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C# | Creating a synchronized (thread-safe) wrapper for the ArrayList

Last Updated : 11 Jul, 2025
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Synchronized(ArrayList) method is used to get an ArrayList wrapper that is synchronized (thread safe). Syntax:
public static System.Collections.ArrayList Synchronized (System.Collections.ArrayList list);
Here, the list is the ArrayList which is to be synchronized. Return Value: It returns an ArrayList wrapper which is synchronized (thread safe). Exception: This method throws ArgumentNullException if the list is null. Below programs illustrate the use of above-discussed method: Example 1: CSharp 
// C# code to check if ArrayList
// Is Synchronized or not
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");

        // Creates a synchronized
        // wrapper around the ArrayList
        ArrayList smyList = ArrayList.Synchronized(myList);

        // Displays the synchronization
        // status of both ArrayList
        Console.WriteLine("myList is {0}.", myList.IsSynchronized ? 
                              "Synchronized" : "Not Synchronized");

        Console.WriteLine("smyList is {0}.", smyList.IsSynchronized ?
                                "Synchronized" : "Not Synchronized");
    }
}
Output: 
myList is Not Synchronized.
smyList is Synchronized.
Example 2: CSharp 
// C# code to check if ArrayList
// Is Synchronized or not
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");

        // it will give error as
        // the parameter is null
        ArrayList smyList = ArrayList.Synchronized(null);
    }
}
Runtime Error:
Unhandled Exception: System.ArgumentNullException: Value cannot be null. Parameter name: list
Note: For the thread safety of the ArrayList, all operations must be done through this wrapper. Reference:

K

Kirti_Mangal
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