Is My `foreach` Loop and Lambda Threading Safe in C#?

Thread safety issues of foreach loops and Lambda expressions in C#

Let’s analyze the following code snippet:

foreach (Foo f in ListOfFoo)
{      
    Thread thread = new Thread(() => f.DoSomething());
    threads.Add(thread);
    thread.Start();
}

foreach (Foo f in ListOfFoo)
{      
    Foo f2 = f;
    Thread thread = new Thread(() => f2.DoSomething());
    threads.Add(thread);
    thread.Start();
}

Which code snippet ensures that each thread calls a method on the Foo instance in the same loop iteration as when the thread was created?

In C# 5 and later, both code snippets are safe due to compiler changes to variable definitions within closure scopes. However, prior to C# 5, only the second snippet was safe.

In the first code snippet, the variable f is declared outside the loop and remains visible throughout execution. This means there is only one instance of f in the closure scope. Therefore, different threads accessing f may conflict, causing method invocation errors.

To alleviate this problem, the second code snippet declares a new variable f2 inside the loop. This ensures that each closure scope has its own copy of the reference to the Foo instance, guaranteeing per-thread safe method execution.

The following example demonstrates this problem:

static void Main()
{
    int[] data = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };

    foreach (int i in data)
    {
        new Thread(() => Console.WriteLine(i)).Start();
    }

    Console.ReadLine();
}

If temporary variables are not used, this code will produce unpredictable and incorrect output. This is because all threads reference the same i variable, and the value of foreach is the last value after the i loop ends.

Therefore, to ensure thread safety, it is best to always create a copy of a local variable when using a Lambda expression within a foreach loop, as shown in the second code snippet.

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