C# Reference

// System.Console is globally imported in modern C# templates
Console.WriteLine("Hello, World!");

# Create a new console project
dotnet new console -o MyApp

# Run the project from its directory
dotnet run

# Build the project into an executable
dotnet build

// Single-line comment for brief notes

/* Multi-line
   comment block for detailed explanations */

/// <summary>
/// XML Documentation comment for methods/types (used for IntelliSense)
/// </summary>

using System;
using System.Collections.Generic;
using System.Linq;
using static System.Math; // Import static members for direct use (e.g., Sqrt(x))

using System;

// Exit with status code (0 for success, non-zero for error)
Environment.Exit(1);

try
{
    int result = 10 / int.Parse("0");
}
catch (DivideByZeroException ex)
{
    // Handle specific exception
    Console.WriteLine($"Error: {ex.Message}");
}
finally
{
    // Always runs, regardless of exception
    Console.WriteLine("Cleanup");
}

var name = "Gemini"; // Implicitly typed (string)
string version = "1.0"; // Explicitly typed
const int MaxRetries = 3; // Compile-time constant
readonly double Pi = 3.14159; // Runtime constant

double d = 9.78;
int i = (int)d; // Explicit cast (truncates)
string s = i.ToString(); // Conversion to string
long l = i; // Implicit cast (widening)

bool isTrue = true;
if (isTrue) { /* ... */ }

// Explicit checks required for other types (no implicit 0/null truthiness)
string? s = null;
if (s != null) { /* ... */ }

// Get variable by name
string path = Environment.GetEnvironmentVariable("PATH") ?? "";
// Set variable for the current process
Environment.SetEnvironmentVariable("APP_ENV", "prod");

using System;
using System.Diagnostics;

// High-resolution measurement
var sw = Stopwatch.StartNew();
// ... (code execution)
Console.WriteLine($"Elapsed: {sw.ElapsedMilliseconds}ms");

// Current UTC time and custom formatting
DateTime now = DateTime.UtcNow;
Console.WriteLine(now.ToString("yyyy-MM-dd HH:mm:ss"));

int a = 10 + 3; // Addition (13)
int b = 10 / 3; // Integer division (3)
double c = 10.0 / 3.0; // Floating point division (3.333...)
bool d = true && false; // Logical AND (false)
string e = "a" + "b"; // String concatenation ("ab")

using System.Text.RegularExpressions;

string email = "user@example.com";
// Check if string matches pattern
bool isMatch = Regex.IsMatch(email, @"^[^@\s]+@[^@\s]+\.[^@\s]+$");
// Extract groups
var match = Regex.Match(email, @"(.*)@(.*)");
Console.WriteLine($"User: {match.Groups[1].Value}");

// Top-level statements provide the 'args' array implicitly
foreach (var arg in args)
{
    if (arg == "--verbose") Console.WriteLine("Verbose mode enabled");
}

// Single-expression method
int Add(int a, int b) => a + b;

// Method with Tuple return (multi-value)
(int sum, int diff) Calculate(int a, int b) => (a + b, a - b);

var (s, d) = Calculate(10, 5);

// ref: pass by reference (must be initialized before passing)
void Increment(ref int x) => x++;

// out: must be assigned within the method
void GetValues(out int x) => x = 42;

// in: pass by reference (read-only for performance)
void Print(in int x) => Console.WriteLine(x);

// unsafe: raw pointers (requires <AllowUnsafeBlocks>true</AllowUnsafeBlocks> in .csproj)
unsafe {
    int val = 10;
    int* p = &val;
    *p = 20; // Modify original via pointer
}

for (int i = 0; i < 3; i++) Console.WriteLine($"for: {i}");

int j = 0;
while (j < 3) { Console.WriteLine($"while: {j}"); j++; }

for (int i = 0; i < 10; i++)
{
    if (i < 2) continue; // Skip iteration
    if (i == 5) break;    // Exit loop
    Console.WriteLine(i);
}

var list = new List<int> { 10, 20 };
foreach (var item in list)
{
    Console.WriteLine(item);
}

string s = " Hello World ";
s = s.Trim().Replace(" ", "_"); // "Hello_World"
string sub = s.Substring(0, 5); // "Hello"
string joined = string.Join(",", new[] { "a", "b" }); // "a,b"

using System.IO;

// Join parts into a path using platform-specific separators
string path = Path.Combine("usr", "local", "bin"); 

using System.IO;

using var fs = new FileStream("data.bin", FileMode.OpenOrCreate);
byte[] buffer = new byte[4096];
int bytesRead;
while ((bytesRead = fs.Read(buffer, 0, buffer.Length)) > 0)
{
    // Process bytesRead from buffer
}

using System.IO;

// Read all bytes or text at once
byte[] bytes = File.ReadAllBytes("file.bin");
string text = File.ReadAllText("file.txt");

using System;
using System.Runtime.InteropServices;

Span<byte> buffer = stackalloc byte[12]; // Stack allocation for efficiency
int val = 42;
// Pack integer as Big-Endian bytes
MemoryMarshal.Write(buffer[..4], ref val);
if (BitConverter.IsLittleEndian) buffer[..4].Reverse();

int a = 0b1010 & 0b1100; // AND (0b1000)
int b = 0b1010 | 0b1100; // OR  (0b1110)
int c = 0b1010 ^ 0b1100; // XOR (0b0110)
int d = ~0b1010; // NOT
int e = 0b0001 << 2; // Shift Left (0b0100)

string hex = 255.ToString("X2"); // Format as uppercase hex: "FF"
int val = Convert.ToInt32("FF", 16); // Parse hex to integer: 255

using System.Collections.Generic;

var list = new List<int> { 1, 2 }; list.Add(3); // List
var dict = new Dictionary<string, int> { ["k"] = 1 }; // Key-Value Map
var set = new HashSet<int> { 1, 2, 2 }; // { 1, 2 } (Unique values)

using System.Text.Json;

var json = "{\"Id\":42}";
using var doc = JsonDocument.Parse(json);
int id = doc.RootElement.GetProperty("Id").GetInt32();
Console.WriteLine($"ID: {id}");

using System.Net.Sockets;
using System.Text;

using var client = new TcpClient("example.com", 80);
using var stream = client.GetStream();
byte[] msg = Encoding.UTF8.GetBytes("GET / HTTP/1.1\r\n\r\n");
stream.Write(msg, 0, msg.Length);

using System.Net.Sockets;
using System.Text;

using var client = new UdpClient();
byte[] msg = Encoding.UTF8.GetBytes("ping");
client.Send(msg, msg.Length, "127.0.0.1", 8080);

using System.Threading.Tasks;

async Task<int> DoWorkAsync()
{
    await Task.Delay(100);
    return 42;
}

int result = await DoWorkAsync();
Console.WriteLine(result);

using Microsoft.Data.Sqlite;

using var connection = new SqliteConnection("Data Source=:memory:");
connection.Open();

var command = connection.CreateCommand();
command.CommandText = "SELECT 42";
var result = command.ExecuteScalar();
Console.WriteLine(result);