Check if ArrayList is Synchronized (thread safe) in C#

The IsSynchronized property in C# is used to check if an ArrayList is synchronized (thread-safe). By default, ArrayList is not synchronized, meaning it's not safe for concurrent access by multiple threads. However, you can create a synchronized wrapper using the ArrayList.Synchronized() method.

Syntax

To check if an ArrayList is synchronized −

bool isSync = arrayList.IsSynchronized;

To create a synchronized ArrayList wrapper −

ArrayList syncList = ArrayList.Synchronized(originalList);

Return Value

The IsSynchronized property returns a bool value:

• true if the ArrayList is synchronized (thread-safe)

• false if the ArrayList is not synchronized

Checking Default ArrayList Synchronization

By default, a regular ArrayList is not synchronized. Here's how to check this −

Example

using System;
using System.Collections;

public class Demo {
   public static void Main(String[] args) {
      ArrayList list1 = new ArrayList();
      list1.Add("A");
      list1.Add("B");
      list1.Add("C");
      list1.Add("D");
      list1.Add("E");
      
      Console.WriteLine("Elements in ArrayList1...");
      foreach (string res in list1) {
         Console.WriteLine(res);
      }
      
      Console.WriteLine("Is ArrayList synchronized? = " + list1.IsSynchronized);
   }
}

The output of the above code is −

Elements in ArrayList1...
A
B
C
D
E
Is ArrayList synchronized? = False

Creating a Synchronized ArrayList

You can create a thread-safe version of ArrayList using the ArrayList.Synchronized() method −

Example

using System;
using System.Collections;

public class Demo {
   public static void Main(String[] args) {
      ArrayList list1 = new ArrayList();
      list1.Add("One");
      list1.Add("Two");
      list1.Add("Three");
      list1.Add("Four");
      list1.Add("Five");
      
      Console.WriteLine("Elements in ArrayList...");
      foreach (string res in list1) {
         Console.WriteLine(res);
      }
      
      ArrayList syncList = ArrayList.Synchronized(list1);
      Console.WriteLine("Original ArrayList synchronized? = " + list1.IsSynchronized);
      Console.WriteLine("Synchronized wrapper synchronized? = " + syncList.IsSynchronized);
   }
}

The output of the above code is −

Elements in ArrayList...
One
Two
Three
Four
Five
Original ArrayList synchronized? = False
Synchronized wrapper synchronized? = True

Thread Safety Comparison

Modern Alternatives

While ArrayList synchronization is available, modern C# applications typically use thread-safe collections from System.Collections.Concurrent namespace like ConcurrentBag<T> or List<T> with manual synchronization for better performance and type safety.

Conclusion

The IsSynchronized property helps determine if an ArrayList is thread-safe. Regular ArrayLists return false, while synchronized wrappers created using ArrayList.Synchronized() return true. Use synchronized ArrayLists only when multiple threads need concurrent access to the same collection.