C# Fundamentals

After this lesson, you will be able to: Use C# fundamentals: types, control flow, methods, namespaces, using statements.

C# is a statically typed, OOP-first language with strong tooling. Coming from Java? Almost identical. From Python? Get used to types.

Prerequisites:.NET Ecosystem

Variables and types

Statically typed. Type can be explicit or inferred.

tsx

// Explicit types
int count = 10;
double price = 9.99;
string name = "Alex";
bool active = true;
char grade = 'A';

// Type inference with var (still statically typed!)
var n = 42;              // n is int
var s = "hello";         // s is string
var list = new List<int>();

// Nullable reference types (C# 8+, enabled by default in new projects)
string nonNullable = "x";       // can't be null
string? nullable = null;        // can be null, the ? matters

// Constants
const double PI = 3.14159;
readonly string AppName = "LastWrite";   // readonly = set once at init

Control flow + methods

If, switch, for, foreach, methods.

tsx

// if / else
int x = 10;
if (x > 5) Console.WriteLine("big");
else if (x == 0) Console.WriteLine("zero");
else Console.WriteLine("small");

// switch expression (modern C# 8+)
string label = x switch {
    > 100 => "huge",
    > 10  => "big",
    0     => "zero",
    _     => "small",
};

// for + foreach
for (int i = 0; i < 5; i++) Console.WriteLine(i);

var nums = new[] { 1, 2, 3 };
foreach (var n in nums) Console.WriteLine(n);

// Methods
int Square(int x) => x * x;     // expression-bodied

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

// Optional + named args
static int Power(int base_, int exp = 2) => (int)Math.Pow(base_, exp);
Power(3);                       // 9
Power(3, exp: 3);               // 27, named arg

Namespaces + using

How C# organizes code.

tsx

// namespace block (legacy)
namespace MyApp.Users {
    public class User { }
}

// File-scoped namespace (C# 10+, preferred)
namespace MyApp.Users;

public class User { }

// using directives at the top of a file
using System;                       // optional in C# 10+ (auto-imported)
using System.Collections.Generic;
using System.Linq;

// Global usings (C# 10+), declare once in any file, applies project-wide
global using System.Threading.Tasks;

Common mistakes

Mixing camelCase and PascalCase incorrectly (looks unprofessional in C# code). Using `var` for non-obvious types (`var x = GetData();`, what type?). Skipping the nullable annotation (`string` vs `string?`) when nullable enabled. Using `List<T>` when array would do; performance is similar but intent differs.

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