dotnet-aspnet-core

ASP.NET Core

Trigger On

• working on ASP.NET Core apps, services, or middleware
• changing auth, routing, configuration, hosting, or deployment behavior
• deciding between ASP.NET Core sub-stacks such as Blazor, Minimal APIs, or controller APIs
• debugging request pipeline issues
• modernizing legacy ASP.NET to ASP.NET Core

Documentation

• ASP.NET Core Overview
• ASP.NET Core Middleware
• ASP.NET Core Best Practices
• Configuration in ASP.NET Core
• Authentication and Authorization

References

• patterns.md - Detailed middleware patterns, security patterns, configuration patterns, DI patterns, error handling patterns, and logging patterns
• anti-patterns.md - Common ASP.NET Core mistakes including HttpClient misuse, async anti-patterns, configuration errors, DI issues, middleware ordering problems, and security vulnerabilities

Workflow

1. Detect the real hosting shape first:

   • top-level Program.cs structure
   • middleware order and registration
   • auth model (Identity, JWT, OAuth, cookies)
   • endpoint registrations and routing

2. Follow the correct middleware order:

   ExceptionHandler → HttpsRedirection → Static Files → Routing
   → CORS → Authentication → Authorization → Rate Limiting
   → Response Caching → Custom Middleware → Endpoints
   

3. Use built-in patterns correctly:

   • Prefer IOptions<T> / IOptionsSnapshot<T> for configuration
   • Use ILogger<T> for structured logging
   • Use IHttpClientFactory for HTTP clients (never new HttpClient())
   • Use IHostedService / BackgroundService for background work

4. Route specialized work to specific skills:

   • UI and components → dotnet-blazor
   • Real-time → dotnet-signalr
   • RPC → dotnet-grpc
   • New HTTP APIs → dotnet-minimal-apis (prefer unless controllers needed)
   • Controller APIs → dotnet-web-api

5. Validate with build, tests, and targeted endpoint checks.

Middleware Patterns

Correct Order Matters

var app = builder.Build();

app.UseExceptionHandler("/error");      // 1. Catch all exceptions
app.UseHsts();                          // 2. Security headers
app.UseHttpsRedirection();              // 3. HTTPS redirect
app.UseStaticFiles();                   // 4. Serve static files
app.UseRouting();                       // 5. Route matching
app.UseCors();                          // 6. CORS policy
app.UseAuthentication();                // 7. Who are you?
app.UseAuthorization();                 // 8. Can you access?
app.UseRateLimiter();                   // 9. Rate limiting
app.UseResponseCaching();               // 10. Response cache
app.MapControllers();                   // 11. Endpoints

Custom Middleware Pattern

public class RequestTimingMiddleware
{
    private readonly RequestDelegate _next;
    private readonly ILogger<RequestTimingMiddleware> _logger;

    public RequestTimingMiddleware(RequestDelegate next, ILogger<RequestTimingMiddleware> logger)
    {
        _next = next;
        _logger = logger;
    }

    public async Task InvokeAsync(HttpContext context)
    {
        var sw = Stopwatch.StartNew();
        await _next(context);
        _logger.LogInformation("Request {Path} completed in {Elapsed}ms",
            context.Request.Path, sw.ElapsedMilliseconds);
    }
}

Configuration Patterns

Strongly-Typed Options

// appsettings.json
{
  "EmailSettings": {
    "SmtpServer": "smtp.example.com",
    "Port": 587
  }
}

// Registration
builder.Services.Configure<EmailSettings>(
    builder.Configuration.GetSection("EmailSettings"));

// Usage
public class EmailService(IOptions<EmailSettings> options)
{
    private readonly EmailSettings _settings = options.Value;
}

Environment-Based Configuration

builder.Configuration
    .AddJsonFile("appsettings.json", optional: false)
    .AddJsonFile($"appsettings.{builder.Environment.EnvironmentName}.json", optional: true)
    .AddEnvironmentVariables()
    .AddUserSecrets<Program>(optional: true);

Security Patterns

Authentication Setup

builder.Services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
    .AddJwtBearer(options =>
    {
        options.TokenValidationParameters = new TokenValidationParameters
        {
            ValidateIssuer = true,
            ValidateAudience = true,
            ValidateLifetime = true,
            ValidateIssuerSigningKey = true,
            ValidIssuer = builder.Configuration["Jwt:Issuer"],
            ValidAudience = builder.Configuration["Jwt:Audience"],
            IssuerSigningKey = new SymmetricSecurityKey(
                Encoding.UTF8.GetBytes(builder.Configuration["Jwt:Key"]!))
        };
    });

Authorization Policies

builder.Services.AddAuthorization(options =>
{
    options.AddPolicy("AdminOnly", policy =>
        policy.RequireRole("Admin"));
    options.AddPolicy("MinAge18", policy =>
        policy.RequireClaim("Age", "18", "19", "20")); // simplified
});

Anti-Patterns to Avoid

Performance Best Practices

1. Use async/await everywhere — avoid sync blocking calls
2. Pool DbContext properly — use scoped lifetime
3. Enable response compression — UseResponseCompression()
4. Use output caching — UseOutputCache() for .NET 7+
5. Profile with diagnostic tools — Visual Studio Diagnostic Tools, PerfView
6. Avoid allocations in hot paths — use Span<T>, pooling

Deliver

• production-credible ASP.NET Core code and config
• a clear request pipeline and hosting story
• verification that matches the affected endpoints and middleware
• security headers and HTTPS configured correctly

Validate

• middleware order is intentional and documented
• security and configuration changes are explicit
• endpoint behavior is covered by tests or smoke checks
• no blocking calls in async context
• secrets are not committed to source control
• health checks are implemented for production readiness