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The Join() method of Thread class is a very useful method in a situation when we want a thread to wait for another thread to finish its execution. The name Join() comes from a concept of making a thread wait until a specific thread joins it after finishing its execution.

A few overloaded versions of Join() method -:

Method	Description
void Join()	This version of Join() method makes a calling thread wait for another thread(whose instance has called this method) to finish its execution.
bool Join(long milliseconds)	This version of Join() method makes a calling thread wait only for a specified time(in milliseconds) for another thread(whose instance has called this method) to finish its execution. This method returns true if the thread has terminated, or false if the thread has not terminated after the amount of time specified.

Example of Join() method

The version of Join() method makes a calling thread wait for another thread(whose instance has called this method) to finish its execution.

//C# Example of Join() method of Thread class

using System;
using System.Threading;

class ThJoin
{
String threadName;

//Constructor of our class
public ThJoin(String name)
{
	threadName = name;
}


//Entry Point of the new thread.
public void Run()  
{
	 //Getting the reference to the currently executing thread.
	Thread th = Thread.CurrentThread;
		
	//Setting the name of our thread
	th.Name = threadName;

	Console.WriteLine(th.Name + " has stated its execution");

	try
	{
		for(int i=0;i<3;i++)
		{
			Thread.Sleep(1000); //This will make this thread sleep for 1000 ms.
			Console.WriteLine(i);	
		}
		Console.WriteLine(th.Name + " has completed");			
	}
	catch(ThreadInterruptedException e)
	{
		Console.WriteLine("Thread Interrupted" + e);
	}
}


public static void Main(String[] ar)
{
	//Getting the reference of the automatically created Main Thread
	Thread th1 = Thread.CurrentThread;

	//Setting the name of the automatically created Main Thread
	th1.Name = "Main Thread";

	Console.WriteLine(th1.Name + " has stated its execution");


	//Creating an object of our class
	//To use it to create our thread
	ThJoin ob = new ThJoin("My Thread");

	//Creating an object of Thread class by calling its constructor,
	//We are specifying the method that will be called to begin execution of our thread.
	Thread th2 = new Thread(ob.Run);


	//Calling the Start() method, which automatically calls the entry-point method of the invoked thread
	th2.Start();
	
	try
	{
		//The Main thread will wait for other thread to complete its execution and join it.
		th2.Join(); 
		Console.WriteLine(th1.Name + " has completed");
	}	
	catch(ThreadInterruptedException e)
	{
		Console.WriteLine(e);
	}
}
}

Output is - :

Main Thread has stated its execution
My Thread has stated its execution
0
1
2
My Thread has completed
Main Thread has completed

Program Analysis

The code is inside the Main() method, hence, the Main thread(default thread) runs automatically.

• In the Main() method, we have created a thread with the name, My Thread, referenced by th2.
• Method call th2.start() starts the execution of the My Thread.
• Method th2.Join() in Main(), makes the Main thread wait for the My Thread to finish its execution.
• In the entry-point method of My Thread i.e. Run() method, a for-loop makes the My thread sleep for 1 second, thrice.
• Once My Thread finishes its execution, the Main thread continues and finishes its execution as well.

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Example of Join(int milliseconds) method

Let us see a different version of Join() method i.e. Join(int milliseconds), which makes a calling thread wait only for a specified milliseconds for another thread(whose instance has called this method) to finish its execution.

//C# Example of Join(int milliseconds) method of Thread class

using System;
using System.Threading;

class ThJoin
{
String threadName;

//Constructor of our class
public ThJoin(String name)
{
	threadName = name;
}


//Entry Point of the new thread.
public void Run()  
{
	 //Getting the reference to the currently executing thread.
	Thread th = Thread.CurrentThread;
		
	//Setting the name of our thread
	th.Name = threadName;

	Console.WriteLine(th.Name + " has stated its execution");

	try
	{
		for(int i=0;i<3;i++)
		{
			Thread.Sleep(1000); //This will make this thread sleep for 1000 ms.
			Console.WriteLine(i);	
		}
		Console.WriteLine(th.Name + " has completed");			
	}
	catch(ThreadInterruptedException e)
	{
		Console.WriteLine("Thread Interrupted" + e);
	}
}


public static void Main(String[] ar)
{
	//Getting the reference of the automatically created Main Thread
	Thread th1 = Thread.CurrentThread;

	//Setting the name of the automatically created Main Thread
	th1.Name = "Main Thread";

	Console.WriteLine(th1.Name + " has stated its execution");


	//Creating an object of our class
	//To use it to create our thread
	ThJoin ob = new ThJoin("My Thread");

	//Creating an object of Thread class by calling its constructor,
	//We are specifying the method that will be called to begin execution of our thread.
	Thread th2 = new Thread(ob.Run);


	//Calling the Start() method, which automatically calls the entry-point method of the invoked thread
	th2.Start();
	
	try
	{
		//The Main thread will wait for other thread to complete its execution only for 2000ms i.e. 2 seconds.
		th2.Join(2000); 
		Console.WriteLine(th1.Name + " has completed");
	}	
	catch(ThreadInterruptedException e)
	{
		Console.WriteLine(e);
	}
}
}

Output is - :

Main Thread has stated its execution
My Thread has stated its execution
0
Main Thread has completed
1
2
My Thread has completed

Program Analysis

The code is inside the Main() method, hence, the Main thread(default thread) runs automatically.

• In the Main() method, we have created a thread with the name, My Thread, pointed by th2.
• Method call th2.start() starts the execution of the My Thread.
• Method th2.Join() in Main(), makes the Main thread wait for the My Thread to finish its execution, only for 2 milliseconds i.e. 2 seconds.
• In the entry-point method of My Thread, a for-loop makes it sleep for 1 second, thrice, at the same time for the which the Main() thread had to wait for MyThread expires and the Main Thread finishes its execution.
• Finally, the My Thread finishes its execution by executing its entry-point method.

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