in page login :
private void SetCookies(string id)
{
SetSession("sesID", usernameUser);
}
private void SetSession(string nameSession, string valuesession)
{
Session[nameSession] = valuesession.Trim();
}
and page logout :
protected void Page_Load(object sender, EventArgs e)
{
FormsAuthentication.SignOut();
Session.RemoveAll();
Session.Abandon();
Response.Redirect("Default.aspx",false);
}