Is Using the `foreach` Variable Directly in C# Multi-threaded Closures Safe?

foreach Variables and multi-threaded closures in C#: Are they safe?

When using foreach to loop over a collection in C#, it is crucial to understand the scope and behavior of iteration variables, especially in a multi-threaded environment.

The following code snippet explores whether it is safe to use foreach variables directly as closure arguments in multi-threading, and whether a local copy needs to be created for each iteration.

Pre-C# 5 versions

In versions prior to C# 5, foreach variables were declared outside the loop. This means that a single variable instance is shared across all iterations.

Consider the first code snippet:

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

In this example, the f variable is shared among all threads created during the loop iteration. This can lead to unpredictable results, as multiple threads may end up calling methods on different instances of Foo, or calling methods on the same instance multiple times.

To avoid such problems, it is recommended to create a local copy of the f variable in each iteration, as shown in the second code snippet:

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

C# 5 and above

C# 5 introduces significant changes. In C# 5 and above, foreach variables are defined within a loop scope. This means each iteration has its own instance of the variable.

Thus, the two code snippets become equivalent and safe. Each thread now has its own f instance within its closure scope, eliminating the risk of confusion or inconsistency.

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