Similar to TestUserInterface used in the previous chapter, the TestInventoryContext class will be used to mock the behavior of our repository by implementing the IInventoryContext interface. This class will support the three methods of the interface, as well as supporting two additional methods for retrieving the books that have been added to the collection during the unit test and for retrieving the books that have been updated during the unit test.

To support the TestInventoryContext class, two collections will be used:

private readonly IDictionary<string, Book> _seedDictionary;
private readonly IDictionary<string, Book> _books;

The first is used to store the starting collection of the books, while the second is used to store the final collection of the books. The constructor is shown in the following code; note how the dictionaries are copies of each other:

public TestInventoryContext(IDictionary<string, Book> books)
{
    _seedDictionary = books.ToDictionary(book => book.Key,
                                         book => new Book { Id = book.Value.Id, 
                                                            Name = book.Value.Name, 
                                                            Quantity = book.Value.Quantity });
    _books = books;
}

The IInventoryContext methods are written to update and return just one of the collections, as follows:

public Book[] GetBooks()
{
    return _books.Values.ToArray();
}

public bool AddBook(string name)
{
    _books.Add(name, new Book() { Name = name });

    return true;
}

public bool UpdateQuantity(string name, int quantity)
{
    _books[name].Quantity += quantity;

    return true;
}

At the end of the unit test, the two remaining methods can be used to determine the difference between the starting and ending collections:

public Book[] GetAddedBooks()
{
    return _books.Where(book => !_seedDictionary.ContainsKey(book.Key))
                                                    .Select(book => book.Value).ToArray();
}

public Book[] GetUpdatedBooks()
{ 
    return _books.Where(book => _seedDictionary[book.Key].Quantity != book.Value.Quantity)
                                                    .Select(book => book.Value).ToArray();
}