Csharp Dotnet

dotnet new sln -n Shop                         # solution
dotnet new webapi -n Shop.Api                  # minimal API (default in .NET 10)
dotnet new webapi -n Shop.Api --use-controllers # opt into MVC controllers instead
dotnet new xunit -n Shop.Api.Tests             # test project
dotnet sln add Shop.Api Shop.Api.Tests         # wire into the solution
dotnet add Shop.Api package Npgsql.EntityFrameworkCore.PostgreSQL
dotnet ef migrations add Initial -p Shop.Api   # create a migration
dotnet ef database update -p Shop.Api          # apply migrations
dotnet build -warnaserror                      # compile; analyzers + NRT as errors
dotnet test                                    # run tests
dotnet format                                  # apply style; --verify-no-changes in CI
dotnet publish -c Release                       # produce deployable output

Shop.sln
Directory.Packages.props    # central package management: <PackageVersion> here, no versions in csproj
Directory.Build.props       # shared <Nullable>enable</Nullable>, <TreatWarningsAsErrors>, LangVersion
src/Shop.Api/               # endpoints grouped by feature module (Products/, Orders/)
src/Shop.Domain/            # entities, value objects (no EF/ASP.NET dependency)
tests/Shop.Api.Tests/

// Bad: mutable class, no value equality, hand-written boilerplate
public class ProductDto { public int Id { get; set; } public string Name { get; set; } }

// Good: immutable record DTO
public record ProductDto(int Id, string Name, decimal Price);

// Bad
if (shape is Circle) { var c = (Circle)shape; return Math.PI * c.R * c.R; }

// Good
return shape switch
{
    Circle c      => Math.PI * c.R * c.R,
    Rectangle r   => r.W * r.H,
    _             => throw new ArgumentOutOfRangeException(nameof(shape)),
};

// Bad: explicit backing field just to trim a string
private string _name = "";
public string Name { get => _name; set => _name = value.Trim(); }

// Good (C# 14)
public string Name { get; set => field = value.Trim(); }

public class OrderService(IOrderRepository repo, ILogger<OrderService> log)  // primary ctor: DI in one line
{
    public required string Region { get; init; }                            // must be set at construction, then immutable
    private static readonly int[] DefaultTiers = [1, 2, 3];                  // collection expression
}

// Bad: blocks the thread on an async call -> thread-pool starvation / deadlock under load
public IActionResult Get() => Ok(_svc.LoadAsync().Result);

// Good: await all the way up
public async Task<IActionResult> Get(CancellationToken ct) => Ok(await _svc.LoadAsync(ct));

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddOpenApi();                 // OpenAPI 3.1, JSON Schema 2020-12
builder.Services.AddProblemDetails();          // RFC 9457 error bodies
builder.Services.AddDbContext<ShopDb>(o => o.UseNpgsql(builder.Configuration.GetConnectionString("Shop")));

var app = builder.Build();
app.MapOpenApi();                              // serves the OpenAPI document
app.UseExceptionHandler();                     // emits ProblemDetails on unhandled errors

var products = app.MapGroup("/products").WithTags("Products");
products.MapGet("/{id:int}", async (int id, ShopDb db, CancellationToken ct) =>
    await db.Products.AsNoTracking()
        .Where(p => p.Id == id)
        .Select(p => new ProductDto(p.Id, p.Name, p.Price))   // project to DTO, no entity leak
        .FirstOrDefaultAsync(ct) is { } dto
        ? Results.Ok(dto)
        : Results.NotFound());
products.MapPost("/", async (CreateProduct req, ShopDb db, CancellationToken ct) =>
{   // DataAnnotations on CreateProduct are validated automatically in .NET 10
    var entity = new Product { Name = req.Name, Price = req.Price };
    db.Products.Add(entity);
    await db.SaveChangesAsync(ct);
    return Results.Created($"/products/{entity.Id}", new ProductDto(entity.Id, entity.Name, entity.Price));
});
app.Run();