C# — Data Types

Type categories

Category	Types
Value types	int, double, bool, char, struct, enum
Reference types	string, array, class, interface, delegate

Value types

using System;

// Integers
byte b = 255;          // 1 byte
short s = -32000;      // 2 bytes
int i = 2147483647;    // 4 bytes
long l = 9223372036854775807L; // 8 bytes
nint ni = 42;          // Native int (C# 9+)

// Unsigned
uint ui = 4294967295U;
ulong ul = 18446744073709551615UL;

// Floating-point
float f = 3.14f;       // 4 bytes
double d = 3.14159;    // 8 bytes (default)
decimal m = 3.14159m;  // 16 bytes (precise)

// Other
bool flag = true;      // 1 byte
char c = 'A';          // 2 bytes (Unicode)

Reference types

using System;

// String (immutable)
string name = "Hello";
string upper = name.ToUpper();

// Array
int[] nums = { 1, 2, 3, 4, 5 };
string[] names = new string[3];

// Object
object obj = 42; // Boxing
int unboxed = (int)obj; // Unboxing

Console.WriteLine(name);
Console.WriteLine(nums.Length);

Structs

using System;

struct Point
{
    public double X;
    public double Y;

    public Point(double x, double y)
    {
        X = x;
        Y = y;
    }

    public double DistanceTo(Point other)
    {
        double dx = X - other.X;
        double dy = Y - other.Y;
        return Math.Sqrt(dx * dx + dy * dy);
    }
}

var p1 = new Point(0, 0);
var p2 = new Point(3, 4);
Console.WriteLine($"Distance: {p1.DistanceTo(p2)}"); // 5

Enums

using System;

// Basic enum
enum Color { Red, Green, Blue }

// Flags enum
[Flags]
enum Permissions
{
    None = 0,
    Read = 1,
    Write = 2,
    Execute = 4,
    All = Read | Write | Execute
}

var favorite = Color.Green;
var perms = Permissions.Read | Permissions.Write;

Console.WriteLine(favorite); // Green
Console.WriteLine(perms);    // Read, Write

Tuples

using System;

// Named tuple
var point = (X: 3.0, Y: 4.0);
Console.WriteLine($"({point.X}, {point.Y})");

// Unnamed tuple
(int, string) person = (30, "Alice");

// Method returning tuple
static (int min, int max, double avg) GetStats(int[] numbers)
{
    return (numbers.Min(), numbers.Max(), numbers.Average());
}

var stats = GetStats(new[] { 1, 2, 3, 4, 5 });
Console.WriteLine($"Min: {stats.min}, Max: {stats.max}, Avg: {stats.avg}");

Collections

using System;
using System.Collections.Generic;

// List<T>
var numbers = new List<int> { 1, 2, 3, 4, 5 };
numbers.Add(6);
numbers.RemoveAt(0);
Console.WriteLine($"Count: {numbers.Count}");

// Dictionary<TKey, TValue>
var ages = new Dictionary<string, int>
{
    ["Alice"] = 30,
    ["Bob"] = 25
};
Console.WriteLine($"Alice: {ages["Alice"]}");

// HashSet<T>
var unique = new HashSet<int> { 1, 2, 3, 2, 1 };
Console.WriteLine($"Unique count: {unique.Count}"); // 3

// Queue<T>
var queue = new Queue<string>();
queue.Enqueue("First");
queue.Enqueue("Second");
Console.WriteLine(queue.Dequeue()); // First

// Stack<T>
var stack = new Stack<int>();
stack.Push(1);
stack.Push(2);
Console.WriteLine(stack.Pop()); // 2

Type aliases

using System;

// Keywords are aliases for .NET types
int i = 42;     // Same as System.Int32
string s = "Hi"; // Same as System.String
bool b = true;  // Same as System.Boolean

Console.WriteLine(typeof(int));     // System.Int32
Console.WriteLine(typeof(string));  // System.String

Mini Practice

Write C# code that:

1. Creates a struct with fields and methods
2. Uses a tuple to return multiple values from a method
3. Creates and manipulates a List<int> and a Dictionary<string, int>
4. Demonstrates enum with [Flags] attribute

Up Next

In the next lesson, you'll learn about Operators — arithmetic, comparison, and logical operations.