Lambda expressions and captured variables

As we saw, a lambda expression is the most powerfulway to pass data to a thread. However, you must becareful about

accidentally modifying captured variablesafter starting the thread, because these variablesare shared. For instance,

consider the following:

1 for (int i = 0; i < 10; i++) 2 new Thread (() => Console.Write (i)).Start();

The output is nondeterministic! Here’s a typical result:

0223557799

The problem is that the ivariable refers to the samememory location throughout the loop’s lifetime. Therefore, each

thread calls Console.Writeon a variable whose value may change as it is running!

This is analogous to the problem we describe in “Captured Variables” in Chapter 8 of C# 4.0 in a Nutshell. The

problem is less about multithreading and more aboutC#'s rules for capturing variables (which are somewhat

undesirable in the case of forand foreachloops).

The solution is to use a temporary variable as follows:

1 for (int i = 0; i < 10; i++) 2 new Thread (() => Console.Write (i)).Start();

 

Variable tempis now local to each loop iteration. Therefore, each thread captures a different memory location and

there’s no problem. We can illustrate the problem in the earlier code more simply with the following example:

string text = "t1";

Thread t1 = new Thread ( () => Console.WriteLine (t ext) );

text = "t2";

Thread t2 = new Thread ( () => Console.WriteLine (t ext) );

t1.Start();

t2.Start();

Because both lambda expressions capture the same textvariable, t2is printed twice:

t2

t2