System.Configuration.ConfigurationSettings.AppSettings is obsolete: This method is obsolete, it has been replaced by ConfigurationManager.AppSettings.
Ever had similar warnings at compile time? Ok, no problem you think : just replace ConfigurationSettings with ConfigurationManager and you’re done. Not! The ConfigurationSettings class exists in the System.dll assembly while the new ConfigurationManager (.NET 2.0) class exists in the System.Configuration.dll assembly. So, you won’t be able to find the ConfigurationManager class without adding a reference to the System.Configuration.dll in your project.
Filed under: Resolve .Net 2.0 Obselete