A Hashtable is a collection of key/value pairs that are arranged based on the hash code of the key. Or in other words, a Hashtable is used to create a collection which uses a hash table for storage. It generally optimized the lookup by calculating the hash code of every key and store into another basket automatically and when you accessing the value from the hashtable at that time it matches the hashcode with the specified key. It is the non-generic type of collection which is defined in System.Collections namespace. 

Important Points: 

• In Hashtable, the key cannot be null, but value can be.
• In Hashtable, key objects must be immutable as long as they are used as keys in the Hashtable.
• The capacity of a Hashtable is the number of elements that Hashtable can hold.
• A hash function is provided by each key object in the Hashtable.
• The Hashtable class implements the IDictionary, ICollection, IEnumerable, ISerializable, IDeserializationCallback, and ICloneable interfaces.
• In hashtable, you can store elements of the same type and of the different types.
• The elements of hashtable that is a key/value pair are stored in DictionaryEntry, so you can also cast the key/value pairs to a DictionaryEntry.
• In Hashtable, key must be unique. Duplicate keys are not allowed.

Creating a Hashtable

The following example demonstrates creating a Hashtable and adding elements.

using System;
using System.Collections;
	
public class Program
{
	public static void Main()
	{
		Hashtable numberNames = new Hashtable();
		numberNames.Add(1,"One"); //adding a key/value using the Add() method
		numberNames.Add(2,"Two");
		numberNames.Add(3,"Three");

		//The following throws run-time exception: key already added.
		//numberNames.Add(3, "Three"); 

		foreach(DictionaryEntry kvp in numberNames)
			Console.WriteLine("Key: {0}, Value: {1}", kvp.Key, kvp.Value);

		//creating a dictionary using collection-initializer syntax
		var cities = new Hashtable(){
			{"UK", "London, Manchester, Birmingham"},
			{"USA", "Chicago, New York, Washington"},
			{"India", "Mumbai, New Delhi, Pune"}
		};

		foreach(DictionaryEntry kvp in cities)
			Console.WriteLine("Key: {0}, Value: {1}", kvp.Key, kvp.Value);		

	}
}

Output

Key: 3, Value: Three
Key: 2, Value: Two
Key: 1, Value: One
Key: India, Value: Mumbai, New Delhi, Pune
Key: UK, Value: London, Manchester, Birmingham
Key: USA, Value: Chicago, New York, Washington

The Hashtable collection can include all the elements of Dictionary, as shown below.

using System;
using System.Collections;
using System.Collections.Generic;

public class Program
{
	public static void Main()
	{
		Dictionary dict = new Dictionary();
		
		dict.Add(1, "one");
		dict.Add(2, "two");
		dict.Add(3, "three");
		
		Hashtable ht = new Hashtable(dict);
		
		Console.WriteLine("Total elements: {0}", ht.Count);
		
	}
}

Output

Total elements: 3

Update Hashtable

You can retrieve the value of an existing key from the Hashtable by passing a key in indexer. The Hashtable is a non-generic collection, so you must type cast values while retrieving it.

using System;
using System.Collections;
					
public class Program
{
	public static void Main()
	{
		//creating a Hashtable using collection-initializer syntax
		var cities = new Hashtable(){
			{"UK", "London, Manchester, Birmingham"},
			{"USA", "Chicago, New York, Washington"},
			{"India", "Mumbai, New Delhi, Pune"}
		};

		string citiesOfUK = (string) cities["UK"]; //cast to string
		string citiesOfUSA = (string) cities["USA"]; //cast to string

		Console.WriteLine(citiesOfUK);
		Console.WriteLine(citiesOfUSA);

		cities["UK"] = "Liverpool, Bristol"; // update value of UK key
		cities["USA"] = "Los Angeles, Boston"; // update value of USA key

		if(!cities.ContainsKey("France")){
			cities["France"] = "Paris";
		}
		
		Console.WriteLine("---After updating values---");
		
		foreach(DictionaryEntry de in cities)
			Console.WriteLine("key: {0}, Value: {1}", de.Key, de.Value);

	}
}

Output

London, Manchester, Birmingham
Chicago, New York, Washington
---After updating values---
key: India, Value: Mumbai, New Delhi, Pune
key: UK, Value: Liverpool, Bristol
key: France, Value: Paris
key: USA, Value: Los Angeles, Boston

Remove Elements in Hashtable

The Remove() method removes the key-value that match with the specified in the Hashtable. It throws the KeyNotfoundException if the specified key not found in the Hashtable, so check for an existing key using the ContainsKey() method before removing.

Use the Clear() method to remove all the elements in one shot.

using System;
using System.Collections;

public class Program
{
	public static void Main()
	{
		var cities = new Hashtable(){
			{"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

		Console.WriteLine("Total Elements: {0}", cities.Count);
		
		if(cities.ContainsKey("France")){ // check key before removing it
			cities.Remove("France");
		}

		cities.Clear(); //removes all elements
		Console.WriteLine("Total Elements after Clear(): {0}", cities.Count);

	}
}

Output

Total Elements: 3
Total Elements: 2
Total Elements after Clear(): 0

---

Difference between Hashtable and Dictionary

The following table lists the differences between Hashtable and Dictionary in C#.

Hashtable	Dictionary
Hashtable is included in the System.Collections namespace.	Dictionary is included in the System.Collections.Generic namespace.
Hashtable is a loosely typed (non-generic) collection, this means it stores key-value pairs of any data types.	Dictionary is a generic collection. So it can store key-value pairs of specific data types.
Hashtable is thread safe.	Only public static members are thread safe in Dictionary.
Hashtable returns null if we try to find a key which does not exist.	Dictionary throws an exception if we try to find a key which does not exist.
Data retrieval is slower than dictionary because of boxing-unboxing.	Data retrieval is faster than Hashtable.