Single user login at a time using asp.net c#

//////////////// at login page load event

public System.Data.DataTable dtSessionGlobal = new DataTable();

    string clientIp = "";

    protected void Page_Load(object sender, EventArgs e)

    {

        Session["OptionDB"] = null;

        if (Cache["UserSessions"] != null)

        {

            dtSessionGlobal = (DataTable)Cache["UserSessions"];

            if (dtSessionGlobal.Columns.Count == 0)

            {

                dtSessionGlobal.Columns.Add("UserName");

                dtSessionGlobal.Columns.Add("SessionID");

            }

        }

        else

        {

            dtSessionGlobal.Columns.Add("UserName");

            dtSessionGlobal.Columns.Add("SessionID");

            Cache["UserSessions"] = dtSessionGlobal;

        }

    }

////////////////////////

if (dtSessionGlobal.Rows.Count > 0)

        {

            System.Data.DataRow[] dr = dtSessionGlobal.Select(" UserName='" + Session["UName"] + "'");

            if (dr.Length > 0)

            {

                /// write code here which you want

                return;

            }

        }