I was reading through the tips and tricks post and I thought I'd try out some of the C# stuff that I'd never done before. Therefore, the following code serves no actual purpose, but is just a 'test function' to see what happens.
Anyway, I have two static private fields:
private static volatile string staticVolatileTestString = "";
[ThreadStatic]
private static int threadInt = 0;
As you can see, I'm testing ThreadStaticAttribute and the volatile keyword.
Anyway, I have a test method that looks like this:
private static string TestThreadStatic() {
// Firstly I'm creating 10 threads (DEFAULT_TEST_SIZE is 10) and starting them all with an anonymous method
List<Thread> startedThreads = new List<Thread>();
for (int i = 0; i < DEFAULT_TEST_SIZE; ++i) {
Thread t = new Thread(delegate(object o) {
// The anon method sets a newValue for threadInt and prints the new value to the volatile test string, then waits between 1 and 10 seconds, then prints the value for threadInt to the volatile test string again to confirm that no other thread has changed it
int newVal = randomNumberGenerator.Next(10, 100);
staticVolatileTestString += Environment.NewLine + "\tthread " + ((int) o) + " setting threadInt to " + newVal;
threadInt = newVal;
Thread.Sleep(randomNumberGenerator.Next(1000, 10000));
staticVolatileTestString += Environment.NewLine + "\tthread " + ((int) o) + " finished: " + threadInt;
});
t.Start(i);
startedThreads.Add(t);
}
foreach (Thread th in startedThreads) th.Join();
return staticVolatileTestString;
}
What I'd expect to see returned from this function is an output like this:
thread 0 setting threadInt to 88
thread 1 setting threadInt to 97
thread 2 setting threadInt to 11
thread 3 setting threadInt to 84
thread 4 setting threadInt to 67
thread 5 setting threadInt to 46
thread 6 setting threadInt to 94
thread 7 setting threadInt to 60
thread 8 setting threadInt to 11
thread 9 setting threadInt to 81
thread 5 finished: 46
thread 2 finished: 11
thread 4 finished: 67
thread 3 finished: 84
thread 9 finished: 81
thread 6 finished: 94
thread 7 finished: 60
thread 1 finished: 97
thread 8 finished: 11
thread 0 finished: 88
However, what I'm getting is this:
thread 0 setting threadInt to 88
thread 4 setting threadInt to 67
thread 6 setting threadInt to 94
thread 7 setting threadInt to 60
thread 8 setting threadInt to 11
thread 9 setting threadInt to 81
thread 5 finished: 46
thread 2 finished: 11
thread 4 finished: 67
thread 3 finished: 84
thread 9 finished: 81
thread 6 finished: 94
thread 7 finished: 60
thread 1 finished: 97
thread 8 finished: 11
thread 0 finished: 88
The second 'half' the output is as expected (which I suppose means that the ThreadStatic field is working like I thought), but it seems like a few of the initial outputs have been 'skipped' from the first 'half'.
Additionally, the threads in the first 'half' are out of order, but I understand that a thread does not immediately run as soon as you call Start(); but instead the internal OS controls will be starting the threads as it sees fit.
EDIT: No they're not, actually, I just thought they were because my brain misses the consecutive numbers
So, my question is: What's going wrong to cause me to lose a few lines in the first 'half' of the output? For example, where is the line 'thread 3 setting threadInt to 84'?
Threads are executing at the same time. What conceptually happens is this:
staticVolatileTestString += Environment.NewLine + "\tthread " + ((int) o) + " setting threadInt to " + newVal;
That causes your lines to be lost. Volatile doesn't help here; the whole process of concatenating the string is not atomic. You need to use a lock around those operations:
private static object sync = new object();
lock (sync) {
staticVolatileTestString += Environment.NewLine + "\tthread " + ((int) o) + " setting threadInt to " + newVal;
}