Why Do Lambda Expressions Capture the Final Value of Iteration Variables, and How Can This Be Avoided?

Risks of using iteration variables in Lambda expressions

Lambda expressions provide a concise way to define inline functions in loops. However, using iteration variables directly in lambdas may lead to unexpected behavior later.

Lambda trap of iterating variables:

Consider the following code:

<code class="language-c#">for (int i = 0; i < 10; i++) {
    Action action = () => Console.WriteLine(i);
    actions.Add(action);
}

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

One might think that each lambda will print the corresponding value of i. However, due to closures, all lambdas share the same captured reference to i.

Unexpected print result:

When the loop exits, i becomes 10 and all lambdas now point to this final value. As a result, executing the lambda will print "10" ten times instead of the expected sequence from 0 to 9.

This behavior results from lambda closures maintaining references to variables declared within their scope, even after the loop has completed.

Avoidances and alternatives:

To solve this problem, create a local variable in the loop and assign it the value of the iteration variable:

<code class="language-c#">for (int i = 0; i < 10; i++) {
    int j = i; // 创建一个局部变量
    Action action = () => Console.WriteLine(j);
    actions.Add(action);
}

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

This ensures that each lambda captures a different value of j, resulting in the expected sequence of prints.

Keep in mind that using iteration variables directly in lambda expressions may lead to unexpected closure-related conditions. Instead, consider creating local variables to capture exactly the value you need.

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