Thread.CurrentPrincipal.Identity is changed after set in firstTime

Question

I have a simple wpf sample(a window with a button) .I set Thread.CurrentPrincipal in constructor like:

//my Window constructor
public Window1(){
InitializeComponent();
Thread.CurrentPrincipal = new WindowsPrincipal(WindowsIdentity.GetCurrent());   
}

and I watch that Thread.CurrentPrincipal is correct.

I put a breakepoint in buttonclick event .after I click button I check Thread.CurrentPrincipal but it's change and my it's identity Name is empty