Handling .lock files - locking

Question

We have a large e-business application server that is out of our control, we can not modify it in any way shape or form. The company behind this application server will also not modify it to our customers requirements. This is due to the fact that they see us as a competitor and they do not wish the customer to use our application as a front end to their application.
Our desktop application uses web services to communicate with this application server and each login from the desktop occupies a user slot on the server. The application server offers no functionality that enables us to figure out if a user slot is taken or not. The best solution would be for us to develop an application that sits in the middle of the application server and our desktop client, this application would manage the allocation and deallocation of user slots and the problem is solved. This solution has be declined as the customer does not want to install our application on the e-business application server as that would void the user support of said e-business server, the alternative would be an extra server but that is something they don't want to deal with.
Which leaves us with the solution that our desktop application has write access to a shared folder. Each desktop application has a UID and we use that UID together with the user slot id and create a file UID.UserSlotId.locked. This would mean that each time a connection is made the desktop application would need to check this shared location and make sure they are not about to use a UserSlotId that is taken.
Now I now that the locked file is a terrible solution but the customer has requested this and no matter how much we inform them that this will never be a water tight solution they still want to go ahead. They are under the assumption that a 98% solution is a good solution. So what can the StackOverflow community offer in the way of advice with dealing with such a file locking system?

Answer 1

Open the lock file in Write mode and keep it open while the application uses a slot.
private static void TakeFistUnusedLock(FileStream[] currentLock)
{
for (int i = 1; i < 5; i++)
{
try
{
var fs = File.OpenWrite(Path.Combine(Path.GetTempPath(), "DbLock", i.ToString() + ".lock"));
currentLock[i - 1] = fs;
Console.WriteLine("Got lock " + i);
break;
}
catch (Exception) { }
}
}
I tested like this
FileStream[] currentLock = new FileStream[5];
var path = Path.Combine(Path.GetTempPath(), "DbLock");
DirectoryInfo di = new DirectoryInfo(path);
di.Create();
TakeFistUnusedLock(currentLock);
TakeFistUnusedLock(currentLock);
TakeFistUnusedLock(currentLock);
currentLock[1].Dispose(); // release lock 2
TakeFistUnusedLock(currentLock);
output was
Got lock 1
A first chance exception of type 'System.IO.IOException' occurred in mscorlib.dll
Got lock 2
A first chance exception of type 'System.IO.IOException' occurred in mscorlib.dll
A first chance exception of type 'System.IO.IOException' occurred in mscorlib.dll
Got lock 3
A first chance exception of type 'System.IO.IOException' occurred in mscorlib.dll
Got lock 2
Instead of keeping the currentLock array you only store one lock per application. You need to store the opened FileStream to make sure the file stays open in write mode. To release the lock you dispose the stored FileStream, then the write lock on the file is released.
This method ensures that the locks are released even if your application crashes.