Making a thread wait

This recipe will show you how a program can wait for some computation in another thread to complete to use its result later in the code. It is not enough to use Thread.Sleep because we don't know the exact time the computation will take.

Getting ready

To work through this recipe, you will need Visual Studio 2012. There are no other prerequisites. The source code for this recipe can be found at BookSamples\Chapter1\Recipe3.

How to do it...

To understand how a program can wait for some computation in another thread to complete to use its result later, perform the following steps:

1. Start Visual Studio 2012. Create a new C# Console Application project.
2. In the Program.cs file, add the following using directives:

   using System;
   using System.Threading;

3. Add the following code snippet below the Main method:

   static void PrintNumbersWithDelay()
   {
     Console.WriteLine("Starting...");
     for (int i = 1; i < 10; i++)
     {
       Thread.Sleep(TimeSpan.FromSeconds(2));
       Console.WriteLine(i);
     }
   }

4. Add the following code snippet inside the Main method:

   Console.WriteLine("Starting...");
   Thread t = new Thread(PrintNumbersWithDelay);
   t.Start();
   t.Join();
   Console.WriteLine("Thread completed");

5. Run the program.

How it works...

When the program is run, it runs a long-running thread that prints out numbers and waits two seconds before printing each number. But in the main program, we called the t.Join method, which allows us to wait for thread t to complete. When it is complete, the main program continues to run. With the help of this technique, it is possible to synchronize execution steps between two threads. The first one waits until another one is complete and then continues to work. While the first thread is waiting, it is in a blocked state (as it is in the previous recipe when you call Thread.Sleep).