How to Add Items to Hashtable Collection in C#

A Hashtable collection in C# stores key-value pairs using a hash-based mechanism. Each key-value pair is organized based on the hash code of the key, which is calculated using a hash function. Unlike modern generic collections, hashtables can store objects of different data types but exhibit lower performance due to boxing and unboxing operations.

In this article, we will explore how to add items to a hashtable collection using various methods available in the Hashtable class.

Syntax

The primary method for adding items to a hashtable is the Add() method

public virtual void Add(object key, object value);

You can also add items using index notation

hashtable[key] = value;

Parameters

• key The key of the element to add (type object). Must be non-null and unique.

• value The value of the element to add (type object). Can be null.

Exceptions

• ArgumentNullException Thrown when the key is null.

• ArgumentException Thrown when an element with the same key already exists.

• NotSupportedException Thrown when the hashtable is read-only or has a fixed size.

Using Add() Method

The Add() method is the primary way to add key-value pairs to a hashtable

using System;
using System.Collections;

class Program {
   static void Main(string[] args) {
      Hashtable mixedHashTable = new Hashtable();
      
      // Add method with different data types
      mixedHashTable.Add("msg", "Collection");
      mixedHashTable.Add("site", "HashTable");
      mixedHashTable.Add(1, 3.14);
      mixedHashTable.Add(2, null);

      // Assign value using indexer
      mixedHashTable[3] = "Tutorial";

      // Demonstrate exception handling for duplicate keys
      try {
         mixedHashTable.Add(2, 750);
      } catch {
         Console.WriteLine("Hashtable already has an element with Key = '2'.");
      }
      
      Console.WriteLine("*********HashTable Elements********");
      foreach (DictionaryEntry elem in mixedHashTable) {
         Console.WriteLine("Key = {0}, Value = {1}", elem.Key, elem.Value);
      }
   }
}

The output of the above code is

Hashtable already has an element with Key = '2'.
*********HashTable Elements********
Key = 2, Value = 
Key = msg, Value = Collection
Key = 3, Value = Tutorial
Key = site, Value = HashTable
Key = 1, Value = 3.14

Using Index Notation

You can add items directly using the indexer syntax, which is more convenient for updating existing keys

using System;
using System.Collections;

class Program {
   public static void Main() {
      Hashtable strHashTable = new Hashtable();

      // Adding elements using Add() method
      strHashTable.Add("4", "Even Number");
      strHashTable.Add("9", "Odd Number");
      strHashTable.Add("5", "Odd and Prime Number");
      strHashTable.Add("2", "Even and Prime Number");

      // Adding using indexer (alternative method)
      strHashTable["6"] = "Even Number";

      // Get collection of keys
      ICollection keys = strHashTable.Keys;

      Console.WriteLine("=========Contents of the Hashtable=========");
      foreach(string key in keys) {
         Console.WriteLine(key + ": " + strHashTable[key]);
      }
   }
}

The output of the above code is

=========Contents of the Hashtable=========
6: Even Number
5: Odd and Prime Number
9: Odd Number
2: Even and Prime Number
4: Even Number

Add() vs Indexer Comparison

Key Rules

• Keys must be non-null and unique within the hashtable.

• Values can be null or any object type.

• Hashtable supports mixed data types for both keys and values.

• Use Add() for strict key uniqueness, indexer for flexible assignment.

Conclusion

Adding items to a hashtable in C# can be accomplished using the Add() method for strict key enforcement or the indexer notation for flexible assignment. The hashtable's ability to store mixed data types makes it versatile, though modern generic collections like Dictionary<TKey, TValue> are generally preferred for better performance and type safety.