Dynamically Writing into Web.config File

Inline

    XmlDocument xDoc = new XmlDocument(); // Gobal Declaration
    string Path; // Gobal Declaration
    int i; // Gobal Declaration
    protected void Page_Load(object sender, EventArgs e)
    {
        i = Convert.ToInt32(ConfigurationManager.AppSettings["Test"].ToString());
    }
    protected void bt1_Click(object sender, EventArgs e)
    {
        Path = Server.MapPath("~/web.config");
        xDoc.Load(Path);
        XmlNodeList NodeList = xDoc.GetElementsByTagName("appSettings");
        XmlNodeList tNodeList = NodeList[0].ChildNodes;
        XmlAttributeCollection xAttColl = tNodeList[0].Attributes;
        xAttColl[0].InnerXml = "Test";
        xAttColl[1].InnerXml = Convert.ToString(i + 1);
        xDoc.Save(Path);
        Response.Write(ConfigurationManager.AppSettings["Test"].ToString());
    }

Web.Config

<configuration>

<appSettings>

<add key="Test" value="0" /> <!-- Add This In Your WebConfig File -->

</appSettings>

</configuration>