To add,update,delete key,value pair of appsetting tag in web.config

To add key,value pair to appsetting tag of web.config

 
            String key = "abc";
            String value = "123";

            bool isPresent=ConfigurationManager.AppSettings.AllKeys.ToList().Contains(key);

            if (isPresent)
            {
              // label lblKeyValue 
                lblKeyValue.Visible = true;
                lblKeyValue.Text = "Key has already entered";
             }
            else
            {

                Configuration config = WebConfigurationManager.OpenWebConfiguration("~");
                config.AppSettings.Settings.Add(key, value);

                config.Save(ConfigurationSaveMode.Modified, true);

                ConfigurationManager.RefreshSection("appSettings");
                           }

To delete key,value pair of appsetting tag in web.config

 
             String key = "abc";
            Configuration config = WebConfigurationManager.OpenWebConfiguration("~");
            config.AppSettings.Settings.Remove( key );
            config.Save(ConfigurationSaveMode.Modified, true);
            ConfigurationManager.RefreshSection("appSettings");

To Update key,value pair of appsetting tag in web.config

 
           string  key = "abc";
           string   value = "456";
            XmlDocument xmlDoc = new XmlDocument();
            xmlDoc.Load(AppDomain.CurrentDomain.SetupInformation.ConfigurationFile);
            int k = xmlDoc.DocumentElement.ChildNodes.Count;
            for (int i = 0; i < k; i++)
            {
                XmlNode childnode = xmlDoc.DocumentElement.ChildNodes[i];
                if (childnode.Name == "appSettings")
                {


                    foreach (XmlNode node in childnode)
                    {
                        if (node.Attributes[0].Value.Equals(key))
                        {
                            node.Attributes[1].Value = value;
                        }
                    }
                }
            }

            xmlDoc.Save(AppDomain.CurrentDomain.SetupInformation.ConfigurationFile);

            ConfigurationManager.RefreshSection("appSettings");

bool isPresent=ConfigurationManager.AppSettings.AllKeys.ToList().Contains("abc")