Agent skill

dotnet-10-csharp-14

Use when building .NET 10 or C# 14 applications; when using minimal APIs, modular monolith patterns, or feature folders; when implementing HTTP resilience, Options pattern, Channels, or validation; when seeing outdated patterns like old extension method syntax

View SKILL.md on GitHub Repository
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Install this agent skill to your Project

npx add-skill https://github.com/majiayu000/claude-skill-registry/tree/main/skills/data/dotnet-10-csharp-14

SKILL.md

.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

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
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.

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