Chapter 118: Making a variable thread safe

Section 118.1: Controlling access to a variable in a Parallel.For loop

using System;

using System.Threading; using System.Threading.Tasks;

class Program

{

static void Main( string[] args )

{

object sync = new object(); int sum = 0;

Parallel.For( 1, 1000, ( i ) => {

lock( sync ) sum = sum + i; // lock is necessary

//As a practical matter, ensure this `parallel for` executes

//on multiple threads by simulating a lengthy operation.

Thread.Sleep( 1 );

} );

Console.WriteLine( "Correct answer should be 499500. sum is: {0}", sum );

}

}

It is not su cient to just do sum = sum + i without the lock because the read-modify-write operation is not atomic.

A thread will overwrite any external modifications to sum that occur after it has read the current value of sum, but before it stores the modified value of sum + i back into sum.