How to use ConfigurationManager

Okay, it took me a while to see this, but there's no way this compiles:

return String.(ConfigurationManager.AppSettings[paramName]);

You're not even calling a method on the String type. Just do this:

return ConfigurationManager.AppSettings[paramName];

The AppSettings KeyValuePair already returns a string. If the name doesn't exist, it will return null.

Based on your edit you have not yet added a Reference to the System.Configuration assembly for the project you're working in.

Go to tools >> nuget >> console and type:

Install-Package System.Configuration.ConfigurationManager 

If you want a specific version:

Install-Package System.Configuration.ConfigurationManager -Version 4.5.0

Your ConfigurationManager dll will now be imported and the code will begin to work.