Finding Position of a CRM user programatically

Hi guys,

I would like to retrieve the position of a systemuser, however I am not able to get the positionid.

WhoAmIRequest whoReq = new WhoAmIRequest();
WhoAmIResponse whoResp = (WhoAmIResponse)_service.Execute(whoReq);

   Guid userid = whoResp.UserId;

Entity queryPositionid = _service.Retrieve("systemuser", userid, new ColumnSet("positionid"));
if (queryPositionid.Contains("positionid"))
{
_positionid = queryPositionid["positionid"].ToString(); //I am not able to get the id here, having exception :Guid //should contain 32 digits with 4 dashes (xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx).

Entity _position = _service.Retrieve("position", new Guid(_positionid), new ColumnSet("name"));
if (_position.Contains("name"))
{
_positionname = _position["name"].ToString();

}

....

Please advise, thanks guys!