Class Variables

Building off of what we learned about basic classes, we can start working with class variables or fields. We know we create an instance of a class using:

myClass myObject = new myClass();

Additionally we know these objects have fields or member variables defined like so:

class Shape{
  public string Type = "square";
  public int Sides = 4;
  public int Sidelength = 1;
  public double Area = 1;
}

The above code defines a class named Shape with 4 different fields. In this case our shape is a square with 4 sides of length 1 and an area of 1. What if we wanted a triangle instead of a square? We can do that by allowing our class to accept different values for its fields. Because we will be defining the values of the fields later, we don't have to assign them here. We give Shape fields without values like this:

class Shape{
  public string Type;
  public int Sides;
  public int Sidelength;
  public double Area;  
}

Now to give those fields values we need to define something called a constructor in our class. Constructors use the same name of their class. Adding an empty constructor, our class looks like this:

class Shape{
  public string Type;
  public int Sides;
  public int Sidelength;
  public double Area;  
  public Shape(){} 
}

Let's give the constructor parameters and set the values of our fields to the passed in parameters:

class Shape{
  public string Type;
  public int Sides;
  public int Sidelength;
  public double Area;  
  public Shape(string type, int sides, int sidelength, double area){
    Type = type;
    Sides = sides;
    Sidelength = sidelength;
    Area = area;
  }
}

Now when we create an instance of Shape, we can give it parameters using the constructor! Putting it all together looks something like this:

using System;
class Shape{
  public string Type;
  public int Sides;
  public int Sidelength;
  public double Area;  
  public Shape(string type, int sides, int sidelength, double area){
    Type = type;
    Sides = sides;
    Sidelength = sidelength;
    Area = area;
  }
}
class MainClass{
  public static void Main(){
    Shape square = new Shape("square", 4, 1, 1);
    Shape bigsquare = new Shape("square", 4, 2, 4);
    Shape triangle = new Shape("triangle", 3, 3, 3.9);
    Console.WriteLine("A {0} with {1} sides of length {2} has an area of {3}", square.Type, square.Sides, square.Sidelength, square.Area);
    Console.WriteLine("A {0} with {1} sides of length {2} has an area of {3}", bigsquare.Type, bigsquare.Sides, bigsquare.Sidelength, bigsquare.Area);
    Console.WriteLine("A {0} with {1} sides of length {2} has an area of {3}", triangle.Type, triangle.Sides, triangle.Sidelength, triangle.Area);
  }
}

Executing the above code will print the following:

A square with 4 sides of length 1 has an area of 1 A square with 4 sides of length 2 has an area of 4 A triangle with 3 sides of length 3 has an area of 3.9

Exercise

Make a class Vehicle with the fields string Type, int NumTires, int Year, and bool Runs, and create:

A car: Type = car with NumTires = 4 from Year = 2000 which Runs = true An oldcar: Type = car with NumTires = 4 from Year = 1980 which Runs = false A bike: Type = bike with NumTires = 2 from Year = 2017 which Runs = true

Watch out for the capitalizations!