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

Last Updated : 11 Jul, 2025

Hashtable.Synchronized(Hashtable) Method is used to return a synchronized (thread-safe) wrapper for the Hashtable.
Syntax:

public static System.Collections.Hashtable Synchronized (System.Collections.Hashtable table);

Here table is the Hashtable which is to be synchronized.
Return Value: This method returns a synchronized (thread-safe) wrapper for the Hashtable.
Exception: This method will throw the ArgumentNullException if the table is null. 
Below programs illustrate the use of above-discussed method:
Example 1:

CSharp
// C# code to get a synchronized (thread-
// safe) wrapper for the Hashtable
using System;
using System.Collections;

class GFG {

    // Main method
    static void Main(string[] args)
    {

        // create and initialize Hash table
        // using Add() method
        Hashtable has1 = new Hashtable();
        has1.Add("1", "Welcome");
        has1.Add("2", "to");
        has1.Add("3", "geeks");
        has1.Add("4", "for");
        has1.Add("5", "geeks");

        // Creating a synchronized packing
        // around the Hashtable
        Hashtable has2 = Hashtable.Synchronized(has1);

        // --------- Using IsSynchronized Property

        // print the status of both Hashtables
        Console.WriteLine("has1 Hashtable is {0}.",
            has1.IsSynchronized ? "synchronized" : 
                              "not synchronized");

        Console.WriteLine("has2 Hashtable is {0}.",
             has2.IsSynchronized ? "synchronized" :
                               "not synchronized");
    }
}

Output: 
 

has1 Hashtable is not synchronized.
has2 Hashtable is synchronized.


Example 2:

CSharp
// C# code to get a synchronized (thread-
// safe) wrapper for the Hashtable
using System;
using System.Collections;

class GFG {

    // Main method
    static void Main(string[] args)
    {

        // create and initialize Hash table
        // there will be no elements
        Hashtable has1 = new Hashtable();

        // it will give runtime error as
        // table parameter can't be null
        Hashtable has2 = Hashtable.Synchronized(null);
    }
}

Runtime Error:
 

Unhandled Exception: 
System.ArgumentNullException: Value cannot be null. 
Parameter name: table 
 

Note:

  • This method is a thread-safe for multiple readers and writers. Furthermore, the synchronized wrapper ensures that there is only one writer writing at a time.
  • Enumerating through a collection is intrinsically not a thread-safe process. Other threads can still modify the collection, which causes the enumerator to throw an exception even when a collection is synchronized.
  • To guarantee thread safety during enumeration, you can either lock the collection during the entire enumeration or catch the exceptions resulting from changes made by other threads.
  • This method is an O(1) operation.


Reference: 


 

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