International Calling Connoisseur - Tests should check for correct handling of duplicate Add().

[purringk]

Add() throws if the key is already in the dictionary. I saw a bunch of passing solutions that don't check for the key before calling Add().

In general, there's lots of ways for dictionaries to throw exceptions when you do naive-but-seemingly-reasonable things, and the "Intro-to-Dictionaries" text might want to call out the common ways to go wrong.

Example test:

var countryCodes = DialingCodes.AddCountryToExistingDictionary(DialingCodes.GetExistingDictionary(), 91, "India");
Assert.Equal("India", countryCodes[91]);

[ErikSchierboom]

I appreciate the suggestion. While you are correct in noting that Add would throw, having the tests verify this would mean that students would have to know about exceptions, which they'll only learn later on as that requires a fair bit more knowledge. As an alternative, I'd be happy to have this mentioned in the concepts about.md document. Would you be willing to submit a PR?