Lock() in a static method

Question

I have a multi threaded application that writes to a settings xml file using a static method. I want to avoid that the file is being updated twice at the same time (causing accesss/write exception).

How do I do that?

This doesn't work:

namespace Program
{
    public class Settings
    {
        private static void SetSettingsValue (string settings, string value)
        {
            // make this thread safe to avoid writing to a locked settings xml file
            lock (typeof(Settings))
            {
                //write data to xml file
            }
        }
    }
}

Answer 1

You should create a separate, static lock object and use that. DO NOT USE A STRING! Strings are automatically interned and there will be only one instance of each programmatically declared string so you can't guarantee exclusive access to the lock.

You should do this:

public class A {
    private static Object LOCK = new Object();

    private static void foo() {
        lock(LOCK) {
            // Do whatever
        }
    }
}

(The syntax may be incorrect; I'm a Java person mostly but the same rules about locking and String interning apply to C#)

The lock keyword enforces a mutual exclusion lock: only one thread can lock any particular object at a time. If a second thread calls foo then it will block until the first thread has exited the lock block.

Take home messages: for a static method lock on a private static variable. Don't lock on Strings or typeof(...) because you cannot guarantee that no-one else is using that object. Always lock on an object you know is not touched by anyone else by making it private and making it new.

Answer 2

The concept of lock() is to use an existing-object it can reference and use to control whether access is granted.

static object SpinLock = new object();

lock(SpinLock)
{
   //Statements
}

When the execution leaves the lock() block the reference is released and any other threads waiting to execute the code block can proceed (one at a time, of course).