ReSharper
 
Get ReSharper
Get your hands on the new features ahead of the release by joining the Early Access Program for ReSharper 2025.1! Learn more

Code inspection: Access to foreach variable in closure

Last modified: 11 February 2024

First of all, let's make sure that you understand what a closure is. To put it simply, a closure in C# is a lambda expression or an anonymous method that captures some variables from an outer scope. Here is the simplest example:

In the example above, print will capture the variable myStr (and not its value) and will only get the value of myStr when you invoke print().

In more complex scenarios, when a closure is defined in a changing context, it may not behave as expected.

One of the situations where it may happen is a closure defined inside a foreach statement compiled with C# 4.0 (Visual Studio 2010) or earlier versions.

Before C# 5.0 (Visual Studio 2012), the compiler would expand a foreach to a while statement with the iteration variable defined outside the loop. Therefore, if this variable was used in a closure, later invocations of this closure would always get the value corresponding to the last iteration of the loop:

ReSharper detects this problem and suggests to fix it by copying the value of the loop variable to the scope where the closure is defined:

This fix makes sure that when you pick an action from myActions and get the context where this action was created, str will hold the value of the corresponding iteration.