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Inheritance
Make Inherited Members Abstract

Now we want to add one more method to Vehicle called Describe(). It will be different for every subclass, so there’s no point in defining a default one in Vehicle. Regardless, we want to make sure that it is implemented in each subclass.

This might sound similar to an interface. Why not add this method to the IAutomobile interface? We want Describe() to be available to all vehicles, not just automobiles.

To do this we need one more modifier: abstract. This line would go into the Vehicle class:

public abstract string Describe();

This is like the Vehicle class telling its subclasses: “If you inherit from me, you must define a Describe() method because I won’t be giving you any default functionality to inherit.” In other words, abstract member have no implementation in the superclass, but they must be implemented in all subclasses.

If one member of a class is abstract, then the class itself can’t really exist as an instance. Imagine calling Vehicle.Describe(). It doesn’t make sense because it doesn’t exist! This means that the entire Vehicle class must be abstract. Label it with abstract as well:

abstract class Vehicle

If you don’t do this, you’ll get an error message like this:

error CS0513: 'Vehicle.Describe()' is abstract but it is contained in non-abstract class 'Vehicle'

Once we write the abstract method and mark the class as abstract, we’ll need to actually implement it in each subclass. For example in Sedan:

public override string Describe() { return $"This Sedan is moving on {Wheels} wheels at {Speed} km/h, with license plate {LicensePlate}."; }

To make it clear that this Describe() method in Sedan is overriding the Describe() method in Vehicle, we will need label it override.

Instructions

1.

Add the abstract method Describe() to the Vehicle class.

  • Describe() should be public and return a string
  • Vehicle will also need to be labeled abstract

You might see an error after this.

2.

You probably saw this error:

error CS0534: 'Bicycle' does not implement inherited abstract member 'Vehicle.Describe()'

Fix this by implementing Describe() methods in Bicycle, Sedan, and Truck. Each method should:

  • mention the type, e.g. the bicycle version of the method returns a string containing "bicycle"
  • mention the license plate, speed, and wheels
  • be labeled with override

For bicycles, the returned string might look like this:

This bicycle is moving on 2 wheels at 10 km/h, with license plate ABCD1234.
3.

In Program.cs, call Describe on each instance and print the result to the console.

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