What is the IsReadOnly property of ArrayList class in C#?

The IsReadOnly property of the ArrayList class in C# returns a boolean value indicating whether the ArrayList is read-only. A read-only ArrayList cannot be modified after creation − you cannot add, remove, or change elements.

Syntax

Following is the syntax for using the IsReadOnly property −

bool isReadOnly = arrayList.IsReadOnly;

Return Value

The IsReadOnly property returns −

• true − if the ArrayList is read-only

• false − if the ArrayList can be modified

Using IsReadOnly with Regular ArrayList

A regular ArrayList created using the default constructor is always modifiable, so IsReadOnly returns false −

Example

using System;
using System.Collections;

class Demo {
   public static void Main() {
      ArrayList arrList = new ArrayList();
      arrList.Add("Apple");
      arrList.Add("Banana");
      
      Console.WriteLine("ArrayList IsReadOnly: " + arrList.IsReadOnly);
      Console.WriteLine("Elements: " + arrList.Count);
      
      // Since it's not read-only, we can modify it
      arrList.Add("Orange");
      Console.WriteLine("After adding Orange: " + arrList.Count);
   }
}

The output of the above code is −

ArrayList IsReadOnly: False
Elements: 2
After adding Orange: 3

Using IsReadOnly with Read-Only ArrayList

You can create a read-only wrapper around an existing ArrayList using ArrayList.ReadOnly() method −

Example

using System;
using System.Collections;

class Demo {
   public static void Main() {
      ArrayList originalList = new ArrayList();
      originalList.Add("Item1");
      originalList.Add("Item2");
      
      // Create a read-only wrapper
      ArrayList readOnlyList = ArrayList.ReadOnly(originalList);
      
      Console.WriteLine("Original ArrayList IsReadOnly: " + originalList.IsReadOnly);
      Console.WriteLine("Read-only ArrayList IsReadOnly: " + readOnlyList.IsReadOnly);
      
      try {
         readOnlyList.Add("Item3"); // This will throw an exception
      }
      catch (NotSupportedException ex) {
         Console.WriteLine("Exception: " + ex.Message);
      }
   }
}

The output of the above code is −

Original ArrayList IsReadOnly: False
Read-only ArrayList IsReadOnly: True
Exception: Collection is read-only.

Common Use Cases

The IsReadOnly property is commonly used to −

• Validate before modification − Check if the ArrayList can be modified before attempting to add or remove elements

• Defensive programming − Ensure your code handles read-only collections properly

• API design − Return read-only collections to prevent external modification of internal data

Conclusion

The IsReadOnly property helps determine whether an ArrayList can be modified. Regular ArrayLists return false, while read-only wrappers created using ArrayList.ReadOnly() return true, preventing any modifications to the collection.