Lock() in a static method

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
            }
        }
    }
}