How do parameters work when a page is refreshed?

 Hello,
The title for this question isn't very good, but it is the best i could do.  Basically i'm confused as to how class paramters are handled.  This is just a simple code behind file i created to show you what i mean.

public class MyPage : Page
{
private string test;

public void change()
{
test = "some value";
//something that makes the page refresh
}

public void show()
{
lblStatus.Text = test;
}
} Say the method change() refreshes the page.  Show() results in a null reference exception.  What happens to paramters like these when the page is redirected or refreshed?  If they just disappear is there a way for me ...