Using static keyword

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August 13, 2021 (08:00:10 AM)

Static classes

One use case for static classes is creating utility classes (or “helper class”) that contain related and frequently-used methods; making those methods easily callable anywhere in the program. Some examples of static classes in C# are Math and Console class.

Pay attention to how these classes are used:

Console object is never instantiated before use
WriteLine method is called referring to the name of the class (not object identifier)

using System;

class Program {
    static void Main() {
        Console.WriteLine("calling a static method");
    }
}

Using your IDE, check what happens if you do the following:

using System;

class Program {
    static void Main() {
        Console test = new Console();       
    }
}

Indeed, it is not possible to instantiate an object when a class is declared static. Further, if a class is declared static, all its members (attributes, methods, constructors, etc.) must also be declared static.

Static Calculator

In your IDE create a new project. Then add a new class file called Calculator.cs

In Calculator.cs:

Declare a static class and name it Calculator.
Add 5 public methods to Calculator class. Each method takes 2 arguments x and y of type double:
1. Add method that returns result of x + y.
2. Subtract method that returns result of x - y.
3. Multiply method that returns result of x * y.
4. Divide method that returns result of x / y.
5. Modulo method that returns result of x % y.

After implementing Calculator,

Open the file that contains program’s Main method

Paste the following code inside Main method:

double x = 10d, y = 2d;

Console.WriteLine($"{x} + {y} = {Calculator.Add(x, y)}");
Console.WriteLine($"{x} - {y} = {Calculator.Subtract(x, y)}");
Console.WriteLine($"{x} * {y} = {Calculator.Multiply(x, y)}");
Console.WriteLine($"{x} / {y} = {Calculator.Divide(x, y)}");
Console.WriteLine($"{x} % {y} = {Calculator.Modulo(x, y)}");

Again, notice how

no instance of Calculator is created before use, and
each Calculator method is called referring to the name of the class.

Execute the program
- If your implementation of Calculator class matches the instructions, you will see meaningful output after executing the program.
- Otherwise review the instructions again and retrace your implementation steps to resolve any issues.

Static members in a non-static class

A non-static class can contain both static and non-static class members.

Study the following program implementation but *do not* execute it. After reading through the implementation, answer the questions below.

Student.cs

using System;

class Student {

    private int id;
    private string name; 
    private static string universityName = "Augusta University";
    private static int studentCount;

    public Student(int id, string name){
        this.id = id;
        this.name = name;
        studentCount++;
    } 

    public static void DisplayStudentCount(){
        // does this work? uncomment to check
        // Console.WriteLine(name);
        
        Console.WriteLine($"Number of students: {studentCount}");
    }

    public override string ToString(){
        return $"id: {id}\n"+
               $"name: {name}\n"+
               $"university: {universityName}";
    }  
}

Program.cs

using System;

class Program {

    static void Main() {

        Student alice = new Student(1111, "Alice");
        Console.WriteLine(alice);

        Student.DisplayStudentCount(); // first time

        Student bob = new Student(1112, "Bob");        
        Console.WriteLine(bob);

        Student.DisplayStudentCount(); // second time
    }
}

How many non-static attributes does Student class have?
How many static attributes does Student class have?
How many non-static methods does Student class have?
How many static methods does Student class have?
What is the output of each of the following lines in “Program.cs”:
1. Console.WriteLine(alice);
2. Student.DisplayStudentCount(); // first time
3. Console.WriteLine(bob);
4. Student.DisplayStudentCount(); // second time
if the studentCount attribute was *not* static, what would be the output of:
1. Student.DisplayStudentCount(); // first time
2. Student.DisplayStudentCount(); // second time
When a class contains both static and non-static members, is it possible to refer to non-static members inside a static method? For example, if we try to refer to name attribute inside DisplayStudentCount, will it work? Why or why not?

Check your answers by creating a matching program in your IDE and executing it.

To check the last question, in Student.cs, uncomment the following line and verify its behavior matches your answer:

// Console.WriteLine(name);