scope of variables and events problem

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C#

event

I am working here with FileSystemWatcher, and i want to keep an arraylist of all the files created during the session of the application.
So after and event on the filesystem i want to add the file to the array.
Now this doesn't work. But how to implement it the right way.

snippit

class Import
{
   private ArrayList a;
   private FileSystemWatcher f;
   public Import()
   {
      // create and configure Arraylist and FileWatcher here
      
      f.Changed += new FileSystemEventHandler(OnChanged);
   }
   private static void OnChanged(object source, FileSystemEventArgs args)
   {
      // Here i cannot access the Arraylist a to add the args. to it.
   }
}

Sorry for the newbie level