title: What exception should I throw if a configuration value is missing?
body: <p>I have a static class that retrieves values from app.config in its constructor. These could potentially be null, and I would like to throw an exception if this is the case.</p>
  
  <p>I would like to throw the error in the static methods that use these values, but <code>ArgumentNullException</code> doesn't seem right as they are not arguments passed to the method.</p>
  
  <p>Is there an alternative, or is this not a good approach?</p>