how to call wcf service asynchronously from asp.net page

protected void btnSearch_Click(object sender, EventArgs e)
{
PageAsyncTask task = new PageAsyncTask(BeginGetrdgDetailsAsyncData, EndGetrdgDetailsAsyncData, null, null);
            Page.RegisterAsyncTask(task);
}



IAsyncResult BeginGetrdgDetailsAsyncData(object src, EventArgs args, AsyncCallback cb, object state)
    {
        StringBuilder sb = new StringBuilder();
        sb.Append("<SalesAnalysis>");
        -----
-------
        sb.Append("</SalesAnalysis>");
        return pharma.BeginSales_Analysis(sb.ToString(), cb, state);
    }

    void EndGetrdgDetailsAsyncData(IAsyncResult ar)
    {
        try
        {
            String result = pharma.EndSales_Analysis(ar, out strerror);
            if (strerror == "")
            {
                DataSet dsresult = new DataSet();
                dsresult = JsonConvert.DeserializeObject<DataSet>(result);

                DataView dv = new DataView(dsresult.Tables[2]);
                DataTable distinctValues = dv.ToTable(true, "SCHEDULE_ID", "SCHEDULE_NAME");
                for (int i = 0; i < distinctValues.Rows.Count; i++)
                {
                    dsresult.Tables[1].Columns.Add(distinctValues.Rows[i]["SCHEDULE_NAME"].ToString());
                    DataRow[] drs = dsresult.Tables[1].Select("SCHEDULE_ID='" + distinctValues.Rows[i]["SCHEDULE_ID"].ToString() + "'");
                    if (drs.Length > 0)
                    {
                        //foreach (DataRow dr in drs)
                        //{
                        //    //assigning usage col values under dynamically created col "EQ_DESC"
                        //    dr[distinctValues.Rows[i]["SCHEDULE_NAME"].ToString()] = dr["Usage"].ToString();

                        //    //usage += Convert.ToDecimal(dr["Usage"].ToString());
                        //}
                    }
                }

                rdgDetails.DataSource = dsresult.Tables[1];
                rdgDetails.DataBind();
            }
            else
            {
                errormsg
            }
        }
        catch (Exception ex)
        {
         
        }
    }
--------------------------------------------------------------------------------
wcf

public String Sales_Analysis(String strParameter, out string error)

        {

            error = string.Empty;

            XmlDocument pdoc = new XmlDocument();

            pdoc.LoadXml(strParameter);

            DataSet ds = new DataSet();

            methodParameters = strParameter;

            try

            {

                if (dbclass.Connectdb(ConfigurationManager.ConnectionStrings["constr"].ConnectionString))

                {

                    dbclass.cmd = new OracleCommand();

                    dbclass.cmd.CommandText = "Proc Name";

                    dbclass.cmd.CommandType = CommandType.StoredProcedure;

                    #region add parameters

 //IN parameter..

                    //Out parameter..

                   

                    #endregion

                    ds = dbclass.ExecuteCommandDataSet(dbclass.con, dbclass.cmd);

                    if (ds.Tables[1].Rows.Count > 0)

                    {

                        return JsonConvert.SerializeObject(ds, Newtonsoft.Json.Formatting.Indented);

                    }

                    else

                    {

                        return "[]";

                    }

                }

            }

            catch (Exception ex)

            {

                error = ex.Message;

            }

            return JsonConvert.SerializeObject(ds, Newtonsoft.Json.Formatting.Indented);

        }