C# | Creating a read-only wrapper for the ArrayList

ArrayList.ReadOnly(ArrayList) Method is used to get a read-only ArrayList wrapper.

Syntax:

public static System.Collections.ArrayList ReadOnly(System.Collections.ArrayList list);

Here, the list is the ArrayList which is to be wrapped.

Return Value: It returns a read-only ArrayList Wrapper around the list.

Exception: This method returns the ArgumentNullException if the list is null.

Below programs illustrate the use of above-discussed method:

Example 1:

// C# code to create a read-only
// wrapper for the ArrayList
using System;
using System.Collections;

class GFG {

    // Driver code
    public static void Main()
    {

        // Creating an ArrayList
        ArrayList myList = new ArrayList();

        // Adding elements to ArrayList
        myList.Add("Geeks");
        myList.Add("for");
        myList.Add("Geeks");
        myList.Add("Noida");
        myList.Add("Geeks Classes");
        myList.Add("Delhi");

        // Creating a Read-Only packing
        // around the ArrayList
        ArrayList myList2 = ArrayList.ReadOnly(myList);

        // --------- Using IsReadOnly Property

        // print the status of both ArrayList
        Console.WriteLine("myList ArrayList is {0}.",
                    myList.IsReadOnly ? "read-only" :
                                    "not read-only");

        Console.WriteLine("myList2 ArrayList is {0}.",
                    myList2.IsReadOnly ? "read-only" :
                                     "not read-only");
    }
}

Output:

myList ArrayList is not read-only.
myList2 ArrayList is read-only.

Example 2:

// C# code to create a read-only
// wrapper for the ArrayList
using System;
using System.Collections;

class GFG {

    // Driver code
    public static void Main()
    {

        // Creating an ArrayList
        ArrayList myList = new ArrayList();

        // Adding elements to ArrayList
        myList.Add("C");
        myList.Add("C++");
        myList.Add("Java");
        myList.Add("C#");
        myList.Add("Python");

        Console.WriteLine("Before Wrapping: ");

        // Displaying the elements in the ArrayList
        foreach(string str in myList)
        {
            Console.WriteLine(str);
        }


        // Creating a Read-Only packing
        // around the ArrayList
        ArrayList myList2 = ArrayList.ReadOnly(myList);

        Console.WriteLine("After Wrapping: ");

        // Displaying the elements
        foreach(string str in myList2)
        {
            Console.WriteLine(str);
        }

        Console.WriteLine("Trying to add new element into myList2:");

        // it will give error
        myList2.Add("HTML");


    }
}

Output:

Before Wrapping: 
C
C++
Java
C#
Python
After Wrapping: 
C
C++
Java
C#
Python
Trying to add new element into myList2:

Runtime Error:

Unhandled Exception:
System.NotSupportedException: Collection is read-only.
at System.Collections.ArrayList+ReadOnlyArrayList.Add

Explanation: In the above program, you can add or remove the elements from the myList i.e original ArrayList which will reflect into the read-only collection.

Note:

• To prevent any modifications to list, expose list only through this wrapper.
• A collection that is read-only is simply a collection with a wrapper that prevents modifying the collection. If changes are made to the underlying collection, the read-only collection reflects those changes.
• This method is an O(1) operation.

Reference:

• https://docs.microsoft.com/en-us/dotnet/api/system.collections.arraylist.readonly?view=netframework-4.7.2#System_Collections_ArrayList_ReadOnly_System_Collections_ArrayList_