Why are Captured Closures Problematic in C# 5.0 For Loops but Not Foreach Loops?

Capturing closures in C# 5.0 loops: The difference between For loops and Foreach loops

Question:

In C# 5.0, capturing closures in foreach loops capture correctly, but why are there still problems with capturing closures in for loops?

Answer:

Logically, the behavior of closures in for loops is reasonable. After breaking the for loop into its component parts (initializer, condition, iterator, and body), the initializer is executed only once. Therefore, it is logical that there is only one "variable instantiation".

Also, in a for loop, the initial value of the variable for each iteration is not predetermined. Consider the following example:

for (int i = 0, j = 10; i < 5; i++, j--) {
    Action action = () => Console.WriteLine(i, j);
    action();
}

In this loop, the value of j may change during the loop. What is the expected behavior in this case?

In contrast, the foreach loop appears to declare a new variable for each iteration, and that variable is read-only. Therefore, it is reasonable to think of a foreach loop as declaring a separate read-only variable on each iteration whose value is taken from the iterator. This explains why closures capture correctly in foreach loops.

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