Question

Here’s an MWE:

Func<Int32, Boolean> MyFunc = (s) => {
    var res = false;
    // Insert function logic to modify the value to res
    return res;
};
var Result = new List<Int32> ();
var LockObj = new Object ();
ParallelEnumerable.Range (1, 100000)
    .ForAll (s => {
        if (MyFunc (s)) {
            lock (LockObj) { // IS THIS NECESSARY?
                Result.Add (s);
            } // End lock
        }
    });

This is what the situation boils down to. Am I correct in thinking I don’t need to lock Result if I am not going to ever query it before the ParallelEnumerable statement has finished executing?

Please note: I know that the MWE is better solved by a “Where” clause, like so:

ParallelEnumerable.Range (1, 100000)
    .Where (s => MyFunc (s));

but for reasons not evident in the MWE, this cannot be done.

EDIT

Thank you to everyone who answered. Thanks also for the comments. I have corrected the error spotted by Tung.

Answer 1

Yes, you have to lock. The Parallel.For will cause concurrent calls to Add().

On a side note:

//var Result = new List<Int32> ();
var Result = new List<Int32> (100000);
ParallelEnumerable.Range (1, 100000)

Will make this a lot more efficient. Less growing also means less contention on the lock.