An example of creating a custom exception to validate xml

Question

I want  creating a custom exception to validate xml file.

Can you me show simple example for checking syntax or structure of xml file. For example:
<toolbar>
  <button id="btn1" img="C:\Users\Dima\Desktop\folder_open.png" text="Button 1" />
  <button id="btn2" img="C:\Users\Dima\Desktop\folder_open.png" text="Button 2"/>
 </toolbar>