int number = 10; string text = "Hello"; var auto = "Type inferred"; const double PI = 3.14159; double? nullable = null;

var name = "World"; var msg = $"Hello, {name}!"; var raw = @"C:\path\to\file";

List<int> list = new() { 1, 2, 3 }; Dictionary<string, int> dict = new(); int[] array = { 1, 2, 3, 4, 5 }; HashSet<string> set = new();

// Query Syntax var result = from item in items where item.Price > 10 orderby item.Name select item; // Method Syntax var result = items .Where(x => x.Price > 10) .OrderBy(x => x.Name) .Select(x => x); // Common Methods .First() / .FirstOrDefault() .Single() / .SingleOrDefault() .Any() / .All() / .Count() .Skip(10).Take(5) .GroupBy(x => x.Category) .Join() / .GroupJoin()

public async Task<string> GetDataAsync() { var data = await client.GetAsync(url); return await data.Content .ReadAsStringAsync(); } // Multiple awaits var task1 = GetData1Async(); var task2 = GetData2Async(); await Task.WhenAll(task1, task2); // Cancellation async Task DoWork( CancellationToken token) { await Task.Delay(1000, token); }

services.AddSingleton<IService, Service>(); services.AddScoped<IRepository, Repo>(); services.AddTransient<IHelper, Helper>();

public class MyController { private readonly IService _service; public MyController(IService service) { _service = service; } }

public class AppDbContext : DbContext { public DbSet<User> Users { get; set; } protected override void OnModelCreating( ModelBuilder builder) { builder.Entity<User>() .HasKey(u => u.Id); } }

// Create context.Users.Add(user); await context.SaveChangesAsync(); // Read var user = await context.Users .FirstOrDefaultAsync(u => u.Id == id); // Update user.Name = "New Name"; await context.SaveChangesAsync(); // Delete context.Users.Remove(user); await context.SaveChangesAsync();

[ApiController] [Route("api/[controller]")] public class UsersController : ControllerBase { [HttpGet] public ActionResult<IEnumerable<User>> GetAll() => Ok(users); [HttpGet("{id}")] public ActionResult<User> Get(int id) { var user = users.Find(id); return user == null ? NotFound() : Ok(user); } [HttpPost] public ActionResult<User> Create(User user) { users.Add(user); return CreatedAtAction( nameof(Get), new { id = user.Id }, user); } [HttpPut("{id}")] public IActionResult Update( int id, User user) => NoContent(); [HttpDelete("{id}")] public IActionResult Delete(int id) => NoContent(); }

app.UseRouting(); app.UseAuthentication(); app.UseAuthorization(); app.UseCors(); app.UseEndpoints(endpoints => { endpoints.MapControllers(); });

public class CustomMiddleware { private readonly RequestDelegate _next; public CustomMiddleware( RequestDelegate next) { _next = next; } public async Task InvokeAsync( HttpContext context) { // Before await _next(context); // After } }

{ "ConnectionStrings": { "DefaultConnection": "Server=..." }, "AppSettings": { "ApiKey": "your-key" } }

// Inject IConfiguration private readonly IConfiguration _config; // Get value var key = _config["AppSettings:ApiKey"]; var conn = _config.GetConnectionString( "DefaultConnection"); // Bind to object var settings = new AppSettings(); _config.GetSection("AppSettings") .Bind(settings);

try { // Code that might throw } catch (SpecificException ex) { // Handle specific _logger.LogError(ex, "Error"); } catch (Exception ex) { // Handle general throw; // Re-throw } finally { // Cleanup } // Global Exception Handler app.UseExceptionHandler("/error"); // Custom Exception Filter [ApiController] [ExceptionFilter] public class MyController : ControllerBase { }

public class MyService { private readonly ILogger<MyService> _log; public MyService( ILogger<MyService> logger) { _log = logger; } public void DoWork() { _log.LogInformation("Info"); _log.LogWarning("Warning"); _log.LogError("Error"); _log.LogDebug("Debug"); _log.LogCritical("Critical"); } }

services.AddAuthentication( JwtBearerDefaults.AuthenticationScheme) .AddJwtBearer(options => { options.TokenValidationParameters = new TokenValidationParameters { ValidateIssuer = true, ValidateAudience = true, ValidateLifetime = true, ValidIssuer = "issuer", ValidAudience = "audience", IssuerSigningKey = key }; });

[Authorize] public class SecureController : ControllerBase { [AllowAnonymous] public IActionResult Public() { } [Authorize(Roles = "Admin")] public IActionResult Admin() { } }

public class CalculatorTests { [Fact] public void Add_TwoNumbers_ReturnsSum() { // Arrange var calc = new Calculator(); // Act var result = calc.Add(2, 3); // Assert Assert.Equal(5, result); } [Theory] [InlineData(1, 1, 2)] [InlineData(2, 3, 5)] public void Add_Multiple_Works( int a, int b, int expected) { var result = calc.Add(a, b); Assert.Equal(expected, result); } }

public interface IRepository<T> { Task<T> GetByIdAsync(int id); Task<IEnumerable<T>> GetAllAsync(); Task AddAsync(T entity); Task UpdateAsync(T entity); Task DeleteAsync(int id); } public class Repository<T> : IRepository<T> where T : class { private readonly DbContext _context; private readonly DbSet<T> _dbSet; }

public interface IUnitOfWork { IRepository<User> Users { get; } Task<int> CompleteAsync(); }

services.AddResponseCaching(); app.UseResponseCaching(); [ResponseCache(Duration = 60)] public IActionResult Get() { }

services.AddMemoryCache(); _cache.TryGetValue(key, out var data); _cache.Set(key, data, TimeSpan.FromMinutes(10));

public record Person( string Name, int Age); var person = new Person("John", 30); var older = person with { Age = 31 };

var result = value switch { null => "Null", 0 => "Zero", > 0 and < 10 => "Single digit", _ => "Other" };

string name = user?.Name ?? "Guest"; name ??= "Default"; var first = list?[0];

var arr = new[] { 1, 2, 3, 4, 5 }; var last = arr[^1]; // Last element var range = arr[1..4]; // Middle slice