Synchronizing Threads in a Multithreaded Application in .Net - C#

The Problem "Concurrency"

When you build a multithreaded application, your program needs to ensure that shared data should be protected from the possibility of multiple thread engagement with its value. What's going to happen if multiple threads were accessing the data at the same point? The CLR can suspend any thread for a while who's going to update the value or is in the middle of updating the value and at same time a thread comes to read that value which is not completely updated, that thread is reading an uncompleted/unstable data.

To illustrate the problem of concurrency let us write some line of code.

class Program
{
    static void Main(string[] args)
    {
        Console.WriteLine("----Synchronnization of Threads-----");
        Console.WriteLine("Main Thread {0}", Thread.CurrentThread.ManagedThreadId);
        Printer p = new Printer();

        Thread[] threads = new Thread[5];

        //Queue 5 threads
        for (int i = 0; i < 5; i++)
        {
            threads[i] = new Thread(new ThreadStart(p.PrintNumbersNonSync));
        }
        foreach (Thread t in threads)
        {
            t.Start();
        }
        Console.ReadLine();
    }
}

class Printer
{
    public void PrintNumbersNonSync()
    {
        Console.WriteLine(" ");
        Console.WriteLine("Executing Thread {0}", Thread.CurrentThread.ManagedThreadId);
        for (int i = 1; i <= 10; i++)
        {
            Console.Write(i + " ");
        }
    }
}

Run this program multiple times and watch the output:

Output 1:

Output 2:

As you can see all the output will vary each time you run the program.

What is happening here is that all the threads are sharing the same object of the Printer class and trying to execute the same function at the same time so every time the shared data is being updated randomly which is an unstable state.

Synchronization of threads Solution to the problem of concurrency

Use the locks whenever there's a Shared section. C# provides a Lock keyword that can be used to lock the section that is being accessed by multiple threads. This is the very basic technique to avoid the instability in a multithreaded environment while programming with C#.

The Lock keyword requires you to specify the token (an object reference) that must be acquired by a thread to enter within the lock scope. In the case of a private method you can lock it down by passing the reference of the current type using the "this" keyword.

For e.g.

public void PrintNumbersSynchronized()
{
    //Synchronization thread
    lock (this)
    {
        Console.WriteLine(" ");
        Console.WriteLine("Executing Thread {0}", Thread.CurrentThread.ManagedThreadId);

        for (int i = 1; i <= 10; i++)
        {
            Console.Write(i + " ");
        }

    }
}

However if you are locking down a region of code within a public member, it is safer (and best practice) to declare a private object member variable to serve as the lock token:

class Printer
{
    // Synchronization token
    private Object ThreadLock = new object();
 
    public void PrintNumbersSynchronized()
    {
        //Synchronization thread
        lock (ThreadLock)
        {
            Console.WriteLine(" ");
            Console.WriteLine("Executing Thread {0}", Thread.CurrentThread.ManagedThreadId);
            for (int i = 1; i <= 10; i++)
            {
                Console.Write(i + " ");
            }



        }
    }
}

Output:

Other methods to perform the synchronization

Monitor is a class in the System.Threading namespace that also provides the same functionality. Actually lock is just a shorthand notation of Monitor class. When the compiler processes a lock it actually resolves to the following:

public void PrintNumbersSync()
{
    Monitor.Enter(ThreadLock);
    try
    {
        Console.WriteLine(" ");
        Console.WriteLine("Executing Thread {0}", Thread.CurrentThread.ManagedThreadId);
        for (int i = 1; i <= 10; i++)
        {
            Console.Write(i + " ");
        }
    }
    finally
    {
        Monitor.Exit(ThreadLock);
    }
}

Here with a little more code you can see having the try..finally block. Other than Enter() and Exit() methods, the Monitor class also provides the methods like Monitor.Pulse() and PulseAll() to inform the waiting threads that it has completed.

Simple operations using the Interlocked Type

To perform some basic operation it's not necessary that you write the block using the lock or Monitor. The System.Threading namespace provides an interesting class that can help you out.

Now you can replace this

lock (ThreadLock)
{
    intVal++;
}

int newVal = Interlocked.Increment(ref intVal);

class Program
{
    static void Main(string[] args)
    {
        Console.WriteLine("----Synchronnization of Threads-----");
        Console.WriteLine("Main Thread {0}", Thread.CurrentThread.ManagedThreadId);
        Printer p = new Printer();
        Thread[] threads = new Thread[5];

        //Queue 5 threads
        for (int i = 0; i < 5; i++)
        {
            threads[i] = new Thread(new ThreadStart(p.PrintNumbersNonSync));
        }
        foreach (Thread t in threads)
        {
            t.Start();
        }

        Console.ReadLine();
    }
}

[Synchronization]
class Printer : ContextBoundObject
{
    public void PrintNumbersNonSync()
    {
        Console.WriteLine(" ");
        Console.WriteLine("Executing Thread {0}", Thread.CurrentThread.ManagedThreadId);
        for (int i = 1; i <= 10; i++)
        {
            Console.Write(i + " ");
        }
    }
}