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C# | Adding an element into the Hashtable

Last Updated : 11 Jul, 2025

The Hashtable class represents a collection of

that are organized based on the hash code of the key. The key is used to access the items in the collection.

is used to adds an element with the specified key and value into the Hashtable.

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

key: It is the specified key of the element to add of type System.Object. value: It is the specified value of the element to add of type System.Object. The value can be null.

• ArgumentNullException : If the key is null.
• ArgumentException : If an element with the same key already exists in the Hashtable.
• NotSupportedException : Either Hashtable is read-only or Hashtable has a fixed size.

Below given are some examples to understand the implementation in a better way :

// C# code for adding an element with the
// specified key and value into the Hashtable
using System;
using System.Collections;

class GFG {

    // Driver code
    public static void Main()
    {

        // Creating a Hashtable
        Hashtable myTable = new Hashtable();

        // Adding elements in Hashtable
        myTable.Add("g", "geeks");
        myTable.Add("c", "c++");
        myTable.Add("d", "data structures");
        myTable.Add("q", "quiz");

        // Get a collection of the keys.
        ICollection c = myTable.Keys;

        // Displaying the contents
        foreach(string str in c)
            Console.WriteLine(str + ": " + myTable[str]);
    }
}

d: data structures
c: c++
q: quiz
g: geeks

// C# code for adding an element with the
// specified key and value into the Hashtable
using System;
using System.Collections;

class GFG {

    // Driver code
    public static void Main()
    {

        // Creating a Hashtable
        Hashtable myTable = new Hashtable();

        // Adding elements in Hashtable
        myTable.Add("4", "Even");
        myTable.Add("9", "Odd");
        myTable.Add("5", "Odd and Prime");
        myTable.Add("2", "Even and Prime");

        // Get a collection of the keys.
        ICollection c = myTable.Keys;

        // Displaying the contents
        foreach(string str in c)
            Console.WriteLine(str + ": " + myTable[str]);
    }
}

5: Odd and Prime
9: Odd
2: Even and Prime
4: Even

• https://learn.microsoft.com/en-us/dotnet/api/system.collections.hashtable.add?view=netframework-4.7.2

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