A Task Management System using Three Layer Architecture

using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Data.SqlClient;

/// <summary>
/// Summary description for DataExecuteClass
/// </summary>
public class DataExecuteClass
{
    public DataExecuteClass()
    {
    }
    public static string st = System.Configuration.ConfigurationManager.ConnectionStrings["TaskManagementSystemDBConnectionString"].ConnectionString;
    SqlConnection con = new SqlConnection(st);
    SqlCommand cmd = null;
    SqlDataAdapter adapter = null;
    DataSet ds = null;


    public DataSet getDataSet(string query)
    {
        try
        {
            con = new SqlConnection(st);
            con.Open();
            ds = new DataSet();
            cmd = new SqlCommand(query, con);
            adapter = new SqlDataAdapter();
            adapter.SelectCommand = cmd;
            adapter.Fill(ds);

        }
        catch (Exception ex)
        {
            throw ex;
        }
        finally
        {
            adapter = null;
            cmd = null;

            query = null;

            con.Close();
        }
        return ds;
    }
    public void ExecuteNonQuery(string query)
    {


        try
        {
            con = new SqlConnection(st);
            cmd = new SqlCommand(query, con);
            con.Open();
            cmd.ExecuteNonQuery();

        }
        catch (Exception ex)
        {
            throw ex;
        }
        finally
        {
            con.Close();
            cmd = null;
            con = null;
            query = null;
        }
    }
    public object getSingleValue(string query)
    {
        con = new SqlConnection(st);

        try
        {
            con = new SqlConnection(st);
            cmd = new SqlCommand(query, con);
            con.Open();
            object retValue = cmd.ExecuteScalar();
            return retValue;
        }
        catch (Exception ex)
        {
            return (object)ex.Message.ToString();
        }
        finally
        {
            con.Close();
            cmd = null;
            con = null;
            query = null;

        }
    }
}