How to add connection string dynamically in web.config - asp.net

Question

I have to create one application which will add connection string in web.config when user clicks on some control.
actually it runs fine,
But now together with it i have to run one SQL script which will create new database.
for that i have write following code..
now it gives me error as
using System;
using System.Collections;
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;
using System.Collections.Generic;
using System.Drawing;
using System.Text.RegularExpressions;
using System.Web.Configuration;
using System.Collections.Generic;
public partial class connectionString : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
}
protected void btnadd_Click(object sender, EventArgs e)
{
try
{
Configuration config = WebConfigurationManager.OpenWebConfiguration("~");
// System.Configuration.Configuration config = System.Web.Configuration.WebConfigurationManager.OpenWebConfiguration("~");
var section = config.GetSection("connectionStrings") as ConnectionStringsSection;
section.ConnectionStrings.Add(new ConnectionStringSettings(txtNAme.Text, txtNAme.Text, "System.Data.SqlClient"));
config.Save();
string sqlConnectionString = #"Integrated Security=SSPI;Persist Security Info=False;Initial Catalog=ccwebgrity;Data Source=SURAJIT\SQLEXPRESS";
FileInfo file = new FileInfo(#"G:\AAA\SCRIPT.sql");
string script = file.OpenText().ReadToEnd();
SqlConnection conn = new SqlConnection(sqlConnectionString);
Server server = new Server(new ServerConnection(conn));
server.ConnectionContext.ExecuteNonQuery(script);
file.OpenText().Close();
}
catch (Exception ex)
{
}
}
}

Answer 1

Define the connection string in web.config
<ConnectionStrings>
<add name="student" connectionString="Server=student;Max Pool Size=300;Initial Catalog=studentDB;User ID=student;Password=st123dent;" providerName="System.Data.SqlClient"/>
</Connectionstrings>
Configuration is read only so you can not do it in obvious way like
ConfigurationManager.ConnectionStrings["student"].ConnectionString = "new value";