Question

I have a simple phone book class (this class is extracted because I inject a different phone book for testing) that has ~10 entries and I’m wondering. Do I create a test for each phone book entry or simply test to ensure a few phone numbers are successfully fetched based on the key?

The interface is quite simple.

public interface IPhoneBook
{
    string Lookup(string key);
}

Answer 1

Have a test to test fetching an entry that should exist.

Have a test to test fetching an entry that should not exist.

That should be enough, if the data structure is as simple as you describe.

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Ask yourself – what value would testing all entries give you over the above tests? If none, don’t do it.