We want to initialize instances of the Circle class with the radius value. In order to do so, we can take advantage of the constructors in C#. Constructors are special class methods that are automatically executed when we create an instance of a given type. The runtime executes the code within the constructor before any other code within a class.

We can define a constructor that receives the radius value as an argument and use it to initialize an attribute with the same name. We can define as many constructors as we want. Therefore, we can provide many different ways of initializing a class. In this case, we just need one constructor.

The following lines create a Circle class and define a constructor within the class body.

class Circle
{
  private double radius;

  public Circle(double radius)
  {
    Console.WriteLine(String.Format("I'm initializing a new Circle instance with a radius value of {0}.", radius));
    this.radius = radius;
  }
}

The constructor is...