I got the following warning
\'System.Configuration.ConfigurationSettings.AppSettings\' is obsolete: \'\"This method is obsolete, it has been replace
I also face same issue, sometimes the assembly reference not loaded properly or if you are using multiple projects it give problems sometimes. You just add reference to assembly. Right Click>Add Reference>.Net>System.configuration> Click OK You can see now there are many configuration options available choose ConfigurationManager.AppSetting["Con"].ToString();
Build and Smile :)