Creating dynamic controls inside a web page using placeholder control.

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

public partial class _Default : System.Web.UI.Page 
{
    Label mylabel = new Label();
    Button mybutton = new Button();
    protected void Page_Load(object sender, EventArgs e)
    {
        mylabel.Text = "hello";
        mybutton.Text = "then";
        PlaceHolder1.Controls.Add(mylabel);
        PlaceHolder1.Controls.Add(mybutton);
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        
        PlaceHolder1.Controls.Remove(mylabel);
        PlaceHolder1.Controls.Remove(mybutton);
        Label mylabel1 = new Label();           // This is new control replaces that old controls
        mylabel1.Text = "hi";
        PlaceHolder1.Controls.Add(mylabel1);
    }
}