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234 lines
4.2 KiB
Markdown
234 lines
4.2 KiB
Markdown
# Static Members, Methods and Classes
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---
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# Static Members
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So far, we have used __non-static__ fields in our classes
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This means, that each instance of the class holds its own version of the field, and changing the value of it only affects that instance:
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class MyAwesomeClass
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{
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public int MyProperty { get; set; }
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}
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class Program
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{
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static void Main(string[] args)
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{
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MyAwesomeClass instance1 = new MyAwesomeClass();
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MyAwesomeClass instance2 = new MyAwesomeClass();
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instance1.MyProperty = 100;
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instance2.MyProperty = 200; // instance1.MyProperty is still 100
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}
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}
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# Static Members (continued)
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Likewise, non-static class methods _have to _ be called through an instance:
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class MyAwesomeClass
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{
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public void PrintText(string text) { Console.WriteLine(text); }
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}
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class Program
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{
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static void Main(string[] args)
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{
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MyAwesomeClass instance = new MyAwesomeClass();
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instance.PrintText("Hello World"); // Outputs "Hello World"
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MyAwesomeClass.PrintText("Hello World"); // Results in an error
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}
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}
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__Static fields are shared between all instances of a class__
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Let's declare "MyProperty" property with the __static __ keyword. Now it can be referenced through the class type name, but not through the instance, as shown below:
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class MyAwesomeClass
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{
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public static int MyProperty { get; set; } = 100;
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}
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class Program
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{
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static void Main(string[] args)
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{
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MyAwesomeClass instance = new MyAwesomeClass();
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Console.WriteLine(MyAwesomeClass.MyProperty); // Outputs "100"
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Console.WriteLine(instance.MyProperty); // Results in an error
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}
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}
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# Static Members - Example
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In this example, a static field is used for keeping count on how many times the class has been instantiated:
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class Person
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{
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public static int totalPersons = 0;
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private string name;
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public Person(string personName) // Person Constructor
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{
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name = personName;
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++totalPersons;
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}
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public void PrintInfo()
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{
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Console.WriteLine("This person is called " + name + ".");
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Console.WriteLine("There are " + totalPersons + " persons total.");
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}
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}
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# Static Members - Example (continued)
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Now let's instantiate a couple of persons and print their info:
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class Program
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{
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static void Main(string[] args)
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{
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Person steve = new Person("Steve");
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Person wendy = new Person("Wendy");
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steve.PrintInfo();
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wendy.PrintInfo();
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}
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}
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# Static Methods
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Methods can also be static
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What happens when you try to call a non-static method from a static method?
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class Program
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{
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void PrintHelloName(string name)
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{
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Console.WriteLine("Hello, " + name);
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}
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static void Main(string[] args)
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{
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PrintHelloName(); // Will throw an error
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}
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}
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---
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This is why until this point all example methods have been inside of the main function
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# Static Classes
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* Classes can also be made static
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* Static classes cannot be instantiated
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* All members of a static class also have to be static
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* static class Styling
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* {
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* public static string fontFamily = "Verdana";
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* public static float fontSize = 12.5f;
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* }
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* class Program
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* {
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* static void Main(string[] args)
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* {
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* Console.WriteLine
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* ("Using font " + Styling.fontFamily + " " + Styling.fontSize + "px");
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* // Outputs "Using font Verdana 12.5px"
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* Styling = new Styling(); // Results in an error
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* }
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* }
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# Exercise 1
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Create a class Message which has two __static __ properties int TotalMessages and string LastMessage, and a __non-static __ property string MessageText.
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Add a constructor which takes a string message as a parameter, increases TotalMessages by one and sets the value of LastMessage to message which is the parameter
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Create a main loop which keeps asking the user for a new message. A new Message instance is then created with the user input message as an argument:
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Message newMessage = new Message(message);
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newMessage is then added to a list of messages, allMessages
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Finally the static values Message.TotalMessages and Message.LastMessage are printed
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