Create Runtime TexBox

USE [DB Name]
GO

SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
SET ANSI_PADDING ON
GO
CREATE TABLE [dbo].[Student](
	[ID] [int] IDENTITY(1,1) NOT NULL,
	[Stud_Name] [varchar](max) COLLATE SQL_Latin1_General_CP1_CI_AS NULL,
	[Stud_Age] [varchar](max) COLLATE SQL_Latin1_General_CP1_CI_AS NULL,
	[Stud_Addr] [varchar](max) COLLATE SQL_Latin1_General_CP1_CI_AS NULL,
	[Stud_Gender] [varchar](max) COLLATE SQL_Latin1_General_CP1_CI_AS NULL,
	[Stud_Qualification] [varchar](max) COLLATE SQL_Latin1_General_CP1_CI_AS NULL
) ON [PRIMARY]

GO
SET ANSI_PADDING OFF

<div>
       Select Id: <asp:dropdownlist id="DropDownList1" runat="server" autopostback="True" appenddatabounditems="true" xmlns:asp="#unknown">
            onselectedindexchanged="DropDownList1_SelectedIndexChanged">
        </asp:dropdownlist>
</div>

<connectionstrings>
   <add name="ConString" connectionstring="Data Source=[SQL serverName];Initial Catalog=[DB name];Persist Security Info=True;User ID=sa;Password=[*****]" providername="System.Data.SqlClient" />
 </connectionstrings>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.SqlClient;
using System.Data;
using System.Configuration;

public partial class Default2 : System.Web.UI.Page
{
    SqlConnection con;
    SqlCommand cmd;
    SqlDataAdapter da;
    DataSet ds;

    protected void Page_Load(object sender, EventArgs e)
    {
        con = new SqlConnection(ConfigurationManager.ConnectionStrings["ConString"].ConnectionString);

        if (!IsPostBack)
        {
            ddl();
        }
    }
    protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
        try
        {
            cmd = new SqlCommand("Select Stud_Name from student where Id='" + DropDownList1.SelectedItem + "'", con);
            SqlDataAdapter da = new SqlDataAdapter(cmd);
            DataSet ds = new DataSet();
            da.Fill(ds);
            string s = ds.Tables[0].Rows[0][0].ToString();
            for (int i = 0; i < s.Length; i++)
            {
                string id = "txt" + i.ToString();
                TextBox txt = new TextBox();
                txt.ID = id;
                txt.Text = s[i].ToString();
                txt.Width = 20;
                txt.MaxLength = 1;
                txt.ReadOnly = true;
                form1.Controls.Add(txt);
            }
        }
        catch
        {
        }
    }
    private void ddl()
    {
        try
        {
            cmd = new SqlCommand("Select Id from student ", con);
            SqlDataAdapter da = new SqlDataAdapter(cmd);
            DataSet ds = new DataSet();
            ds.Clear();
            DropDownList1.Items.Add("Select...");
            da.Fill(ds);

            DropDownList1.DataSource = ds;
            DropDownList1.DataTextField = "Id";
            DropDownList1.DataValueField = "Id";
            DropDownList1.DataBind();
        }
        catch
        {
        }
    }
}