🧠 C# Fundamentals Every .NET Developer Must Know

Introduction

C# is the heart of the .NET ecosystem. Whether you are building web APIs, desktop apps, or cloud services, everything starts with strong C# fundamentals.

In this article, you’ll learn:

• Core building blocks of C#
• How object-oriented programming works in C#
• Why interfaces are important
• A beginner-friendly introduction to Dependency Injection

This is Week 3 of our .NET weekly blog series.

1️⃣ Variables and Data Types

C# is a strongly typed language, meaning every variable has a specific type.

Common Data Types

int age = 25;
double price = 99.99;
bool isActive = true;
string name = "John";

var Keyword

C# can infer the type automatically:

var count = 10;       // int
var title = "DotNet"; // string

✔ Use var when the type is obvious
❌ Avoid it when readability suffers

2️⃣ Control Flow (Conditions & Loops)

If–Else Statement

if (age >= 18)
{
    Console.WriteLine("Adult");
}
else
{
    Console.WriteLine("Minor");
}

Loops

for (int i = 0; i < 5; i++)
{
    Console.WriteLine(i);
}

foreach (var item in items)
{
    Console.WriteLine(item);
}

3️⃣ Methods (Functions)

Methods help you reuse code and keep it clean.

static int Add(int a, int b)
{
    return a + b;
}

Calling the method:

int result = Add(5, 3);

4️⃣ Classes and Objects (OOP Basics)

C# follows Object-Oriented Programming (OOP).

Creating a Class

class User
{
    public string Name { get; set; }
    public int Age { get; set; }

    public void Greet()
    {
        Console.WriteLine($"Hello, {Name}");
    }
}

Creating an Object

var user = new User
{
    Name = "Alice",
    Age = 30
};

user.Greet();

5️⃣ Interfaces (Why They Matter)

Interfaces define what a class should do, not how it does it.

Interface Example

public interface IEmailService
{
    void SendEmail(string to, string message);
}

Implementing an Interface

public class EmailService : IEmailService
{
    public void SendEmail(string to, string message)
    {
        Console.WriteLine("Email sent");
    }
}

✔ Helps with loose coupling
✔ Makes code testable and maintainable

6️⃣ Introduction to Dependency Injection (DI)

Dependency Injection is a core concept in modern .NET apps.

❌ Without DI (Bad Practice)

public class OrderService
{
    private EmailService _emailService = new EmailService();
}

✅ With DI (Recommended)

public class OrderService
{
    private readonly IEmailService _emailService;

    public OrderService(IEmailService emailService)
    {
        _emailService = emailService;
    }
}

Registering in .NET

builder.Services.AddScoped<IEmailService, EmailService>();

7️⃣ Value Types vs Reference Types

| Type           | Example              |
| -------------- | -------------------- |
| Value Type     | int, bool, struct    |
| Reference Type | class, array, string |

int a = 5;
int b = a;
b = 10; // a remains 5

var obj1 = new User();
var obj2 = obj1;
obj2.Name = "Bob"; // affects obj1

8️⃣ Common Beginner Mistakes

❌ Ignoring null values
❌ Not using interfaces
❌ Writing large methods
❌ Skipping DI concepts

Best Practices for C# Beginners

✅ Follow naming conventions
✅ Keep methods small
✅ Use interfaces
✅ Learn async/await early
✅ Read error messages carefully

What You’ve Learned

✔ Variables & data types
✔ Control flow
✔ Methods
✔ Classes & objects
✔ Interfaces
✔ Dependency Injection basics

These concepts are mandatory for ASP.NET Core development.