Classes for C#

In previous programming studies, we have come across classes and objects. Classes and objects in C# are similar to classes and objects in other languages.

A Minimal Class and Object

Classes may contain fields and methods. Fields contain the data of a class and methods define actions a class can take. We say that fields and methods are members of a class.

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   public class HelloWorld 
   {

      public string message = "Hello World";

      public void SayHello() 
      {
         Console.WriteLine(message);
      }

   }

The only field in the HelloWorld class is the string message, while the only method is SayHello(), which prints the value of the message field and doesn’t return anything.

To execute SayHello(), we’ll need to create an instance of the class HelloWorld. We refer to an object created from a particular class as an instance of that class.

Here’s how this might look with our HelloWorld class:

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   HelloWorld hello = new HelloWorld();
   hello.SayHello();

In order to call the SayHello() method of HelloWorld, we must first have an instance of HelloWorld, which we create using the syntax new HelloWorld(). As with built-in classes, classes that we create define their own types. So the object hello is a variable of type HelloWorld.

We introduced this HelloWorld class as a means of illustrating the simplest representation of some basic concepts in C#. The goal of the next few lessons is to build up the machinery to create a wide variety of interesting classes that can be used to create complex programs and elegantly solve difficult problems.

The this Keyword

In HelloWorld above, we could have written SayHello this way, with the same net effect:

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   public void SayHello() 
   {
      Console.WriteLine(this.message);
   }

In this context, inside of the class, we can refer to fields (and methods) that belong to the class using the special object, this. Whenever you use this, it always refers to the object that the given code is currently within. In other words, this will always be an instance of the given class. Since it is not legal to create code outside of a class in C#, this nearly always makes sense to use (there’s one exception, that we’ll encounter soon).

You are allowed to create local variables (variables declared within a method) with the same name as a field of the given class. In this case, in order to refer to the field, we must use this.

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   public class HelloWorld 
   {

      public string message = "Hello World";

      public void SayHello() 
      {

         string message = "Goodbye World";

         // The line below prints "Goodbye World"
         Console.WriteLine(message);

         // The line below prints "Hello World"
         Console.WriteLine(this.message);
      }
   }

Check Your Understanding

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   public class Greeting 
   {

      public String name = "Jess"

      public void SayHello() 
      {
         Console.WriteLine("Hello " + here.name + "!");

   }