Read file content on file when file is changed

Question

I have some archive file with data, I have some thread that writes to this archive file, at the main thread I have file system watcher that reads the file content when there is some change:

static FileSystemWatcher watcher = new FileSystemWatcher();
string archPath=@"C:\arch.xml";

public MyClass()
{
   watcher.Path = dir;
   watcher.NotifyFilter = NotifyFilters.LastWrite | NotifyFilters.CreationTime;
   watcher.Filter = Path.GetFileName("*" + archPath);
   watcher.Changed += OnFileChanged;
   watcher.Created += OnFileCraeted;
   watcher.EnableRaisingEvents = true;
}

public void OnFileChanged(object source, FileSystemEventArgs e)
{
        try
        {
            watcher.EnableRaisingEvents = false;
            LoadArchive();               
        }

        finally
        {
            watcher.EnableRaisingEvents = true;
        }
}

void LoadArchive()
{                                                        
     DataSet ds = new DataSet();
     ds.ReadXml(archPath); /// <----this line throws exeption                                        
}

System.IO.IOException: The process cannot access the file 'C:\arch.xml' because it is being used by another process.

my question: I want to read the content when there is some change, how to avoid this exeption?