Question

What exactly is the Outer Variable Trap?
Explanation and examples in C# are appreciated.

EDIT: Incorporating Jon Skeet’s diktat 🙂

Eric Lippert on the Outer Variable Trap

Answer 1

The "Outer Variable Trap" occurs when a developer expects the value of a variable to be captured by a lambda expression or anonymous delegate, when actually the variable is captured itself.

Example:

var actions = new List<Action>();
for (var i = 0; i < 10; i++)
{
    actions.Add(() => Console.Write("{0} ", i));
}
foreach (var action in actions)
{
    action();
}

Possible output #1:

0 1 2 3 4 5 6 7 8 9

Possible output #2:

10 10 10 10 10 10 10 10 10 10

If you expected output #1, you’ve fallen into the Outer Variable Trap. You get output #2.

Fix:

Declare an "Inner Variable" to be captured repeatedly instead of the "Outer Variable" which is captured only once.

var actions = new List<Action>();
for (var i = 0; i < 10; i++)
{
    var j = i;
    actions.Add(() => Console.Write("{0} ", j));
}
foreach (var action in actions)
{
    action();
}

For more details, see also Eric Lippert’s blog.