dotnet-10-csharp-14

.NET 10 & C# 14 Best Practices

.NET 10 (LTS, Nov 2025) with C# 14. Covers minimal APIs, not MVC.

Official docs: .NET 10 | C# 14 | ASP.NET Core 10

Detail Files File Topics csharp-14.md Extension blocks, field keyword, null-conditional assignment minimal-apis.md Validation, TypedResults, filters, modular monolith, vertical slices security.md JWT auth, CORS, rate limiting, OpenAPI security, middleware order infrastructure.md Options, resilience, channels, health checks, caching, Serilog, EF Core, keyed services testing.md WebApplicationFactory, integration tests, auth testing anti-patterns.md HttpClient, DI captive, blocking async, N+1 queries libraries.md MediatR, FluentValidation, Mapster, ErrorOr, Polly, Aspire Quick Start net10.0 14 enable

var builder = WebApplication.CreateBuilder(args);

// Core services builder.Services.AddValidation(); builder.Services.AddProblemDetails(); builder.Services.AddOpenApi();

// Security builder.Services.AddAuthentication().AddJwtBearer(); builder.Services.AddAuthorization(); builder.Services.AddRateLimiter(opts => { / see security.md / });

// Infrastructure builder.Services.AddHealthChecks(); builder.Services.AddOutputCache();

// Modules builder.Services.AddUsersModule();

var app = builder.Build();

// Middleware (ORDER MATTERS - see security.md) app.UseExceptionHandler(); app.UseHttpsRedirection(); app.UseCors(); app.UseRateLimiter(); app.UseAuthentication(); app.UseAuthorization(); app.UseOutputCache();

app.MapOpenApi(); app.MapHealthChecks("/health"); app.MapUsersEndpoints(); app.Run();

Decision Flowcharts Result vs Exception digraph { "Error type?" [shape=diamond]; "Expected?" [shape=diamond]; "Result/ErrorOr" [shape=box]; "Exception" [shape=box]; "Error type?" -> "Expected?" [label="domain"]; "Error type?" -> "Exception" [label="infrastructure"]; "Expected?" -> "Result/ErrorOr" [label="yes"]; "Expected?" -> "Exception" [label="no"]; }

IOptions Selection digraph { "Runtime changes?" [shape=diamond]; "Per-request?" [shape=diamond]; "IOptions" [shape=box]; "IOptionsSnapshot" [shape=box]; "IOptionsMonitor" [shape=box]; "Runtime changes?" -> "IOptions" [label="no"]; "Runtime changes?" -> "Per-request?" [label="yes"]; "Per-request?" -> "IOptionsSnapshot" [label="yes"]; "Per-request?" -> "IOptionsMonitor" [label="no"]; }

Channel Type digraph { "Trust producer?" [shape=diamond]; "Can drop?" [shape=diamond]; "Bounded+Wait" [shape=box,style=filled,fillcolor=lightgreen]; "Bounded+Drop" [shape=box]; "Unbounded" [shape=box]; "Trust producer?" -> "Unbounded" [label="yes"]; "Trust producer?" -> "Can drop?" [label="no"]; "Can drop?" -> "Bounded+Drop" [label="yes"]; "Can drop?" -> "Bounded+Wait" [label="no"]; }

Key Patterns Summary C# 14 Extension Blocks extension(IEnumerable source) { public bool IsEmpty => !source.Any(); }

.NET 10 Built-in Validation builder.Services.AddValidation(); app.MapPost("/users", (UserDto dto) => TypedResults.Ok(dto));

TypedResults (Always Use) app.MapGet("/users/{id}", async (int id, IUserService svc) => await svc.GetAsync(id) is { } user ? TypedResults.Ok(user) : TypedResults.NotFound());

Module Pattern public static class UsersModule { public static IServiceCollection AddUsersModule(this IServiceCollection s) => s .AddScoped();

public static IEndpointRouteBuilder MapUsersEndpoints(this IEndpointRouteBuilder app)
{
    var g = app.MapGroup("/api/users").WithTags("Users");
    g.MapGet("/{id}", GetUser.Handle);
    return app;
}

}

HTTP Resilience builder.Services.AddHttpClient() .AddStandardResilienceHandler();

Error Handling (RFC 9457) builder.Services.AddProblemDetails(); app.UseExceptionHandler(); app.UseStatusCodePages();

MANDATORY Patterns (Always Use These) Task ✅ ALWAYS Use ❌ NEVER Use Extension members C# 14 extension() blocks Traditional this extension methods Property validation C# 14 field keyword Manual backing fields Null assignment obj?.Prop = value if (obj != null) obj.Prop = value API returns TypedResults.Ok() Results.Ok() Options validation .ValidateOnStart() Missing validation HTTP resilience AddStandardResilienceHandler() Manual Polly configuration Timestamps DateTime.UtcNow DateTime.Now Quick Reference Card ┌─────────────────────────────────────────────────────────────────┐ │ .NET 10 / C# 14 PATTERNS │ ├─────────────────────────────────────────────────────────────────┤ │ EXTENSION PROPERTY: extension(IEnumerable s) { │ │ public bool IsEmpty => !s.Any(); │ │ } │ ├─────────────────────────────────────────────────────────────────┤ │ FIELD KEYWORD: public string Name { │ │ get => field; │ │ set => field = value?.Trim(); │ │ } │ ├─────────────────────────────────────────────────────────────────┤ │ OPTIONS VALIDATION: .BindConfiguration(Section) │ │ .ValidateDataAnnotations() │ │ .ValidateOnStart(); // CRITICAL! │ ├─────────────────────────────────────────────────────────────────┤ │ HTTP RESILIENCE: .AddStandardResilienceHandler(); │ ├─────────────────────────────────────────────────────────────────┤ │ TYPED RESULTS: TypedResults.Ok(data) │ │ TypedResults.NotFound() │ │ TypedResults.Created(uri, data) │ ├─────────────────────────────────────────────────────────────────┤ │ ERROR PATTERN: ErrorOr or user?.Match(...) │ ├─────────────────────────────────────────────────────────────────┤ │ IOPTIONS: IOptions → startup, no reload │ │ IOptionsSnapshot → per-request reload │ │ IOptionsMonitor → live + OnChange() │ └─────────────────────────────────────────────────────────────────┘

Anti-Patterns Quick Reference Anti-Pattern Fix new HttpClient() Inject HttpClient or IHttpClientFactory Results.Ok() TypedResults.Ok() Manual Polly config AddStandardResilienceHandler() Singleton → Scoped Use IServiceScopeFactory GetAsync().Result await GetAsync() Exceptions for flow Use ErrorOr Result pattern DateTime.Now DateTime.UtcNow Missing .ValidateOnStart() Always add to Options registration

See anti-patterns.md for complete list.

Libraries Quick Reference Library Package Purpose MediatR MediatR CQRS FluentValidation FluentValidation.DependencyInjectionExtensions Validation Mapster Mapster.DependencyInjection Mapping ErrorOr ErrorOr Result pattern Polly Microsoft.Extensions.Http.Resilience Resilience Serilog Serilog.AspNetCore Logging

See libraries.md for usage examples.