how to do two threads can not acces the same folder

Question

I am writing a multithreaded application it is windows service. I have 20 folders. I create 15 threads onstart method. I want to achieve that; 15 threads go to folders 1,2,3,...,15 sequentially. When one thread finished, it creates another thread. This created thread must go 16.th folder. It must not go to working folders. How can I do this? That is, how can I be sure that two threads do not go the same folder?

do you mean 15 threads go to every 15 folder or one folder per thread.. if the latter, why do you need threads for what would become synchronous? — Sayse, Sep 03 '14 at 12:36
just pass a parameter for determining the number. Or use parallelfor — Sebastian L, Sep 03 '14 at 12:43
Yes it might be that my solution with Task doesn't really run on 15 threads as well. By default Tasks uses the threadpool which has a limited number of threads (although you can set your own number of max threads). Anyway you would be able to have a static counter for the folder names even if you created the Threads yourself. — mortb, Sep 03 '14 at 13:47

mortb · Answer 1 · 2014-09-03T12:54:20.127

1

Could you not just have a static variable that would be a counter for the folder name?

Something like:

private static int _folderNameCounter = 0;
private static readonly object _padlock = new object();
public static int GetFolderCounter()
{
     lock(_padlock)
     {
         _folderNameCounter++;
         return _folderNameCounter;
     }
}

public static void Main()
{
        for(int i = 0; i < 20; i++)
        {

            Task.Factory.StartNew(() => 
             {
                var path = @"c:\temp\" + GetFolderCounter();
                Directory.CreateDirectory(path);
                // add your own code for the thread here
             });
        }

}

edited Sep 03 '14 at 12:54

answered Sep 03 '14 at 12:46

mortb

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No, it does not really start 20 threads. The taskscheduler determines how many. If you "explictly" want to start 20 threads you can specify the TaskCreationOptions.LongRunning option to Task.Factory.StartNew , (it does not seem 100% guaranteed that you get that many threads): http://stackoverflow.com/questions/13570579/is-it-possible-always-to-force-a-new-thread-with-task – mortb Sep 04 '14 at 09:21

score 0 · Answer 2 · edited May 23 '17 at 12:12

Note: I've used the TPL instead of using Threads directly since I think that the TPL is a better solution. You can of course have specific requirements which can mean that Threads is the better solution for your case.

Use a BlockingCollection<T> and fill the collection with the folder numbers. Each task handles an item of the collection, and the collection itself handles the multi-threading aspect so that each item is only handled by one consumer.

// Define the blocking collection with a maximum size of 15.
const int maxSize = 15;
var data = new BlockingCollection<int>(maxSize);

// Add the data to the collection.
// Do this in a separate task since BlockingCollection<T>.Add()
// blocks when the specified capacity is reached.
var addingTask = new Task(() => {
    for (int i = 1; i <= 20; i++) {
        data.Add(i);
    }
).Start();

// Define a signal-to-stop bool
var stop = false;

// Create 15 handle tasks.
// You can change this to threads if necessary, but the general idea is that
// each consumer continues to consume until the stop-boolean is set.
// The Take method returns only when an item is/becomes available.
for (int t = 0; t < maxSize; t++) {
    new Task(() => {
        while (!stop) {
            int item = data.Take();
            // Note: the Take method will block until an item comes available.
            HandleThisItem(item);
        }
    }).Start();
};

// Wait until you need to stop. When you do, set stop true
stop = true;

how to do two threads can not acces the same folder

2 Answers2