Abstract Factory design pattern

To illustrate our first pattern, let's walk through the development of the help command and the initial console application. The initial version of the console application is shown as follows:

private static void Main(string[] args)
{
    Greeting();
    
    // note: inline out variable introduced as part of C# 7.0
    GetCommand("?").RunCommand(out bool shouldQuit); 

    while (!shouldQuit)
    { 
        // handle the commands
        ...
    }

    Console.WriteLine("CatalogService has completed."); 
}

When the application starts, both a greeting and the result of a help command are shown. The application will then process entered commands until the quit command is entered.

The following shows the detail of handling commands:

    while (!shouldQuit)
    { 
        Console.WriteLine(" > ");
        var input = Console.ReadLine...