Parallel.ForEach - Access To Modified Closure Applies?

Question

I've read a number of other questions about Access to Modified closure so I understand the basic principle. Still, I couldn't tell - does Parallel.ForEach have the same issues?

Take the following snippet where I recompute the usage stats for users for the last week as an example:

var startTime = DateTime.Now;
var endTime = DateTime.Now.AddHours(6);
for (var i = 0; i < 7; i++)
{
    // this next line gives me "Access To Modified Closure"
    Parallel.ForEach(allUsers, user => UpdateUsageStats(user, startTime, endTime));

    // move back a day and continue the process
    startTime = startTime.AddDays(-1);
    endTime = endTime.AddDays(-1);
}

From what I know of this code the foreach should run my UpdateUsageStats routine right away and start/end time variables won't be updated till the next time around the loop. Is that correct or should I use local variables to make sure there aren't issues?

Answer 1

You are accessing a modified closure, so it does apply. But, you are not changing its value while you are using it, so assuming you are not changing the values inside UpdateUsageStats you don't have a problem here.

Parallel.Foreach waits for the execution to end, and only then are you changing the values in startTime and endTime.