Вот ещё неплохое решение для объединения разных видов авторизаций

public class Login : Page {
  protected Label ErrorMessageLabel; 
  protected TextBox UsernameTextBox; 
  protected TextBox PasswordTextBox; 
  protected CheckBox CheckBox1; 

  public void Page_Load(object o, EventArgs e) {
    if (IsPostBack) {
      string authenticatedUser = null; 
      if (CheckBox1.Checked) // Use their network credentials 
      {
        string user = Request.ServerVariables["LOGON_USER"];
        if (user.Length == 0) // They haven't provided credentials yet
        { 
          Response.StatusCode = 401;
          Response.StatusDescription = "Unauthorized";
          Response.End(); 
        }
        else // They have
        {
          authenticatedUser = user; 
        }
      }
      else // Use the username and password they provide
      {
         if (IsPasswordOK(UsernameTextBox.Text, PasswordTextBox.Text)) {
          authenticatedUser = UsernameTextBox.Text; 
        }
      }
      if (authenticatedUser != null) // They authenticated successfully
      {
        // Issue the Forms Auth cookie and send them on their way
        FormsAuthentication.RedirectFromLoginPage(authenticatedUser, false);
      }
      else // They didn't
      {
        ErrorMessageLabel.Text = "Invalid username or bad password. Please try again."; 
      }
    }
  }
}