Problem
My.Settings.YourSettingName is a read only property for application scope configuration
Explanation
By design you can’t modify application scope configuration settings, e.g. My.Settings.Setting1 is always read-only.
Solution
You can change the scope of the setting from Application to User, but then the setting will be stored in user.config instead of app.config
Another solution is to access application configuration file directly via System.Configuration.Configuration class. Here a small sample that updates LastDownloadDate configuration setting inside applicationSettings/APP.My.MySettings section:
Dim config As System.Configuration.Configuration = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None)
Dim sec As ClientSettingsSection = config.GetSection("applicationSettings/APP.My.MySettings")
sec.Settings.Get("LastDownloadDate").Value.ValueXml.InnerXml = Now.ToString
sec.SectionInformation.ForceSave = True
config.Save(ConfigurationSaveMode.Modified)