Decouple code using Interfaces

In object-oriented programming, interfaces define a contract that classes can implement. They specify method signatures and properties that implementing classes must provide. This allows for consistent behavior across different types while enabling flexibility in implementation. In C#, interfaces are defined using the interface keyword, and classes implement them using the : InterfaceName syntax.

In this exercise, you will refactor a tightly coupled console application to use interfaces. By introducing interfaces, you will decouple the application logic from specific implementations, making the code more flexible and easier to maintain.

This exercise takes approximately 20-25 minutes to complete.

This exercise demonstrates how to use interfaces in C# to create flexible, reusable, and loosely coupled code. You’ll learn to define and implement interfaces with default methods, use interfaces as method parameters, and work with system-defined interfaces like IComparable and IEnumerable. By the end, you’ll apply these concepts to a scenario.

Before you start

Before you can start this exercise, you need to:

Ensure that you have the latest short term support (STS) version of the .NET SDK installed on your computer. You can download the latest versions of the .NET SDK using the following URL: Download .NET.
Ensure that you have Visual Studio Code installed on your computer. You can download Visual Studio Code using the following URL: Download Visual Studio Code.
Ensure that you have the C# Dev Kit configured in Visual Studio Code.

For additional help configuring the Visual Studio Code environment, see Install and configure Visual Studio Code for C# development.

Exercise scenario

Suppose you’re a software developer at a tech company working on a new project. Your task is to design a system that models people in different roles, such as teachers and students, while ensuring the code is flexible, reusable, and easy to maintain. You’ll achieve this by leveraging interfaces and system-defined features in C#. In this exercise, you will build a console application that demonstrates how to use interfaces to decouple code, implement default methods, and create a dynamic classroom system that supports sorting and iteration.

Task 1: Create a new C# project

To start, you need to create a new C# project in your development environment. This project will serve as the foundation for creating decoupled code using Interfaces.

Open Visual Studio Code.
Open the terminal in Visual Studio Code by selecting View > Terminal.
Navigate to the directory where you want to create your project.
Run the following command to create a new console application:
```
dotnet new console -n DecoupleWithInterfaces  
```
This command creates a new console application named DecoupleWithInterfaces, which will serve as the starting point for the exercise.
Navigate into the newly created project directory:
```
cd DecoupleWithInterfaces  
```
This step ensures that you are working within the correct project directory.
Open the project in Visual Studio Code:
```
code .  
```
Opening the project in Visual Studio Code allows you to edit and manage the files easily.

Paste the following command at the DecoupleWithInterfaces directory terminal prompt and press “Enter”:

     echo namespace DecoupleWithInterfaces; > Classroom.cs
     echo <Project Sdk="Microsoft.NET.Sdk"> > DecoupleWithInterfaces.csproj
     echo namespace DecoupleWithInterfaces; > IPerson.cs
     echo namespace DecoupleWithInterfaces; > PersonUtilities.cs
     echo namespace DecoupleWithInterfaces; > Program.cs
     echo namespace DecoupleWithInterfaces; > Student.cs
     echo namespace DecoupleWithInterfaces; > Teacher.cs
     echo "Done"

Verify files are created in the Visual Studio Code Explorer.

- `Classroom.cs` for the `Classroom` class.
- `DecoupleWithInterfaces.csproj` for the Project file.
- `IPerson.cs` for the `IPerson` interface.
- `PersonUtilities.cs` for the utility class.
- `Program.cs` for the main entry point of the application.
- `Teacher.cs` for the `Teacher` class.
- `Student.cs` for the `Student` class.

Task 2: Extend the IPerson Interface

You start by adding a new property and a default method to the IPerson interface. Default methods allow you to provide functionality directly in the interface, which can be overridden by implementing classes if needed.

Add the code for the IPerson class to the file named IPerson.cs:

     namespace DecoupleWithInterfaces;
        
     public interface IPerson
     {
         string Name { get; set; }
         int Age { get; set; }
         void DisplayInfo();
        
         // New property
         string Role { get; }
        
         // Default method
         void Greet()
         {
             Console.WriteLine($"Hello, my name is {Name} and I am a {Role}.");
         }
     }

This code introduces a default method and a new property to the interface, enabling shared functionality across implementing classes.

Notice the IPerson interface includes the Role property and the Greet method with a default implementation.

Task 3: Update Teacher and Student Classes

The Teacher and Student classes now implement the new Role property. The Teacher class overrides the default Greet method, while the Student class uses the default implementation.

Add the code for the Teacher class to the file named Teacher.cs:

 namespace DecoupleWithInterfaces;
    
 public class Teacher : IPerson
    
 {
     public string Name { get; set; } = string.Empty;
     public int Age { get; set; } = 0;
    
     public string Role => "Teacher";
    
     public void DisplayInfo()
     {
         Console.WriteLine($"Teacher Name: {Name}, Age: {Age}");
     }
    
     public void Greet()
     {
         Console.WriteLine($"Hello, I am {Name}, and I am a teacher.");
     }
 }

This code demonstrates how the Teacher class implements the IPerson interface, defines the Role property, and overrides the default Greet method.

Add the code for the Student class to the file named Student.cs:

 namespace DecoupleWithInterfaces;
    
 public class Student : IPerson, IComparable  
 {  
     public string Name { get; set; } = string.Empty;  
     public int Age { get; set; } = 0;  
    
     public string Role => "Student";  
    
     public void DisplayInfo()  
     {  
         Console.WriteLine($"Student Name: {Name}, Age: {Age}");  
     }  
    
     public int CompareTo(Student? other)  
     {  
         if (other == null) return 1;  
         return this.Age.CompareTo(other.Age);  
     }  
 }

This code shows how the Student class implements the IPerson interface, defines the Role property, and uses the default Greet method while adding support for sorting through the IComparable interface.

Observe that:
- The Teacher class overrides the default Greet method with a custom implementation.
- The Student class uses the default Greet method and implements the IComparable interface for sorting.

Task 4: Use Interfaces as Method Parameters

In this task, you will create a utility class that uses an interface as a method parameter. This demonstrates how interfaces allow you to handle multiple object types generically, enabling flexibility and reusability in your code.

**Add the code for the PersonUtilities class to the file named PersonUtilities.cs: **

     namespace DecoupleWithInterfaces;
        
     public class PersonUtilities
     {
         public static void PrintPersonDetails(IPerson person)
         {
             person.DisplayInfo();
             person.Greet();
         }
     }

This code demonstrates how to use an interface as a method parameter to handle multiple object types generically.

Observe the use of Interfaces as Method Parameters:

The PrintPersonDetails method accepts an IPerson object as a parameter. This allows the method to work with any class that implements the IPerson interface, such as Teacher or Student. Inside the method, the DisplayInfo and Greet methods are called on the IPerson object. These methods are defined in the IPerson interface and implemented by the Teacher and Student classes.

Task 5: Create a Classroom with IEnumerable

In this task, you will create a Classroom class that uses List<T> to store students dynamically and implements IEnumerable to allow iteration over the collection. You will also test the Classroom class by adding, sorting, and displaying students.

Create the Classroom Class in the file named Classroom.cs and add the following code:

     namespace DecoupleWithInterfaces;
     using System.Collections;
     using System.Collections.Generic;
        
     public class Classroom : IEnumerable<Student>
     {
         private List<Student> students = new List<Student>();
        
         public void AddStudent(Student student)
         {
             students.Add(student);
         }
        
         public void SortStudentsByAge()
         {
             students.Sort(); // Uses the IComparable implementation in Student
         }
        
         public IEnumerator<Student> GetEnumerator()
         {
             return students.GetEnumerator();
         }
        
         IEnumerator IEnumerable.GetEnumerator()
         {
             return GetEnumerator();
         }
     }

This code shows how to create a custom collection class that supports dynamic storage (can grow or shrink as needed), sorting, and iteration using List<T> and IEnumerable.

Observe the Classroom class:
- The Classroom class implements IEnumerable<Student>, which means it provides an enumerator through the GetEnumerator method. This allows the foreach loop to iterate over the students list directly without needing a separate method like GetStudents().
- The foreach loop automatically uses the GetEnumerator method to retrieve the enumerator, which handles the iteration process internally by calling MoveNext() and accessing the Current property of the enumerator.

Task 6: Update the Program Class

Update the Program.cs file to demonstrate the functionality of the Classroom class.

     namespace DecoupleWithInterfaces;
        
     class Program
     {
         static void Main(string[] args)
         {
             IPerson teacher = new Teacher { Name = "Helen Karu", Age = 35 };
             IPerson student1 = new Student { Name = "Eba Lencho", Age = 20 };
             IPerson student2 = new Student { Name = "Frederiek Eppink", Age = 22 };
        
             // Use the utility class
             PersonUtilities.PrintPersonDetails(teacher);
             PersonUtilities.PrintPersonDetails(student1);
        
             // Create a classroom and add students
             Classroom classroom = new Classroom();
             classroom.AddStudent((Student)student1);
             classroom.AddStudent((Student)student2);
        
             // Sort students by age
             classroom.SortStudentsByAge();
        
             Console.WriteLine("\nSorted Students by Age:");
             foreach (Student student in classroom)
             {
                 student.DisplayInfo();
             }
        
             // Demonstrate ArgumentException for incompatible comparison
             try
             {
                 Console.WriteLine("\nAttempting to compare a Student with a Teacher...");
                 Student student = (Student)student1;
                 int comparisonResult = student.CompareTo(teacher); // This will throw an exception
             }
             catch (ArgumentException ex)
             {
                 Console.WriteLine($"Error: {ex.Message}");
             }
         }
     }

This code demonstrates how to use the Classroom class to store, sort, and display students dynamically.

Review the Program class
- Program.cs Creates Teacher and Student objects using the IPerson interface and passes them to the PersonUtilities.PrintPersonDetails method.
- Adds Student objects to the Classroom collection, which implements IEnumerable<Student> for iteration.
- Sorts the students by age using the SortStudentsByAge method, which relies on the IComparable implementation in the Student class.
- Iterates over the Classroom collection using a foreach loop to display the sorted student details.

Task 7: Build and run the program to examine the output

Open the terminal in Visual Studio Code.
Run the following command to build the program:
```
 dotnet build  
```
Run the following command to execute the program:
```
 dotnet run  
```

The following is the expected console output:

     Teacher Name: Helen Karu, Age: 35
     Hello, I am Helen Karu, and I am a teacher.
     Student Name: Eba Lencho, Age: 20
     Hello, my name is Eba Lencho and I am a Student.
        
     Sorted Students by Age:
     Student Name: Eba Lencho, Age: 20
     Student Name: Frederiek Eppink, Age: 22
        
     Attempting to compare a Student with a Teacher...
     Error: obj (Parameter 'The object being compared must be of type Student.')

The Teacher and Student details are printed using the PersonUtilities class. The students in the Classroom are displayed in ascending order of age after sorting. The foreach loop successfully iterates over the Classroom collection.

Using interfaces helps decouple components, making your application more flexible and maintainable. Interfaces define clear contracts between parts of the system, and working with system-defined interfaces like IComparable and IEnumerable further enhances your code’s functionality. These interfaces enable sorting and iteration capabilities, allowing your application to leverage built-in .NET features for handling collections and comparisons. These practices improve the structure of your code, making it easier to extend, test, and adapt to future requirements.

Now that you’ve finished the exercise, consider archiving your project files for review at a later time. Having your own projects available for review can be a valuable resource when you’re learning to code. Additionally, building a portfolio of projects can be a great way to demonstrate your skills to potential employers.