How to create a shallow copy of Hashtable in C#

Hashtable.Clone Method is used to create a shallow copy of the Hashtable. When we make a shallow copy, only the elements of the collection get copied irrespective of its type, it doesn’t copy the objects referred to by the references. Basically, it creates a new object and that object points to the original references.

Syntax:

HashtableName.Clone();

The examples below illustrate the use of this method.

Example 1: Create an object H1 of type Hashtable class which is predefined in the System.Collections namespace. Then we add keys and values of type string using the Hashtable. Add method. The Hashtable.clone method is used to create a shallow copy of H1. We use the foreach loop to display the elements of ShallowH1. The DictionaryEntry property is used to set or get the value of the key, value ordered a pair.

// C# Program to demonstrate
// the Hashtable.Clone Method
using System;
using System.Collections;

namespace hashtable {

class GFG {

    // Main Method
    public static void Main(string[] args)
    {
        // Declaring a Hashtable named H1
        // Calls the default constructor
        Hashtable H1 = new Hashtable();

        // Adding keys and values
        // to the hashtable
        H1.Add("1", "This");
        H1.Add("2", "is");
        H1.Add("3", "a");
        H1.Add("4", "Hash");
        H1.Add("5", "Table");

        // Creating a shallow copy of H1
        Hashtable ShallowH1 = new Hashtable();
        ShallowH1 = (Hashtable)H1.Clone();

        // Displaying values of key, value pairs
        // Using DictionaryEntry which is predefined
        foreach(DictionaryEntry item in ShallowH1)
        {
            Console.WriteLine("Key " + item.Key +
                         " Value " + item.Value);
        }
    }
}
}

Output:

Key 5 Value Table
Key 1 Value This
Key 4 Value Hash
Key 3 Value a
Key 2 Value is

Example 2: Now let’s try making some change in the shallow copy. As shown in the code below the changes of the shallow copy are not reflected back in the original Hashtable.

// C# Program to demonstrate
// the Hashtable.Clone Method
using System;
using System.Collections;

namespace hashtable2 {

class GFG {

    // Main Method
    public static void Main(String[] args)
    {
        // Declaring a Hashtable
        // Calls the constructor
        Hashtable H1 = new Hashtable();

        // Add the values to H1
        H1.Add("1", "Orange");
        H1.Add("2", "Apple");
        H1.Add("3", "Blueberry");

        // Create a shallow copy
        Hashtable H2 = new Hashtable();
        H2 = (Hashtable)H1.Clone();

        // Making changes in the shallow copy
        H2.Add("4", "Mango");
        H2.Remove("1");

        // Displaying original Hashtable
        foreach(DictionaryEntry item in H1)
            Console.WriteLine("Key {0} Value {1}",
                            item.Key, item.Value);

        Console.WriteLine();

        // Displaying shallow copy
        // of the Hashtable
        foreach(DictionaryEntry item in H2)
            Console.WriteLine("Key {0} Value {1}",
                            item.Key, item.Value);
    }
}
}

Output:

Key 3 Value Blueberry
Key 2 Value Apple
Key 1 Value Orange

Key 4 Value Mango
Key 3 Value Blueberry
Key 2 Value Apple

Reference:

• https://docs.microsoft.com/en-us/dotnet/api/system.collections.hashtable.clone?view=netframework-4.7.2#System_Collections_Hashtable_Clone