递归 失误

递归 出错
public static string GetGroupTopParent3(string user_Dept)
  {

  Forms_AjaxFunctions f = new Forms_AjaxFunctions();
  string sql = "select Group_ID,Group_ParentID from SYS_BPS_GROUPS where Group_Name = '" + user_Dept + "'";
  DataTable dt = DbHelperSQL.Query(sql).Tables[0];

  string aa = f.GetGroupCode(Convert.ToString(dt.Rows[0]["Group_ParentID"]).Trim(), Convert.ToString(dt.Rows[0]["Group_ID"]).Trim());

  return aa;
  }

  public string GetGroupCode(string parentID, string groupCode)
  {
  DataTable dt = new DataTable();
  DataTable dtReturn = new DataTable();
  if (!string.IsNullOrEmpty(parentID))
  {
  if (Convert.ToInt32(parentID)!=0)
  {
  string sqlP = "select Group_ID,Group_ParentID from SYS_BPS_GROUPS where Group_ID = " + parentID + "";
  dt = DbHelperSQL.Query(sqlP).Tables[0];
  GetGroupCode(Convert.ToString(dt.Rows[0]["Group_ParentID"]).Trim(), Convert.ToString(dt.Rows[0]["Group_ID"]).Trim());
  }
 

  }
  return groupCode;
  }

为什么 parentID=0时 return groupCode 完了 还会继续走GetGroupCode(xx,xx)? 求解

[解决办法]
GetGroupTopParent3会一直递归下去，因为没有条件可以让递归退出，知道溢出报错

加上退出条件
GetGroupTopParent3()
{
if(xxxxx)
return;
}
[解决办法]