在 C# 中，系统线程命名空间下的线程连接类包含许多处理线程的方法。其中一种方法是 Thread.Join()。此方法用于使所有调用线程等待，直到当前线程终止或完成其任务。线程中的Join()方法可以用来实现同步，也可以用来确保特定线程已经终止。
C#提供了三种Join()方法的重载，如下：
语法
Join() 方法的所有三个重载的语法如下：
public void Join();
上面的 Join() 方法不接受任何参数并阻塞调用线程，直到当前实例表示的线程完成执行或在执行标准 COM 和 SendMessage 泵送时终止。
public bool Join(int millisecondsTimeout);
上面的 Join() 方法接受一个整数作为参数，并阻塞调用线程，直到当前实例表示的线程完成执行或终止，或者在执行标准时经过使用整数参数“millisecondsTimeout”指定的时间（毫秒） COM 和 SendMessage 泵送。
public bool Join(TimeSpan timeout);
此 Join() 方法采用 TimeSpan 类型的参数并阻塞调用线程，直到当前实例表示的线程完成执行或终止，或者在执行标准 COM 和 SendMessage 泵送时使用参数“超时”指定的时间过去.
要在 C# 中使用线程，我们需要首先导入 System.在我们的代码中使用 Threading 命名空间，以便我们可以访问 Threading 命名空间中存在的类。
Threading 命名空间下的 Thread 类包含 Join() 方法，该方法用于通过阻塞调用线程来使多个线程以同步方式工作，直到已经调用 Join() 方法的线程完成其任务并终止。如果调用Join()方法的线程没有终止，则调用线程将被无限期地阻塞。因此，join() 方法可以帮助我们确保特定线程已终止。
Join() 方法适用于处于活动状态的线程，我们可以使用 Thread.IsAlive 属性来检查这一点。如果我们在线程启动之前调用 Join() 方法，那么 Join() 方法将立即返回。同样，如果在线程已经终止时调用 Join() 方法，那么 Join() 方法也会立即返回。因此，如果线程没有运行，Join() 方法会立即返回。
我们不应该调用指示当前线程的线程对象的Join()方法。这将使我们的应用程序无响应，因为当前线程将无限期地等待自身。
请在下面的表格中找到有关 Join() 方法的三个重载的重要详细信息：
| Method | Parameters | Returns |
| public void Join() | It does not take any arguments. | Returns void. |
| public bool Join(int millisecondsTimeout) | An integer representing the number of milliseconds required to wait for the thread to terminate. | Returns Boolean value; returns true if the thread has terminated and returns false if the time specified by the parameter has elapsed and the thread has not terminated. |
| public bool Join(TimeSpan timeout) | A TimeSpan which indicates the amount of time required to wait for the thread to terminate. | Returns Boolean value; returns true if the thread has terminated and returns false if the time specified by the parameter has elapsed and the thread has not terminated. |
Below are the examples of c# thread join:
Example showing the use of Join() method on first thread and other two threads in the code wait until the first thread completes its execution.
Code:
using System;
using System.Threading;
namespace ConsoleApp4
{
public class Program
{
public void Display()
{
for (int i = 1; i <= 5; i++)
{
Console.WriteLine(Thread.CurrentThread.Name+" : " + i);
//suspending thread for specified time
Thread.Sleep(200);
}
}
}
public class ThreadDemo
{
public static void Main()
{
Program obj = new Program();
Thread thread1 = new Thread(new ThreadStart(obj.Display));
thread1.Name = "Thread1";
Thread thread2 = new Thread(new ThreadStart(obj.Display));
thread2.Name = "Thread2";
Thread thread3 = new Thread(new ThreadStart(obj.Display));
thread3.Name = "Thread3";
//starting thread1
thread1.Start();
//calling Join() on thread1
thread1.Join();
//starting thread2
thread2.Start();
//starting thread3
thread3.Start();
Console.ReadLine();
}
}
}Output:
Example calling Join() method on all threads.
Code:
using System;
using System.Threading;
namespace ConsoleApp4
{
public class Program
{
public static void Main(string[] args)
{
Console.WriteLine("Main thread started");
Thread thread1 = new Thread(Method1);
Thread thread2 = new Thread(Method2);
Thread thread3 = new Thread(Method3);
thread1.Start();
thread2.Start();
thread3.Start();
thread1.Join();
thread2.Join();
thread3.Join();
Console.WriteLine("Main thread ended");
Console.ReadLine();
}
public static void Method1()
{
Console.WriteLine("Method1 - Thread1 starting execution");
Thread.Sleep(1000);
Console.WriteLine("Method1 - Thread1 execution completed");
}
public static void Method2()
{
Console.WriteLine("Method2 - Thread2 starting execution");
Thread.Sleep(2500);
Console.WriteLine("Method2 - Thread2 execution completed");
}
public static void Method3()
{
Console.WriteLine("Method3 - Thread3 starting execution");
Thread.Sleep(5000);
Console.WriteLine("Method3 - Thread3 execution completed");
}
}
}Output:
Example of Join(int millisecondsTimeout) method.
Code:
using System;
using System.Threading;
namespace ConsoleApp4
{
public class Program
{
public static void Main(string[] args)
{
Console.WriteLine("Main thread started");
Thread thread1 = new Thread(Method1);
Thread thread2 = new Thread(Method2);
Thread thread3 = new Thread(Method3);
thread1.Start();
thread2.Start();
thread3.Start();
if (thread3.Join(2000))
{
Console.WriteLine("Execution of thread3 completed in 2 seconds.");
}
else
{
Console.WriteLine("Execution of thread3 not completed in 2 seconds.");
}
Console.WriteLine("Main thread ended");
Console.ReadLine();
}
public static void Method1()
{
Console.WriteLine("Method1 - Thread1 execution started");
Thread.Sleep(1000);
Console.WriteLine("Method1 - Thread1 execution completed");
}
public static void Method2()
{
Console.WriteLine("Method2 - Thread2 execution started");
Thread.Sleep(2500);
Console.WriteLine("Method2 - Thread2 execution completed");
}
public static void Method3()
{
Console.WriteLine("Method3 - Thread3 execution started");
Thread.Sleep(5000);
Console.WriteLine("Method3 - Thread3 execution completed");
}
}
}Output:
Join() method in C# makes one thread wait till another thread completes its task and terminates. There are three overloads of the Join() method in C#. Join() method works on the thread which is in the alive state. It helps to achieve synchronization while working with multiple threads.
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