Access to modified closure... but why?

Question

Saw several similar questions here, but none of them seemed to quite be my issue...

I understand (or thought I understood) the concept of closure, and understand what would cause Resharper to complain about access to a modified closure, but in the below code I don't understand how I'm breaching closure.

Because primaryApps is declared within the context of the for loop, primary isn't going to change while I'm processing primaryApps. If I had declared primaryApps outside the for loop, then absolutely, I have closure issues. But why in the code below?

var primaries = (from row in openRequestsDataSet.AppPrimaries
                 select row.User).Distinct();

    foreach (string primary in primaries) {

        // Complains because 'primary' is accessing a modified closure
        var primaryApps = openRequestsDataSet.AppPrimaries.Select(x => x.User == primary);

Is Resharper just not smart enough to figure out it's not an issue, or is there a reason closure is an issue here that I'm not seeing?

Answer 1

The problem is in the following statement

Because primaryApps is declared within the context of the for loop, primary isn't going to change while I'm processing primaryApps.

There is simply no way for Resharper to 100% verify this. The lambda which references the closure here is passed to function outside the context of this loop: The AppPrimaries.Select method. This function could itself store the resulting delegate expression, execute it later and run straight into the capture of the iteration variable issue.

Properly detecting whether or not this is possible is quite an undertaking and frankly not worth the effort. Instead ReSharper is taking the safe route and warning about the potentially dangerous capture of the iteration variable.