[c#] App.Config change value

To update entries other than appsettings, simply use XmlDocument.

public static void UpdateAppConfig(string tagName, string attributeName, string value)
{
    var doc = new XmlDocument();
    doc.Load(AppDomain.CurrentDomain.SetupInformation.ConfigurationFile);
    var tags = doc.GetElementsByTagName(tagName);
    foreach (XmlNode item in tags)
    {
        var attribute = item.Attributes[attributeName];
        if (!ReferenceEquals(null, attribute))
            attribute.Value = value;
    }
    doc.Save(AppDomain.CurrentDomain.SetupInformation.ConfigurationFile);
}

This is how you call it:

Utility.UpdateAppConfig("endpoint", "address", "http://localhost:19092/NotificationSvc/Notification.svc");

Utility.UpdateAppConfig("network", "host", "abc.com.au");

This method can be improved to cater for appSettings values as well.