C# ArrayList - ReadOnly() Method

---

---

The C# ArrayList ReadOnly() method is used to create a read-only wrapper around an existing ArrayList.

This method makes the ArrayList immutable by making sure that elements cannot be added, removed, or modified. This method takes O(n) operation.

Syntax

Following is the syntax of the C# ArrayList ReadOnly() method −

public static ArrayList ReadOnly (ArrayList list);

Parameters

This method accepts an ArrayList that needs to be wrapped in read-only.

Return value

This method returns a read-only ArrayList wrapper that prevents modifications.

Example 1: Demonstrate the Use of ReadOnly() Method

Following is the basic example of the ReadOnly() method to demonstrate how to use it−

using System;
using System.Collections;
class Example {
   public static void Main() {
      // create and initialize new ArrayList
      ArrayList mylist = new ArrayList {1, 2, 3, 4, 5};
   
     // creating a read-only wrapper
   
      ArrayList readonlList = ArrayList.ReadOnly(mylist);
   
      // Display the elements
      Console.WriteLine("Read Only List: ");
      foreach(var items in readonlList)
         Console.Write(items + " ");
   }
}

Output

Following is the output −

Read Only List:
1 2 3 4 5

Example 2: Modifying Read-Only ArrayList in C#

Here, in this example, we create an ArrayList. We then use the ReadOnly() method to wrap in read-only. Then we attempt the modifications −

using System;
using System.Collections;
class Example {
   public static void Main() {
      // create and initialize new ArrayList
      ArrayList mylist = new ArrayList {1, 2, 3, 4, 5};
      
      // creating a read-only wrapper
      ArrayList readonlList = ArrayList.ReadOnly(mylist);
      
      // Display the elements
      Console.WriteLine("Read Only List: ");
      foreach(var items in readonlList)
         Console.Write(items + " ");
         
      Console.Write("\n");
         
      // Trying to modify the read-only list
      try
      {
         readonlList.Add(6);
      }
      catch (NotSupportedException e)
      {
         Console.WriteLine("Exception: " + e.Message);
      }
   }
}

Output

Following is the output −

Read Only List: 
1 2 3 4 5 
Exception: Collection is read-only.

Example 3: Updating Read-Only ArrayList in C#

Here, in this example, we create an ArrayList. We then use the ReadOnly() method to wrap in read-only. Then, we attempt to update the elements of ArrayList −

using System;
using System.Collections;
class Example {
   public static void Main() {   
      // create and initialize new ArrayList
      ArrayList mylist = new ArrayList {1, 2, 3, 4, 5};
   
      // creating a read-only wrapper
      ArrayList readonlList = ArrayList.ReadOnly(mylist);
   
      // Display the elements
      Console.WriteLine("Read Only List: ");
      foreach(var items in readonlList){
        Console.Write(items + " ");
      }
      Console.Write("\n");
      try
      {
         readonlList[2] = 10;
      }
      catch (NotSupportedException e)
      {
         Console.WriteLine("Exception: " + e.Message);
      }
   }
}

Output

Following is the output −

Read Only List: 
1 2 3 4 5 
Exception: Collection is read-only.

Example 4: Read-Only Wrapper with Dynamic Updates in List

This example demonstrates that the read-only ArrayList does not allow modifications; however, any changes made to the original ArrayList are still reflected in the read-only version −

using System;
using System.Collections;

class Example {
   static void Main() {

      ArrayList list = new ArrayList() {
         "Apple",
         "Banana",
         "Cherry"
      };

      ArrayList readOnlyList = ArrayList.ReadOnly(list);

      Console.WriteLine("Read-Only List:");
      foreach(var item in readOnlyList) {
         Console.Write(item + " ");
      }
      Console.WriteLine();

      // trying to modify the read-only list
      try {
         readOnlyList.Add("Mango");
      } catch (NotSupportedException e) {
         Console.WriteLine("Exception: " + e.Message);
      }

      // Modifying the original list
      // Allowed because myList is not read-only
      list.Add("Mango");

      Console.WriteLine("Updated Original List:");
      foreach(var item in list) {
         Console.Write(item + " ");
      }
      Console.WriteLine();

      Console.WriteLine("Updated Read-Only List (Reflects changes in original):");
      foreach(var item in readOnlyList) {
         Console.Write(item + " ");
      }
   }
}

Output

Following is the output −

Read-Only List:
Apple Banana Cherry 
Exception: Collection is read-only.
Updated Original List:
Apple Banana Cherry Mango 
Updated Read-Only List (Reflects changes in original):
Apple Banana Cherry Mango