The document explains the C# Dictionary, a generic collection that stores key-value pairs with unique keys and potentially duplicate values. It details how to create, access, update, and remove elements from a Dictionary, including example code snippets. Additional methods such as ContainsKey and TryGetValue are introduced for handling key access safely.

1. C# Dictionary

2. C# - Dictionary<TKey, TValue>
• a generic collection that
stores key-value pairs in no
particular order.

3. Dictionary Characteristics
• Dictionary<TKey, TValue> stores key-value
pairs.
• Comes under System.Collections.Generic
namespace.
• Implements IDictionary<TKey, TValue>
interface.
• Keys must be unique and cannot be null.
• Values can be null or duplicate.
• Values can be accessed by passing associated
key in the indexer e.g. myDictionary[key]
• Elements are stored as KeyValuePair<TKey,
TValue> objects.

4. Creating a Dictionary
• You can create the Dictionary<TKey, TValue>
object by passing the type of keys and values
it can store.
using System;
using System.Collections.Generic;
public class Program
{
public static void Main()
{
IDictionary<int, string> numberNames = new Dictionary<int, string>();
numberNames.Add(1,"One"); //adding key/value using the Add() method
numberNames.Add(3,"Three");
numberNames.Add(2,"Two");
foreach(KeyValuePair<int, string> kvp in numberNames)
Console.WriteLine("Key: {0}, Value: {1}", kvp.Key, kvp.Value);

5. Creating a Dictionary
//creating a dictionary using collection-initializer syntax
var cities = new Dictionary<string, string>(){
{"UK","London, Manchester, Birmingham"},
{"USA","Chicago, New York, Washington"},
{"India","Mumbai, New Delhi, Pune"}
};
foreach(var kvp in cities)
Console.WriteLine("Key: {0}, Value: {1}", kvp.Key, kvp.Value);
}
}

6. Access Dictionary Elements
• The Dictionary can be accessed using indexer.
Specify a key to get the associated value. You can
also use the ElementAt() method to get a
KeyValuePair from the specified index.
using System;
using System.Linq;
using System.Collections.Generic;
public class Program
{
public static void Main()
{
var cities = new Dictionary<string, string>(){
{"UK", "London, Manchester, Birmingham"},
{"USA", "Chicago, New York, Washington"},
{"India", "Mumbai, New Delhi, Pune"}
};
Console.WriteLine(cities["UK"]); //prints value of UK key
Console.WriteLine(cities["USA"]);//prints value of USA key
//Console.WriteLine(cities["France"]); // run-time exception: Key does not exist

7. Access Dictionary Elements
//use ContainsKey() to check for an unknown key
if(cities.ContainsKey("France")){
Console.WriteLine(cities["France"]);
}
//use TryGetValue() to get a value of unknown key
string result;
if(cities.TryGetValue("France", out result))
{
Console.WriteLine(result);
}
Console.WriteLine("---access elements using for loop---");
//use ElementAt() to retrieve key-value pair using index
for (int i = 0; i < cities.Count; i++)
{
Console.WriteLine("Key: {0}, Value: {1}",
cities.ElementAt(i).Key,
cities.ElementAt(i).Value);
}
}
}

8. Update Dictionary
• Update the value of a key by
specifying a key in the indexer.
It will throw the
KeyNotFoundException if a key
does not exist in the dictionary,
therefore use the ContainsKey()
method before accessing unknown
keys.

9. Update Dictionary
using System;
using System.Linq;
using System.Collections.Generic;
public class Program
{
public static void Main()
{
var cities = new Dictionary<string, string>(){
{"UK", "London, Manchester, Birmingham"},
{"USA", "Chicago, New York, Washington"},
{"India", "Mumbai, New Delhi, Pune"}
};

10. Remove Elements in Dictionary
• The Remove() method deletes an
existing key-value pair from a
dictionary. The Clear() method
deletes all the elements of the
dictionary.

11. Remove Elements in Dictionary
using System;
using System.Collections.Generic;
public class Program
{
public static void Main()
{
var cities = new Dictionary<string, string>(){
{"UK", "London, Manchester, Birmingham"},
{"USA", "Chicago, New York, Washington"},
{"India", "Mumbai, New Delhi, Pune"}
};
Console.WriteLine("Total Elements: {0}", cities.Count);
cities.Remove("UK"); // removes UK
//cities.Remove("France"); //throws run-time exception: KeyNotFoundException

12. Remove Elements in Dictionary
if(cities.ContainsKey("France")){ // check key before removing it
cities.Remove("France");
}
Console.WriteLine("Total Elements: {0}", cities.Count);
cities.Clear(); //deletes all elements
Console.WriteLine("Total Elements after Clear(): {0}", cities.Count);
}
}