Thanks for reply Andy,

But there exists something strange.
I have a custom Principal class implement IPrincipal interface.
i have two delegate ike below

protected delegate object AsynchronousCall(params object[] args);
protected delegate void AsynchronousCallEnd(object arg);

and my background thread method is

IPrincipal Logon(string username, string password)
{
   Principal myPrincipal = new Principal();
   //Some login operations and populate myPrincipal.
   return myPrincipal;
}

and i call this Logon method on button click
private void okButton_Click(object sender, System.EventArgs e)
{
 AsynchronousCall asyncCall = new AsynchronousCall(Logon);
 asyncCall.BeginInvoke(new object[] {uname.pwd}, new
AsyncCallback(OnLogonEnd), asyncCall);
}

and when my background thread finished its work , OnLogonEnd method
called back.

protected void OnLogonEnd(IAsyncResult ar)
{
 AsynchronousCall d = (AsynchronousCall)ar.AsyncState;
 object returnValue = d.EndInvoke(ar);
 this.OnAsynchronousCallEnd(new object[] {returnValue});
 this.Invoke(new AsynchronousCallEnd(this.OnAsynchronousCallEnd),
returnValue);
}

protected override void OnAsynchronousCallEnd(object arg)
{
 System.Threading.Thread.CurrentPrincipal = (IPrincipal) arg;
 new XForm().Show();
}

i debugged many times and when code entered XForm OnLoad method
System.Threading.Thread.CurrentPrincipal has correct value.

private void XForm_Load(object sender, System.EventArgs e)
{
 System.Security.Principal.IPrincipal p =
System.Threading.Thread.CurrentPrincipal;
 MessageBox.Show(p.Identity.Name);
} // has correct value

But i click on the XForm, i saw the incorrect GenericPrincipal in the
CurrentPrincipal.

private void XForm_Click(object sender, System.EventArgs e)
{
 System.Security.Principal.IPrincipal p =
System.Threading.Thread.CurrentPrincipal;
 MessageBox.Show(p.Identity.Name);
} // has INCORRECT value

Thanks a lof for your interest, again.