csharp-basics/10-static-members-methods-and-classes.md

marp	paginate	math	theme	title
true	true	mathjax	buutti	10. Static Members, Methods and Classes

Static Members, Methods and Classes

Static members

Non-static members

• So far, we have used non-static fields in our classes
• Meaning that each instance of the class holds its own version of the field, and changing the value of it only affects that instance:

  class MyAwesomeClass
  {
    public int MyProperty { get; set; }
  }
  class Program
  {
    static void Main(string[] args)
    {
      MyAwesomeClass instance1 = new MyAwesomeClass();
      MyAwesomeClass instance2 = new MyAwesomeClass();
      instance1.MyProperty = 100;
      instance2.MyProperty = 200;     // instance1.MyProperty is still 100
    }
  }
  

---

• Likewise, non-static class methods have to be called through an instance:

  class MyAwesomeClass
  {
    public void PrintText(string text)
    {
      Console.WriteLine(text);
    }
  }
  class Program
  {
    static void Main(string[] args)
    {
      MyAwesomeClass instance = new MyAwesomeClass();
      instance.PrintText("Hello World");          // Outputs "Hello World"
      MyAwesomeClass.PrintText("Hello World");    // Results in an error
    }
  }
  

Static members

• Let's declare a property MyProperty with the static keyword
• It can be referenced through the class, but not through the instance:

  class MyAwesomeClass
  {
    public static int MyProperty { get; set; } = 100;
  }
  class Program
  {
    static void Main(string[] args)
    {
      MyAwesomeClass instance = new MyAwesomeClass();
      Console.WriteLine(MyAwesomeClass.MyProperty); 	// Outputs "100"
      Console.WriteLine(instance.MyProperty);       	// Results in an error
    }
  }
  

Static members: An example

• In this example, a static field is used for keeping count on how many times the class has been instantiated:

  class Person
  {
    public static int totalPersons = 0;
    private string name;
    public Person(string personName)	// Person Constructor
    {
      name = personName;
      ++totalPersons;
    }
    public void PrintInfo()
    {
      Console.WriteLine("This person is called " + name + ".");
      Console.WriteLine("There are " + totalPersons + " persons total.");
    }
  }
  

---

• Now let's instantiate a couple of persons and print their info:

class Program
{
  static void Main(string[] args)
  {
    Person steve = new Person("Steve");
    Person wendy = new Person("Wendy");
    steve.PrintInfo();
    wendy.PrintInfo();
  }
}

Static methods

• Methods can also be static — for example, Console.WriteLine
• What happens when you try to call a non-static method from a static method?

  class Program
  {
    void PrintHelloName(string name)
    {
      Console.WriteLine("Hello, " + name);
    }
    static void Main(string[] args)
    {
      PrintHelloName();   // Will throw an error
    }
  }
  

Static classes

• Whole classes can also be static
• Static classes cannot be instantiated, and all its members have to be static as well

  static class Styling
  {
    public static string fontFamily = "Verdana";
    public static float fontSize = 12.5f;
  }
  class Program
  {
    static void Main(string[] args)
    {
      Console.WriteLine
        ("Using font " + Styling.fontFamily + " " + Styling.fontSize + "px");
      // Outputs "Using font Verdana 12.5px"
      Styling = new Styling();    // Results in an error
    }
  }
  

Exercise 1

• Create a class Message that has two static properties int TotalMessages and string LastMessage, and a non-static property string MessageText.
• Add a constructor that takes a string message as a parameter, increases TotalMessages by one and sets the value of LastMessage to message.
• Create a main loop that keeps asking the user for a new message. A new Message instance is then created with the user input message as an argument:

  Message newMessage = new Message(message);
  

• newMessage is then added to a list allMessages.
• Finally, the static values Message.TotalMessages and Message.LastMessage are printed.