Confusion about value capturing in C# lambda

Question

To illustrate my confusion, see the following example:

int a = 0;
Action act = () => ++a;
act();
Console.WriteLine(a);

I have difficulty figuring out how modification to the captured variable within the lambda could possibly affect the local variable a . First of all, the implicitly generated lambda function object could not store a reference to the local variable a . Otherwise, if act is returned and invoked later, the referenced local variable a would already be vanished. A solution to this problem would be copy-by-value, either by copying the int value directly or through boxing, so that the function object would have its own copy the local variable a . But this doesn't explain the example just given. So, what is the underlying mechanism? Would it be that the seemingly local variable a is actually no longer a local variable, but translated by the compiler to be a reference to an int field within the generated lambda function object?

Answer 1

The point here is closure . After compilation a is not a local variable anymore - it's a field of auto-generated class, both in function scope and in lambda.