
                                                // on a value present in a dictionary

            foreach(int i in D.Keys) {          // Prints all the Keys
                Console.Write(i + " ");
            }
            Console.WriteLine();
        }
    }
}

Output:

3

2

True

False

11

2 10

2 5 9

Task

In the editor below, perform the different tasks as directed.