Multitasking c#| Getting Array index out of bound

Question

I’m writing a method which will download files from both LAN shared and FTP Shared directories. I have list of multiple vendors. I am using tasks to achieve this. But I am getting ArrayIndexOutofBond Exception. Although my loop condition is right?

Here is code segment.

 vendorWiseTasksListening = new List<Task>();
                    for (int i = 0; i < allVendorIDs.Length; i++)
                    {
                        Task task = new Task(() => FTPMode(allVendorIDs[i]));
                        vendorWiseTasksListening.Add(task);
                        task.Start();
                        task = new Task(() => SharedMode(allVendorIDs[i]));
                        vendorWiseTasksListening.Add(task);
                        task.Start();
                    }
                    Task.WaitAll(vendorWiseTasksListening.ToArray());

Any help would be appreciated.

Well, unless your shared objects are made smart enough to be thread safe ... they are not thread safe which means you get errors accessing them from multiple threads. — TomTom, Apr 11 '16 at 06:46
Locking of SOME sort. Programming in a way that does not require shared objects and does the collection at the end. LOCKING is easiest (read up the lock keyword). At the end: you ask us to teach you programming. Too broad. — TomTom, Apr 11 '16 at 06:54
Learn about thread safety. http://stackoverflow.com/questions/11623039/how-to-make-objects-threadsafe-on-c/11623449 — L-Four, Apr 11 '16 at 06:57

score 2 · Answer 1 · edited May 23 '17 at 11:52

Your problem might be, that the value of you variable i is NOT saved once you create the new task, but all your tasks use a reference to the same variable i. And this i is incremented until allVendorIDs.Length.

If some Task accesses the value of i after the loop in the main thread has finished, it will get allVendorIDs.Length.

Make a local copy of i and use this in your tasks:

 for (int i = 0; i < allVendorIDs.Length; i++)
                {
                    int j = i;
                    Task task = new Task(() => FTPMode(allVendorIDs[j]));
                    vendorWiseTasksListening.Add(task);

                    task.Start();
                    task = new Task(() => SharedMode(allVendorIDs[j]));
                    vendorWiseTasksListening.Add(task);
                    task.Start();
                }

See for example this question for a simpler example and explanation.

Multitasking c#| Getting Array index out of bound

1 Answers1