Join

In .NET we use methods to perform threading. On the Thread type, we find the Join instance method. It enables us to wait until a thread finishes.

Method use

We use the C# Join method on an array of threads to implement useful threading functionality. Join() can be called on a Thread instance.

This program creates an array of 4 threads and then calls a method on each thread. The method called (Start) requires 10,000 milliseconds to complete because it calls Thread.Sleep.

using System;
using System.Diagnostics;
using System.Threading;

class Program
{
    static void Main()
    {
        var stopwatch = Stopwatch.StartNew();
        // Create an array of Thread references.
        Thread[] array = new Thread[4];
        for (int i = 0; i < array.Length; i++)
        {
            // Start the thread with a ThreadStart.
            array[i] = new Thread(new ThreadStart(Start));
            array[i].Start();
        }
        // Join all the threads.
        for (int i = 0; i < array.Length; i++)
        {
            array[i].Join();
        }
        Console.WriteLine("DONE: {0}", stopwatch.ElapsedMilliseconds);
    }

    static void Start()
    {
        // This method takes ten seconds to terminate.
        Thread.Sleep(10000);
    }
}
DONE: 10001

The results show the effectiveness of threading. The four threads would have slept for ten seconds each, but the entire program only took ten seconds total to execute.

ThreadPool

Instead of using a Thread array and implementing the threading functionality, you can use the ThreadPool type. This has further improvements that can improve CPU utilization.

Summary

Join() from the .NET System.Threading namespace is an important C# threading method. It provides blocking functionality that waits for the specified thread to complete.