C#: From Fundamentals to Advanced — A Comprehensive Cheat Sheet

Basic Structure
Data Types
Variables
Constants
Conditional Statements
Loops
Arrays
Lists
Dictionaries
Methods
Classes & Objects
Exception Handling
Delegates, Events & Lambdas
LINQ (Language-Integrated Query)
Attributes
Async/Await
Miscellaneous
String Manipulation
File I/O
Date & Time
Generics
Nullables
Attributes & Reflection
Extension Methods
Dependency Injection
Partial Classes
Interoperability
Anonymous Types
Tuples
Pattern Matching
Local Functions
Records
with Expressions
Indexers and Ranges
using Declaration
Nullable Reference Types (NRTs)
Pattern-Based Using
Property Patterns
Default Interface Implementations
Dynamic Binding

1. Basic Structure

All C# programs follow a fundamental structure, outlined below:

using System;

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

Starting with .NET 5, top-level statements simplify the Program.cs content:

Console.WriteLine("Hello, World");

2. Data Types

C# supports various data types such as:

Value Types: int, char, float
Reference Types: string, class, array

3. Variables

Variables are symbolic names for values:

int age = 30; // integer variable
string name = "John"; // string variable
double PI = 3.14159; // double for floating-point numbers
bool isLoggedIn = true; // boolean variable

Use ‘var’ for type inference:

var number = 5; // compiler infers type as int
var message = "This is a message"; // compiler infers type as string

4. Constants

Constants hold immutable values:

const double GRAVITY = 9.81; // constant for gravitational acceleration
const string COMPANY_NAME = "MyCompany"; // constant company name

5. Conditional Statements

Control program flow based on conditions:

int age = 20;

if (age >= 18)
{
    Console.WriteLine("You are eligible to vote.");
}
else
{
    Console.WriteLine("You are not eligible to vote.");
}

switch (variable) { /*...*/ } // Switch statement

6. Loops

Execute code repeatedly:

for (int i = 1; i <= 5; i++)
{
    Console.WriteLine(i);
}

foreach (var item in collection) { /*...*/ } // Foreach loop

while (condition) { /*...*/ } // While loop

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

7. Arrays

Fixed-size collections of elements:

string[] names = new string[3] { "Alice", "Bob", "Charlie" };
Console.WriteLine(names[1]); // Output: Bob (accessing element at index 1)

8. Lists

Dynamic collections similar to arrays:

List<int> numbers = new List<int>();
numbers.Add(1);
numbers.Add(2);
numbers.Add(3);

foreach (var number in numbers)
{
    Console.WriteLine(number);
}

9. Dictionaries

Key-value pairs for data association:

Dictionary<string, string> phonebook = new Dictionary<string, string>();
phonebook.Add("John Doe", "123-456-7890");
phonebook.Add("Jane Doe", "987-654-3210");

Console.WriteLine(phonebook["John Doe"]); // Output: 123-456-7890

10. Methods

Encapsulate reusable logic:

public class Rectangle
{
    public double Width { get; set; }
    public double Height { get; set; }

    public double GetArea()
    {
        return Width * Height;
    }
}

public class Program
{
    public static void Main(string[] args)
    {
        Rectangle rect = new Rectangle();
        rect.Width = 5;
        rect.Height = 10;

        double area = rect.GetArea();
        Console.WriteLine($"Area of rectangle: {area}");
    }
}

11. Classes & Objects

Classes define blueprints for objects:

public class MyClass // Class definition
{
    public string PropertyName { get; set; } // Properties store data
    public void MethodName() { /*...*/ } // Methods define actions
}

MyClass obj = new MyClass(); // Object creation

12. Exception Handling

Manage runtime errors gracefully:

public static int GetNumberInput()
{
  while (true)
  {
    try
    {
      Console.WriteLine("Enter a number: ");
      string input = Console.ReadLine();
      return int.Parse(input);
    }
    catch (FormatException)
    {
      Console.WriteLine("Invalid input. Please enter a number.");
    }
  }
}

public static void Main(string[] args)
{
  int number = GetNumberInput();
  Console.WriteLine($"You entered: {number}");
}

13. Delegates, Events & Lambda

For event-driven programming and method handling:

public delegate void MyDelegate(); // Delegate declaration

event MyDelegate MyEvent; // Event declaration

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

public static void Main(string[] args)
{
  List<Person> people = new List<Person>()
  {
    new Person { Name = "Alice", Age = 30 },
    new Person { Name = "Bob", Age = 25 },
    new Person { Name = "Charlie", Age = 40 },
  };

  people.Sort((p1, p2) => p1.Name.CompareTo(p2.Name));

  foreach (var person in people)
  {
    Console.WriteLine(person.Name); // Output: Alice, Bob, Charlie (sorted by name)
  }
}

14. LINQ (Language-Integrated Query)

Query capabilities for data manipulation:

using System.Linq;

public static void Main(string[] args)
{
  List<int> numbers = new List<int>() { 1, 2, 3, 4, 5, 6 };
  var evenNumbers = numbers.Where(x => x % 2 == 0);

  foreach (var number in evenNumbers)
  {
    Console.WriteLine(number); // Output: 2, 4, 6
  }
}

15. Attributes

Add metadata to code elements:

[Obsolete("Use the new DoSomethingV2 method instead.")]
public void DoSomething()
{
  // Implementation here
}

public void DoSomethingV2()
{
  // New and improved implementation
}

16. Async/Await

For non-blocking code execution:

using System.Threading.Tasks;

public static async Task DownloadFileAsync(string url, string filePath)
{
  // Simulate downloading data asynchronously
  await Task.Delay(2000); // Simulate a 2-second download

  // Write downloaded data to the file
  File.WriteAllText(filePath, "Downloaded content");
  Console.WriteLine($"File downloaded to: {filePath}");
}

public static void Main(string[] args)
{
  string url = "https://example.com/data.txt";
  string filePath = "downloaded_data.txt";

  DownloadFileAsync(url, filePath);

  // Continue program execution while download happens in the background
  Console.WriteLine("Downloading file...");
  Console.WriteLine("Meanwhile, you can do other things...");
}

17. Miscellaneous

Additional language features:

enum, interface, class, record, struct
dynamic, is, as, var, nameof

18. String Manipulation

Powerful string handling methods:

string.Concat(); // Combine strings
string.Join(); // Join elements
str.Split(); // Split string
str.ToUpper(); // Convert to uppercase
str.ToLower(); // Convert to lowercase

19. File I/O

Operations with files:

using System.IO; // Required for File I/O

File.ReadAllText(path); // Read file content
File.WriteAllText(path, content); // Write to file
File.Exists(path); // Check file existence

20. Date & Time

Date and time manipulation:

using System;

public static void Main(string[] args)
{
  DateTime startDate = DateTime.Parse("2024-03-10");
  DateTime endDate = DateTime.Now;

  TimeSpan difference = endDate - startDate;
  Console.WriteLine($"Time difference: {difference.Days} days, {difference.Hours} hours");
}

21. Generics

Type-safe data structures:

public class Stack<T>
{
  private List<T> items = new List<T>();

  public void Push(T item)
  {
    items.Add(item);
  }

  public T Pop()
  {
    T item = items[items.Count - 1];
    items.RemoveAt(items.Count - 1);
    return item;
  }
}

public static void Main(string[] args)
{
  Stack<string> messages = new Stack<string>();
  messages.Push("Hello");
  messages.Push("World");

  string message = messages.Pop();
  Console.WriteLine(message); // Output: World
}

22. Nullables

Allow value types to be null:

int? nullableInt = null; // Nullable integer

23. Attributes & Reflection

Metadata and type introspection:

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

public static void Main(string[] args)
{
  Type personType = typeof(Person);
  PropertyInfo[] properties = personType.GetProperties();

  foreach (PropertyInfo property in properties)
  {
    Console.WriteLine(property.Name); // Output: Name, Age
  }
}

24. Extension Methods

Add methods to existing types:

public static class StringExtensions
{
  public static string ToUppercase(this string str)
  {
    return str.ToUpper();
  }
}

public static void Main(string[] args)
{
  string message = "Hello, world!";
  string uppercased = message.ToUppercase(); // Using the extension method
  Console.WriteLine(uppercased); // Output: HELLO, WORLD!
}

25. Dependency Injection

Loosely coupled code design:

public interface ILogger
{
  void LogMessage(string message);
}

public class MyService
{
  private readonly ILogger _logger;

  public MyService(ILogger logger)
  {
    _logger = logger;
  }

  public void DoSomething()
  {
    _logger.LogMessage("Doing something...");
  }
}

// Implementing the ILogger interface (example)
public class ConsoleLogger : ILogger
{
  public void LogMessage(string message)
  {
    Console.WriteLine(message);
  }
}

public static void Main(string[] args)
{
  ILogger logger = new ConsoleLogger();
  MyService service = new MyService(logger);
  service.DoSomething();
}

26. Partial Classes

Splitting a single class definition:

public partial class MyClass { /*...*/ } // Partial class definition

27. Interoperability

Interop with other languages:

using System;
using System.Runtime.InteropServices;

[DllImport("user32.dll")]
public static extern int MessageBox(IntPtr hWnd, string lpText, string lpCaption, uint uType);

public static void Main(string[] args)
{
  MessageBox(IntPtr.Zero, "Hello from C#!", "Interop Example", 0);
}

28. Anonymous Types

Creating unnamed types:

var person = new { Name = "John", Age = 30 };
Console.WriteLine($"Name: {person.Name}, Age: {person.Age}");

29. Tuple

Data structures with a specific number of elements:

(string Name, int Age) person = ("Alice", 30);
Console.WriteLine($"Name: {person.Name}, Age: {person.Age}"); // Accessing elements using Item1 and Item2

30. Pattern Matching

Simplifies certain programming tasks:

object obj = new Person { Name = "Bob", Age = 25 };

if (obj is Person { Name: "Bob", Age >= 18 })
{
  Console.WriteLine("Bob is an adult.");
}

31. Local Functions

Encapsulate logic within methods:

public static int Calculate(int number)
{
  int Factorial(int n)
  {
    if (n == 0) return 1;
    return n * Factorial(n - 1);
  }

  return Factorial(number);
}

public static void Main(string[] args)
{
  int result = Calculate(5);
  Console.WriteLine($"5! = {result}");
}

32. Records

Concise syntax for reference types:

public record Person(string Name, int Age);

public static void Main(string[] args)
{
  Person person1 = new Person("Alice", 30);
  Person person2 = new Person("Alice", 30);

  // Records provide default equality comparison
  if (person1 == person2)
  {
    Console.WriteLine("People are equal");
  }
}

33. with Expressions

Non-destructive mutation for records:

var john = new Person("John", 30);
var jane = john with { Name = "Jane" }; // Non-destructive mutation

34. Indexers and Ranges

Flexible data access:

int[] arr = {0, 1, 2, 3, 4, 5};
var subset = arr[1..^1]; // Indexer and range usage

35. using Declaration

Dispose of IDisposable objects:

using var reader = new StreamReader("file.txt"); // using declaration

36. Nullable Reference Types (NRTs)

Avoid null reference exceptions:

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

public static void Main(string[] args)
{
  Person person = new Person() { Age = 30 };

  // NRTs require null checks before accessing properties
  if (person?.Name != null)
  {
    Console.WriteLine(person.Name);
  }
  else
  {
    Console.WriteLine("Name is null");
  }
}

37. Pattern-Based Using

More patterns in the using statement:

public ref struct ResourceWrapper { /*...*/ } // Resource wrapper

using var resource = new ResourceWrapper(); // Pattern-based using

38. Property Patterns

Deconstruct objects in pattern matching:

if (obj is Person { Name: "John", Age: var age }) { /*...*/ } // Property pattern matching

39. Default Interface Implementations

Interfaces with default method implementations:

public interface IPerson { /*...*/ } // Interface with default method
public class MyClass : IPerson { /*...*/ } // Class implementing interface

40. Dynamic Binding

Runtime type resolution:

dynamic d = 5; // Dynamic binding
d = "Hello"; // No compile-time type checking

Conclusion

This structured C# Cheat Sheet concludes with advanced topics and techniques, providing a comprehensive reference for developers aiming to enhance their C# programming skills. For detailed examples and further exploration, refer to the specific sections outlined in this guide. Happy coding!

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C#: From Fundamentals to Advanced Techniques — A Comprehensive Cheat Sheet

The comprehensive C# Cheat Sheet is designed to aid developers in mastering key syntax and concepts related to C# programming.

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