locking static variable

Question

I have a following class at server-side.

public class Sample { private enum Status { NotEvaluated, Yes, No } private static object _lockObj = new object(); private static Status _status = Status.NotEvaluated; public static Status GetStatus() { if (_status == Status.NotEvaluated) { lock (_lockObj) { if (_status == Status.NotEvaluated) { //some evaluation code which sets status to either Yes/No; _status = Status.Yes; } } } return _status; } }

Is anything wrong in locking mechanism above? do i need to lock at all? Because it is server-side (multiple requests will be there) and the variable is static i would think it should be locked at the time of evaluation.

correct me if i am wrong.

Thanks

gangelo · Accepted Answer · 2011-06-30 16:13:43Z

You do not/should not have the outer check for "if (_status == Status.NotEvaluated)". While it appears that nothing "bad" would happen if you left it it, there is the possibility that a second thread may enter that "if" needlessly, just prior to the first thread setting _status to Status.Yes. And yes, you need to lock:

"As a basic rule, you need to lock around accessing any writable shared field." http://www.albahari.com/threading/part2.aspx

lock (_lockObj) { if (_status == Status.NotEvaluated) { // some evaluation code which sets status to either Yes/No; _status = Status.Yes; } return _status; }

I'm guessing OP was trying to optimize by not locking when he KNOWS that he doesn't have to, but the lock() is fairly lightweight - this is not an optimization that you need to do.

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