how to create web server control using designer file

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Text;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

using AjaxControlToolkit;

[assembly: WebResource("SmartControl.SmartDateTime.Calendar_scheduleHS.png", "img/png")]
namespace SmartControl
{
    [DefaultProperty("Text")]
    [ToolboxData("<{0}:SmartDateTime runat=server ></{0}:SmartDateTime>")]
    public class SmartDateTime : WebControl
    {
        [Bindable(true)]
        [Category("Appearance")]
        [DefaultValue("")]
        [Localizable(true)]
        public string Text
        {
            get
            {
                String s = (String)ViewState["Text"];
                return ((s == null) ? String.Empty : s);
            }

            set
            {
                ViewState["Text"] = value;
            }
        }
        public string DataField
        {
            get
            {
                String s = (String)ViewState["DataField"];
                return ((s == null) ? String.Empty : s);
            }
            set
            {
                ViewState["DataField"] = value;
            }
        }
        public DateTime DataValue
        {
            get
            {
                return Convert.ToDateTime(this.Text);
            }
            set
            {
                DataValue = Convert.ToDateTime(this.Text);
            }
        }

        private TextBox SmartDateTimeTextBox;
        private ImageButton SmartImageButton;
        //private ScriptManager SmartScriptManager;
        private CalendarExtender SmartCalendarExtender;

        protected override void CreateChildControls()
        {
            string img = Page.ClientScript.GetWebResourceUrl(this.GetType(), "SmartControl.SmartDateTime.Calendar_scheduleHS.png");

            //SmartScriptManager = new ScriptManager();
            //SmartScriptManager.ID = "SmartScriptManager1";
            //SmartScriptManager.EnableScriptGlobalization = true;

            SmartDateTimeTextBox = new TextBox();
            SmartDateTimeTextBox.ID = "SmartDateTimeTextBox1";

            SmartImageButton = new ImageButton();
            SmartImageButton.ID = "SmartImageButton1";
            SmartImageButton.ImageUrl = img;

            SmartCalendarExtender = new CalendarExtender();
            SmartCalendarExtender.ID = "SmartCalendarExtender1";
            SmartCalendarExtender.Format = "dd/MM/yyyy";
            SmartCalendarExtender.TargetControlID = SmartDateTimeTextBox.ID;
            SmartCalendarExtender.PopupButtonID = SmartImageButton.ID;


            //this.Controls.Add(SmartScriptManager);
            this.Controls.Add(SmartDateTimeTextBox);
            this.Controls.Add(SmartImageButton);
            this.Controls.Add(SmartCalendarExtender);
            
        }

        protected override void RenderContents(HtmlTextWriter output)
        {
            //output.Write(Text);
            foreach (Control c in this.Controls)
            {
                c.RenderControl(output);
            }
        }
    }
}

using System;
using System.Globalization;

namespace SmartControl
{
    public class SmartDateTimeDesigner : System.Web.UI.Design.ControlDesigner
    {
        public SmartDateTimeDesigner()
        {
            
        }

        public override string GetDesignTimeHtml()
        {
            SmartDateTime control = (SmartDateTime)Component;
           
            return "<div style='background-color:ButtonFace;white-space:nowrap;border-style:ridge;border-width:1px;font-family:Calibri;font-size:14px;position-relative;width:" + control.Width + ";height:" + control.Height + ";'><b>" + control.GetType().Name + " - " + "</b>" + this.ID + "</div>";

        }
        
    }

}