Business IT Support Cape Town

Basic Syntax

Program Structure

using System;
using System.Collections.Generic;
using System.Linq;

namespace MyApplication
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Hello, World!");
        }
    }
}

// Top-level programs (C# 9.0+)
using System;
Console.WriteLine("Hello, World!");

Data Types

// Value types
bool flag = true;
byte b = 255;
char letter = 'A';
int number = 42;
long bigNumber = 1234567890L;
float decimal = 3.14f;
double precision = 3.14159;
decimal money = 99.99m;

// Reference types
string text = "Hello World";
object obj = new object();
int[] array = {1, 2, 3, 4, 5};

// Nullable types
int? nullableInt = null;
string? nullableString = null; // C# 8.0+

// var keyword (implicit typing)
var name = "John";      // string
var age = 30;           // int
var list = new List(); // List

Constants & Variables

// Constants
const int MAX_SIZE = 100;
const string APP_NAME = "MyApp";

// Read-only fields
readonly DateTime createdDate = DateTime.Now;

// Static variables
static int instanceCount = 0;

// Properties
public string Name { get; set; }
public int Age { get; private set; }
public string FullName => $"{FirstName} {LastName}";

// Auto-implemented properties
public string FirstName { get; set; } = "Default";

Control Structures

Conditionals

// If statement
if (condition)
{
    // code
}
else if (anotherCondition)
{
    // code
}
else
{
    // code
}

// Ternary operator
string result = condition ? "true" : "false";

// Switch statement
switch (value)
{
    case 1:
        // code
        break;
    case 2:
    case 3:
        // code for 2 or 3
        break;
    default:
        // default code
        break;
}

// Switch expression (C# 8.0+)
string dayType = day switch
{
    DayOfWeek.Monday or DayOfWeek.Tuesday => "Weekday",
    DayOfWeek.Saturday or DayOfWeek.Sunday => "Weekend",
    _ => "Unknown"
};

Loops

// For loop
for (int i = 0; i < 10; i++)
{
    Console.WriteLine(i);
}

// Foreach loop
int[] numbers = {1, 2, 3, 4, 5};
foreach (int num in numbers)
{
    Console.WriteLine(num);
}

// While loop
while (condition)
{
    // code
}

// Do-while loop
do
{
    // code
} while (condition);

// Break and continue
break;    // exit loop
continue; // skip to next iteration

Methods & Functions

Method Declaration

// Basic method
public int Add(int a, int b)
{
    return a + b;
}

// Method with default parameters
public void Greet(string name, string greeting = "Hello")
{
    Console.WriteLine($"{greeting}, {name}!");
}

// Method overloading
public int Add(int a, int b) => a + b;
public double Add(double a, double b) => a + b;
public string Add(string a, string b) => a + b;

// Static method
public static double CalculateArea(double radius)
{
    return Math.PI * radius * radius;
}

// Extension method
public static class StringExtensions
{
    public static bool IsValidEmail(this string email)
    {
        return email.Contains("@");
    }
}

// Usage: "test@example.com".IsValidEmail();

Lambda Expressions & Delegates

// Lambda expressions
Func add = (x, y) => x + y;
Action print = message => Console.WriteLine(message);
Predicate isEven = x => x % 2 == 0;

// Delegates
public delegate void MyDelegate(string message);
MyDelegate del = Console.WriteLine;
del += message => File.WriteAllText("log.txt", message);

// Events
public event Action OnMessageReceived;
OnMessageReceived?.Invoke("Hello");

// Anonymous methods
Button.Click += delegate(object sender, EventArgs e)
{
    MessageBox.Show("Button clicked!");
};

Collections

Generic Collections

List<T>

Dictionary<TKey, TValue>

HashSet<T>

Queue<T>

Stack<T>

LinkedList<T>

SortedList<TKey, TValue>

SortedDictionary<TKey, TValue>

// List
var list = new List {"apple", "banana", "cherry"};
list.Add("date");
list.Remove("banana");
list.Insert(1, "blueberry");

// Dictionary
var dict = new Dictionary
{
    {"apple", 5},
    {"banana", 3}
};
dict["cherry"] = 7;
dict.TryGetValue("apple", out int value);

// HashSet
var set = new HashSet {1, 2, 3, 3, 4}; // {1, 2, 3, 4}
set.Add(5);
bool contains = set.Contains(3);

// Queue and Stack
var queue = new Queue();
queue.Enqueue("first");
string first = queue.Dequeue();

var stack = new Stack();
stack.Push(1);
int top = stack.Pop();

Classes & Objects

Class Definition

public class Person
{
    // Fields
    private string _name;
    private int _age;
    
    // Properties
    public string Name 
    { 
        get => _name; 
        set => _name = value ?? throw new ArgumentNullException(); 
    }
    
    public int Age { get; set; }
    
    // Auto-implemented property
    public string Email { get; set; }
    
    // Constructors
    public Person() : this("Unknown", 0) { }
    
    public Person(string name, int age)
    {
        Name = name;
        Age = age;
    }
    
    // Methods
    public virtual void Introduce()
    {
        Console.WriteLine($"Hi, I'm {Name} and I'm {Age} years old.");
    }
    
    // Static method
    public static Person CreateAdult(string name)
    {
        return new Person(name, 18);
    }
    
    // Override ToString
    public override string ToString()
    {
        return $"Person: {Name}, Age: {Age}";
    }
}

Inheritance & Interfaces

// Interface
public interface IDrawable
{
    void Draw();
    string Color { get; set; }
}

// Abstract class
public abstract class Shape : IDrawable
{
    public string Color { get; set; }
    public abstract double Area { get; }
    public abstract void Draw();
    
    public virtual void Move(int x, int y)
    {
        Console.WriteLine($"Moving shape to ({x}, {y})");
    }
}

// Inheritance
public class Circle : Shape
{
    public double Radius { get; set; }
    
    public override double Area => Math.PI * Radius * Radius;
    
    public override void Draw()
    {
        Console.WriteLine($"Drawing a {Color} circle with radius {Radius}");
    }
    
    // Method hiding
    public new void Move(int x, int y)
    {
        Console.WriteLine($"Moving circle to ({x}, {y})");
    }
}

LINQ

LINQ Methods

Where(predicate)

Select(selector)

OrderBy(keySelector)

OrderByDescending(keySelector)

GroupBy(keySelector)

Join(inner, outerKey, innerKey, result)

First() / FirstOrDefault()

Last() / LastOrDefault()

Single() / SingleOrDefault()

Any(predicate)

All(predicate)

Count() / Count(predicate)

Sum() / Average() / Min() / Max()

Take(count) / Skip(count)

Distinct()

ToList() / ToArray()

var numbers = new[] {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};

// Method syntax
var evenNumbers = numbers
    .Where(n => n % 2 == 0)
    .Select(n => n * n)
    .OrderByDescending(n => n)
    .ToList();

// Query syntax
var evenSquares = from n in numbers
                  where n % 2 == 0
                  orderby n descending
                  select n * n;

// Complex LINQ
var people = new List();
var adults = people
    .Where(p => p.Age >= 18)
    .GroupBy(p => p.Age)
    .Select(g => new { Age = g.Key, Count = g.Count() })
    .OrderBy(x => x.Age);

Exception Handling

Try-Catch-Finally

// Basic exception handling
try
{
    int result = 10 / 0;
}
catch (DivideByZeroException ex)
{
    Console.WriteLine($"Division by zero: {ex.Message}");
}
catch (Exception ex)
{
    Console.WriteLine($"General exception: {ex.Message}");
}
finally
{
    Console.WriteLine("This always executes");
}

// Using statement (automatic disposal)
using (var file = new FileStream("file.txt", FileMode.Open))
{
    // File is automatically closed when leaving this block
}

// Using declaration (C# 8.0+)
using var file = new FileStream("file.txt", FileMode.Open);
// File is automatically closed at end of scope

// Custom exceptions
public class CustomException : Exception
{
    public CustomException(string message) : base(message) { }
    public CustomException(string message, Exception innerException) 
        : base(message, innerException) { }
}

Modern C# Features

C# 6.0+ Features

// String interpolation (C# 6.0)
string name = "John";
int age = 30;
string message = $"Hello, {name}! You are {age} years old.";

// Null-conditional operators (C# 6.0)
string? text = null;
int? length = text?.Length;
string result = text ?? "default";

// Expression-bodied members (C# 6.0)
public string FullName => $"{FirstName} {LastName}";
public void PrintName() => Console.WriteLine(FullName);

// Pattern matching (C# 7.0+)
if (obj is string str)
{
    Console.WriteLine($"String: {str}");
}

switch (obj)
{
    case int i when i > 0:
        Console.WriteLine($"Positive integer: {i}");
        break;
    case string s:
        Console.WriteLine($"String: {s}");
        break;
    case null:
        Console.WriteLine("Null value");
        break;
}

// Tuples (C# 7.0)
(string name, int age) person = ("John", 30);
var (name2, age2) = person;

// Local functions (C# 7.0)
int Add(int x, int y)
{
    return x + y;
    
    int Multiply(int a, int b) => a * b; // Local function
}

// Records (C# 9.0)
public record Person(string Name, int Age);
var person1 = new Person("John", 30);
var person2 = person1 with { Age = 31 };

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