public bool IsSignedOn()

{
  //Find the portal to sign in with using its Uri...
  var uri = new Uri("http://myportal.esri.com/portal/", UriKind.Absolute);
  var aPortal = ArcGISPortalManager.Current.GetPortal(uri);
  if (!aPortal.IsSignedOn())
  {
    //Calling "SignIn" will trigger the OAuth popup if your credentials are
    //not cached (eg from a previous sign in in the session)
    if (portal.SignIn().success)
    {
      //Set this portal as my active portal
      ArcGISPortalManager.Current.SetActivePortal(portal);
    }
  }
}

{
  //Force login
  if (!portal.IsSignedOn())
  {
    portal.SignIn();
  }
  var user = portal.GetSignOnUsername();
}