Why Do Foreach Loops and Closures Require Special Handling in Multithreaded C# Code?

Understanding foreach Loops and Closures in Multithreaded C#

Multithreading in C# can introduce complexities when using foreach loops and closures. The problem arises when a closure within a foreach loop references the loop's iteration variable.

Consider this example:

<code class="language-csharp">var threads = new List<Thread>();
foreach (Foo f in ListOfFoo)
{      
    Thread thread = new Thread(() => f.DoSomething());
    threads.Add(thread);
    thread.Start();
}</code>

Here, each thread's closure captures a reference to the single f variable. As the loop progresses, f's value changes, leading to unpredictable results—threads might operate on unexpected Foo objects.

Ensuring Thread Safety

To avoid this, create a new local variable within each iteration:

<code class="language-csharp">var threads = new List<Thread>();
foreach (Foo f in ListOfFoo)
{      
    Foo f2 = f; // Create a copy for each thread
    Thread thread = new Thread(() => f2.DoSomething());
    threads.Add(thread);
    thread.Start();
}</code>

Now, each thread has its own independent reference (f2), ensuring that DoSomething() is called on the correct Foo instance.

C# 5 and Beyond

The behavior changed slightly in C# 5 and later. The compiler treats the foreach variable as effectively declared within the loop's scope, even though it appears outside. Therefore, both code snippets above are thread-safe in C# 5 and subsequent versions. However, the explicit copy remains good practice for clarity and backward compatibility.

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