Adding an element into the Hashtable in C#

A Hashtable in C# is a collection that stores key-value pairs using a hash-based structure for fast lookups. To add elements to a Hashtable, you use the Add() method which takes a key and a value as parameters.

Syntax

Following is the syntax for adding elements to a Hashtable −

Hashtable hashtableName = new Hashtable();
hashtableName.Add(key, value);

You can also specify the initial capacity when creating the Hashtable −

Hashtable hashtableName = new Hashtable(capacity);

Parameters

• key − The key of the element to add (must be unique and not null).

• value − The value associated with the key (can be null).

Using Add() Method

Example

using System;
using System.Collections;

public class Demo {
    public static void Main(){
        Hashtable hash = new Hashtable(10);
        hash.Add("1", "A");
        hash.Add("2", "B");
        hash.Add("3", "C");
        hash.Add("4", "D");
        hash.Add("5","E");
        hash.Add("6", "F");
        hash.Add("7", "G");
        hash.Add("8","H");
        hash.Add("9", "I");
        hash.Add("10", "J");
        Console.WriteLine("Hashtable Key and Value pairs...");
        foreach(DictionaryEntry entry in hash){
            Console.WriteLine("{0} and {1} ", entry.Key, entry.Value);
        }
        Console.WriteLine("Count of entries in Hashtable = "+ hash.Count);
        hash.Add("11", "K");
        Console.WriteLine("Hashtable Key and Value pairs...UPDATED");
        foreach(DictionaryEntry entry in hash){
            Console.WriteLine("{0} and {1} ", entry.Key, entry.Value);
        }
        Console.WriteLine("Count of entries in Hashtable (updated) = "+hash.Count);
    }
}

The output of the above code is −

Hashtable Key and Value pairs...
10 and J 
1 and A 
2 and B 
3 and C 
4 and D 
5 and E 
6 and F 
7 and G 
8 and H 
9 and I 
Count of entries in Hashtable = 10
Hashtable Key and Value pairs...UPDATED
10 and J 
1 and A 
2 and B 
3 and C 
4 and D 
5 and E 
6 and F 
7 and G 
8 and H 
9 and I 
11 and K 
Count of entries in Hashtable (updated) = 11

Using Enumerator to Add and Display Elements

Example

using System;
using System.Collections;

public class Demo {
    public static void Main(){
        Hashtable hash = new Hashtable(10);
        hash.Add("1", "SUV");
        hash.Add("2", "Electric Cars");
        hash.Add("3", "AUV");
        hash.Add("4", "Utility Vehicle");
        hash.Add("5","Compact Car");
        hash.Add("6", "Sedan");
        hash.Add("7","Crossover");
        Console.WriteLine("Enumerator to iterate through the Hashtable...");
        IDictionaryEnumerator demoEnum = hash.GetEnumerator();
        while (demoEnum.MoveNext())
            Console.WriteLine("Key = " + demoEnum.Key + ", Value = " + demoEnum.Value);
    }
}

The output of the above code is −

Enumerator to iterate through the Hashtable...
Key = 1, Value = SUV
Key = 2, Value = Electric Cars
Key = 3, Value = AUV
Key = 4, Value = Utility Vehicle
Key = 5, Value = Compact Car
Key = 6, Value = Sedan
Key = 7, Value = Crossover

Key Rules

• Keys must be unique − Adding a duplicate key will throw an ArgumentException.

• Keys cannot be null − The key parameter must have a value.

• Values can be null − The value parameter can be null.

• Order is not guaranteed − Elements may appear in a different order than they were added.

Conclusion

The Add() method in C# Hashtable allows you to insert key-value pairs efficiently. Remember that keys must be unique and non-null, while values can be null. The Hashtable provides fast lookups but does not maintain insertion order.