.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
general.md	Local development workflow, migrations, coding philosophy, general rules
csharp.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	Mediator, FluentValidation, ErrorOr, Polly

Quick Start

xml

<Project Sdk="Microsoft.NET.Sdk.Web">
  <PropertyGroup>
    <TargetFramework>net10.0</TargetFramework>
    <LangVersion>14</LangVersion>
    <Nullable>enable</Nullable>
  </PropertyGroup>
</Project>

csharp

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

dot

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

IOptions Selection

dot

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

Channel Type

dot

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

csharp

extension<T>(IEnumerable<T> source)
{
    public bool IsEmpty => !source.Any();
}

.NET 10 Built-in Validation

csharp

builder.Services.AddValidation();
app.MapPost("/users", (UserDto dto) => TypedResults.Ok(dto));

TypedResults (Always Use)

csharp

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

Module Pattern

csharp

public static class UsersModule
{
    public static IServiceCollection AddUsersModule(this IServiceCollection s) => s
        .AddScoped<IUserService, UserService>();

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

HTTP Resilience

csharp

builder.Services.AddHttpClient<IApi, ApiClient>()
    .AddStandardResilienceHandler();

Error Handling (RFC 9457)

csharp

builder.Services.AddProblemDetails();
app.UseExceptionHandler();
app.UseStatusCodePages();

MANDATORY Patterns (Always Use These)

Task	✅ ALWAYS Use	❌ NEVER Use
Extension members	C# 14 `extension<T>()` 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<T>(IEnumerable<T> 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<User> or user?.Match(...)          │
├─────────────────────────────────────────────────────────────────┤
│ IOPTIONS:            IOptions<T>        → startup, no reload    │
│                      IOptionsSnapshot<T> → per-request reload   │
│                      IOptionsMonitor<T>  → 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<T>` 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
FluentValidation	`FluentValidation.DependencyInjectionExtensions`	Validation
ErrorOr	`ErrorOr`	Result pattern
Polly	`Microsoft.Extensions.Http.Resilience`	Resilience
Serilog	`Serilog.AspNetCore`	Logging

See libraries.md for usage examples.

Code Philosophy Quick Reference

Don't favor backwards compatibility, favor:

Features: Build new functionality and improve the product
Proper cleanup: Remove deprecated code, simplify architecture, eliminate technical debt
Fixing root causes: When something breaks, fix the root

General Rules

Read before edit: Always read existing code before proposing changes
Preserve patterns: Follow existing code style and conventions
Don't over-engineer: Keep changes minimal and focused
Never push: Never push to remote

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NPX Install

Tags

SKILL.md Content

.NET 10 & C# 14 Best Practices

Detail Files

Quick Start

Decision Flowcharts

Result vs Exception

IOptions Selection

Channel Type

Key Patterns Summary

C# 14 Extension Blocks

.NET 10 Built-in Validation

TypedResults (Always Use)

Module Pattern

HTTP Resilience

Error Handling (RFC 9457)

MANDATORY Patterns (Always Use These)

Quick Reference Card

Anti-Patterns Quick Reference

Libraries Quick Reference

Code Philosophy Quick Reference

General Rules