C# Sericalization crashes in the middle

Hi all,
I have develepod a basic serialization program with the instruction of my instructor.
basically what the program does is,a c# app and a java app access a same XML file. i.e. if i click the button on the c# app, the button will go false and the button and java app will become true. and once i click the java button, its button will go false and the c# button will become true. like wise it goes.

the problem is, in the middle of the repetition c# program crashes saying the xml file is being used by another program.

any help would be greatly appreciated.
i have attached the source code for your information.
(if it doesn't open plz change the extention .zip to .rar

thanks a lot.