VB -What is the primary advantage to defining an interface type in an application? Solution using interface make it possible to treat different type of object the same way. you could define an Interface IDrivable could define method such as StartEngine, MoveFoward, MoveBackward, TurnLeft and TurnRight. you could then have a Car class which implement the IDrivable interface as well as Bike Class. and in your code, you could have a loop to mixed array of Car/Bike and obj[0] = new Car(); obj[1] = new Car(); obj[2] = new Bike(); and have a for (int i = 0; i < obj.Count; i++) { ((IDrivable)obj[i]).Start(); ((IDrivable)obj[i]).MoveBackward(); } This is only a simple application of what an Interface can do ! Upon that few more advantages are:- -Knowing for certain that specific methods and/or properties are available -Ablility to call those methods/properties without knowing anything about the derived class Hope this help! .