Why Does Using Iteration Variables in Lambda Expressions Lead to Unexpected Results?

Understanding the Unexpected Behavior of Iteration Variables in Lambda Expressions

A developer recently encountered a compiler warning regarding the use of loop iteration variables inside lambda expressions. This highlights a common pitfall leading to unexpected program behavior. Let's explore why this occurs.

Consider this code example:

<code class="language-csharp">List<Action> actions = new List<Action>();

for (int i = 0; i < 10; i++)
{
    actions.Add(() => Console.WriteLine(i));
}

foreach (Action action in actions)
{
    action();
}</code>

One might expect this to print numbers 0 through 9 sequentially. Instead, it prints "10" ten times. This is because the lambda expressions don't capture a copy of i for each iteration. Instead, they capture a reference to the variable i. By the time the foreach loop executes, the loop has completed, and i holds its final value of 10. Each lambda expression, therefore, prints this final value.

This unexpected outcome underscores the importance of avoiding this coding pattern. The compiler warning serves as a crucial safeguard. To achieve the desired sequential output, create a new variable inside the loop and assign the iteration variable's value to it:

<code class="language-csharp">List<Action> actions = new List<Action>();

for (int i = 0; i < 10; i++)
{
    int j = i; // Capture a copy of i
    actions.Add(() => Console.WriteLine(j));
}

foreach (Action action in actions)
{
    action();
}</code>

This revised code correctly prints 0 to 9 because each lambda expression now captures a unique, independent copy of the value j. This simple change ensures the expected and consistent behavior.

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