S O L I D Single Responsibility Principle //In the method below, I take care to follow the Single Responsibility Principle // that states every class function etc. should have a single responsibility. // In the example below, DisplayWeeklyForecast is responsible only for the // writing of the forecast to the console. The unSOLID way might incorporate // code located in another method, such as GenerateForecast. public static void DisplayWeeklyForecast(List list) { for (int i = 0; i < list.Count; i++) { Console.WriteLine("Day {0} Forecast: {1} and {2}.", (i + 1), list[i].Weather.ForecastedTemperature, list[i].Weather.ForecastedAtmosphere); } } Liskov Substitution Principle(LSP) // In the implementation of my Item : Cup, Lemon, SugarUnit, IceCube class / object relationship, I use the LSP. The principle, which maintains that a child and parent should be interchangable without losing functionability. In the Inventory class, I interchange Item with Lemon, Cup, IceCube, and SugarUnit in the AlterItem method below: public List AddItem(int quantity, string type) { List list = new List(); switch (type) { case "cups": for(int i = 0; i < quantity; i++) { Cup cup = new Cup(); list.Add(cup); } return list; case "ice cubes": for (int i = 0; i < quantity; i++) { IceCube iceCube = new IceCube(); list.Add(iceCube); } return list; case "sugar units": for (int i = 0; i < quantity; i++) { SugarUnit sugarUnit = new SugarUnit(); list.Add(sugarUnit); } return list; case "lemons": for(int i = 0; i < quantity; i++) { Lemon lemon = new Lemon(); list.Add(lemon); } return list; default: return list; } }