Capturing outer variables closures

Context: Lambdas can capture variables from the enclosing scope. The captured variables are stored in a closure and live as long as the lambda.

int factor = 2;
Func<int, int> multiplier = x => x * factor;
Console.WriteLine(multiplier(5)); // 10

factor = 3;
Console.WriteLine(multiplier(5)); // 15 (captures by reference)

Capturing local variables

List<Action> actions = new List<Action>();
for (int i = 0; i < 3; i++)
{
    int captured = i;
    actions.Add(() => Console.WriteLine(captured));
}
foreach (var a in actions) a(); // 0,1,2 (not 3,3,3)

Performance implications

Capturing can cause allocations (closure objects). Avoid capturing in performance‑critical loops.

Real-world usage example

Task continuation: Capturing userId in an async task continuation.

int userId = GetUserId();
Task.Run(() => ProcessUser(userId));

Example: In ASP.NET Core, lambdas in middleware often capture configuration values.

Related sections

• 7.3 Lambda expressions
• 7.3.2 Statement lambdas