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

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title: 10. Static Members, Methods and Classes
---

# Static Members, Methods and Classes

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## 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:
  ```csharp
  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:
  ```csharp
  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
    }
  }
  ```
<div class='centered'>

![w:500px](imgs/10%20Static%20Members%2C%20Methods%20and%20Classes_1.png)

</div>

### Static members

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

<div class='centered'>

![w:600px](imgs/10%20Static%20Members%2C%20Methods%20and%20Classes_2.png)

</div>


### Static members: An example

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

<div class='columns21' markdown='1'>
<div markdown='1'>

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

</div>
<div markdown='1'>

![w:400px](imgs/10%20Static%20Members%2C%20Methods%20and%20Classes_3.png)

</div>
</div>

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

<div class='centered'>

![w:800px](imgs/10%20Static%20Members%2C%20Methods%20and%20Classes_4.png)

</div>

## Static classes

* Whole classes can also be static
* Static classes cannot be instantiated, and all its members have to be static as well
  ```csharp
  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
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* 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:
  ```csharp
  Message newMessage = new Message(message);
  ```
* `newMessage` is then added to a list `allMessages`.
* Finally, the static values `Message.TotalMessages` and  `Message.LastMessage` are printed.