System.Data.SqlTypes.SqlDateTime.ToString C# (CSharp) Method

        public override string ToString()
        {
            if (IsNull)
                return SQLResource.s_nullString;
            DateTime dateTime = ToDateTime(this);
            return dateTime.ToString((IFormatProvider)null);
        }

    private void getAgentsByCash()
    {
        DataTable      dtAgents = new DataTable();
        SqlDataAdapter daAgents = new SqlDataAdapter();
        SqlCommand     cmd      = new SqlCommand();
        String         agentid  = ddlAgents.SelectedValue;

        if (!String.IsNullOrEmpty(txt_fromDate.Text.ToString()) && !String.IsNullOrEmpty(txt_toDate.Text.ToString()))
        {
            DateTime ff = DateTime.ParseExact(txt_fromDate.Text, "dd/MM/yyyy", null);
            System.Data.SqlTypes.SqlDateTime dtSql = System.Data.SqlTypes.SqlDateTime.Parse(ff.ToString("yyyy-MM-dd"));
            from = dtSql.ToString();
            from = txt_fromDate.Text.ToString();
            to   = txt_toDate.Text.ToString();
        }

        try
        {
            cmd.Connection = con;
            con.Open();
            cmd.CommandType = CommandType.StoredProcedure;
            cmd.CommandText = "getAgentsByCash";
            cmd.Parameters.AddWithValue("@from", from);
            cmd.Parameters.AddWithValue("@to", to);
            cmd.Parameters.AddWithValue("@agentid", agentid);

            daAgents = new SqlDataAdapter(cmd);
            daAgents.Fill(dtAgents);
            if (dtAgents != null)
            {
                if (dtAgents.Rows.Count > 0)
                {
                    Session["dtProduct"] = dtAgents;
                    repAgents.DataSource = dtAgents;
                    repAgents.DataBind();
                    btnExcelExport.Visible = true;
                }
                else
                {
                    repAgents.DataSource = null;
                    repAgents.DataBind();
                    btnExcelExport.Visible = false;
                }
            }
            else
            {
                repAgents.DataSource = null;
                repAgents.DataBind();
                btnExcelExport.Visible = false;
            }
        }
        catch (Exception ex)
        {
            ErrHandler.writeError(ex.Message, ex.StackTrace);
        }
        finally
        {
            con.Close();
        }
    }