2.4. Objects and Methods, A Primer¶

There is a lot to say about objects in C# that we’ll cover in due time. Here, we highlight some introductory concepts of how C# works as an object-oriented programming language.

2.4.1. Objects¶

In C#, objects are structures that have a state and a set of behaviors. The state of an object includes properties/data that the coder can define and modify. Behaviors are actions that run when requested, and they can be used to evaluate, manipulate, or return data.

As we’ve said several times now, every variable in C# refers to an object.

The string data type is an object. For string language = "C#", the data would be the characters. The string manipulation section gives several of the behaviors available to the language object. For example, language.Length is 2, which tells us how many characters are present in the string.

An array is also an example of an object. It contains data, which are the values stored as the individual elements. The behaviors are methods not unlike those listed in the string manipulation table that perform actions related to the elements in the array. We haven’t provided an array methods table, but you can explore array methods in the docs.

2.4.2. Static Methods¶

In pure object-oriented languages like C# and Java, we don’t have functions in the sense you may be used to. Functions may not be declared outside of a class. Within the context of a class, functions are referred to as methods. We’ll adopt this terminology from now on.

We’ll dive into learning about classes and objects in C# soon enough, but first let’s learn a little about static methods, which behave somewhat similarly to stand-alone functions. A static method is one that can be called without creating an object instance of the class to which it belongs.

Example

Define the class Cat and include the static keyword before the makeNoise method name:

public class Cat {
   public static void MakeNoise(String[] args) {
      // some code
   }
}

Since makeNoise is static, we do NOT need to create a Cat object to access it.

Instead of doing this:

Cat myCat = new Cat();     // Create a new Cat object.
myCat.MakeNoise("purr");   // Call the MakeNoise method.

We can call the method directly:

Cat.MakeNoise("roar");

Until we get further into object oriented programming, every method you write should use the static keyword. Leaving off static will prevent or complicate the process of calling the methods you defined.

We will explore exactly what static does in more detail in later lessons.

2.4.3. `HelloMethods`¶

Let’s examine two classes in C# to explore defining and using methods. Open the HelloMethods project in csharp-web-dev-datatypes.

The first class is defined in the HelloMethods/Program.cs file, and it has a Main method. The second class is defined in a separate HelloMethods/Message.cs file, and it contains a GetMessage method that we want to call from within Main.

Examples

HelloMethods/Program.cs:

using System;

namespace HelloMethods
{
   public class Program
   {
      public static void Main(string[] args)
      {
            string message = Message.GetMessage("fr");
            Console.WriteLine(message);
            Console.ReadLine();
      }
   }
}

HelloMethods/Message.cs:

namespace HelloMethods
{
   public class Message
   {
      public static string GetMessage(string lang)
      {
            if (lang.Equals("sp")) {
               return "Hola Mundo";
            }
            else if (lang.Equals("fr"))
            {
               return "Bonjour le monde";
            }
            else
            {
               return "Hello World";
            }
      }
   }
}

We won’t explore every new aspect of this example, but rather will focus on the two methods.

The Main method in the HelloMethods/Program.cs class has the same structure as that of our temperature conversion example.
Take a look at the Message class. Note that it does NOT have a Main method, so it can’t be run on its own. Code within the Message class must be called from elsewhere in order to execute.
The Message class contains the GetMessage method. Like Main, it has the static keyword. Unlike Main, GetMessage has a return type of string instead of void.
GetMessage takes a single string parameter, lang.

Since C# is statically typed, each method must declare its return type - that is, the data type of what it will return - along with the type of each parameter. One consequence of this that may not be immediately obvious is that methods in C# may not return different types of data. For example, we would not be able to replace the last return statement of GetMessage with something like return 42;. This would be flagged as a compiler error.

2.4.3.1. `Main` Methods¶

In a C# project, only one Main method is allowed. When the project is compiled and run, the Main method indicates what should be executed, and if there were multiple Main methods this would be ambiguous.

2.4.3.2. Public Methods¶

Finally, let’s note how a static method is called. The first line of Main in the Program class is:

Message.GetMessage("fr");

To call a static method, we must use the name of the class in which it is defined, followed by ., followed by the name of the method.

ClassName.methodName(arguments);

We are able to call this method from another class because it is declared to be public. If we wanted to restrict the method from being called by another class, we could instead use the private modifier. We’ll explore access modifiers in more depth in coming lessons.

Note

As you have been following along with these examples, you may have noticed that each class file, for example Message.cs and Program.cs, is named exactly the same as the class it holds (Message and Program, respectively).

There is NOT a rule in C# dictating that a file must be named the same as the class it contains, but it is considered best practice.

2.4.3.3. Try It¶

Poke around with the HelloMethods project in Visual Studio and experiment with the following:

Figure out how to alter the HelloMethods code to change the message returned.
Add another “Hello, World” language option.
Change one public keyword to private to see what happens. Repeat for each occurrence of public.

2.4.4. Check Your Understanding¶

Question

Which of the following defines a method that takes an integer as a parameter and returns a string value?

public static void MethodName(string parameterName)
public static void MethodName(int parameterName)
public static int MethodName(string parameterName)
public static string MethodName(int parameterName)