AGIBuild/middleware-patterns

ASP.NET Core Middleware Patterns

Rationale

Middleware is the backbone of ASP.NET Core request processing. Properly designed middleware enables cross-cutting concerns like logging, authentication, and caching. Understanding the pipeline order and middleware patterns is critical for building robust applications.

---

Pipeline Ordering

Middleware executes in the order it is registered. The order is critical -- placing middleware in the wrong position causes subtle bugs.

Recommended Order

var app = builder.Build();

// 1. Exception handling (outermost -- catches everything below)
app.UseExceptionHandler("/error");

// 2. HSTS (before any response is sent)
if (!app.Environment.IsDevelopment())
{
    app.UseHsts();
}

// 3. HTTPS redirection
app.UseHttpsRedirection();

// 4. Static files (short-circuits for static content before routing)
app.UseStaticFiles();

// 5. Routing (matches endpoints but does not execute them yet)
app.UseRouting();

// 6. CORS (must be after routing, before auth)
app.UseCors();

// 7. Authentication (identifies the user)
app.UseAuthentication();

// 8. Authorization (checks permissions against the matched endpoint)
app.UseAuthorization();

// 9. Custom middleware (runs after auth, before endpoint execution)
app.UseRequestLogging();

// 10. Endpoint execution (terminal -- executes the matched endpoint)
app.MapControllers();
app.MapRazorPages();

Why Order Matters

| Mistake | Consequence | |---------|-------------| | UseAuthorization() before UseRouting() | Authorization has no endpoint metadata -- all requests pass | | UseCors() after UseAuthorization() | Preflight requests fail because they lack auth tokens | | UseExceptionHandler() after custom middleware | Exceptions in custom middleware are unhandled | | UseStaticFiles() after UseAuthorization() | Static files require authentication unnecessarily |

---

Pattern 1: Convention-Based Middleware

Convention-based middleware uses a constructor with RequestDelegate and an InvokeAsync method.

public sealed 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 stopwatch = Stopwatch.StartNew();

        try
        {
            await _next(context);
        }
        finally
        {
            stopwatch.Stop();
            _logger.LogInformation(
                "Request {Method} {Path} completed in {ElapsedMs}ms with status {StatusCode}",
                context.Request.Method,
                context.Request.Path,
                stopwatch.ElapsedMilliseconds,
                context.Response.StatusCode);
        }
    }
}

public static class RequestTimingMiddlewareExtensions
{
    public static IApplicationBuilder UseRequestTiming(this IApplicationBuilder app)
        => app.UseMiddleware<RequestTimingMiddleware>();
}

// Usage in Program.cs
app.UseRequestTiming();

---

Pattern 2: Factory-Based (IMiddleware)

For middleware that requires scoped services, implement IMiddleware. This uses DI to create middleware instances per-request:

public sealed class TenantMiddleware : IMiddleware
{
    private readonly TenantDbContext _db;

    public TenantMiddleware(TenantDbContext db)
    {
        _db = db;
    }

    public async Task InvokeAsync(HttpContext context, RequestDelegate next)
    {
        var tenantId = context.Request.Headers["X-Tenant-Id"].FirstOrDefault();

        if (tenantId is not null)
        {
            var tenant = await _db.Tenants.FindAsync(tenantId);
            context.Items["Tenant"] = tenant;
        }

        await next(context);
    }
}

// IMiddleware requires explicit DI registration
builder.Services.AddScoped<TenantMiddleware>();
app.UseMiddleware<TenantMiddleware>();

Convention-Based vs IMiddleware

| Aspect | Convention-based | IMiddleware | |--------|-----------------|---------------| | Lifetime | Singleton (created once) | Per-request (from DI) | | Scoped services | Via InvokeAsync parameters only | Via constructor injection | | Registration | UseMiddleware<T>() only | Requires services.Add*<T>() + UseMiddleware<T>() | | Performance | Slightly faster | Resolved from DI each request |

---

Pattern 3: Inline Middleware

For simple, one-off logic:

app.Use -- Pass-Through

app.Use(async (context, next) =>
{
    context.Response.Headers["X-Request-Id"] = context.TraceIdentifier;
    await next(context);
});

app.Run -- Terminal

app.Run(async context =>
{
    await context.Response.WriteAsync("Fallback response");
});

app.Map -- Branch by Path

app.Map("/api/diagnostics", diagnosticApp =>
{
    diagnosticApp.Run(async context =>
    {
        var data = new
        {
            MachineName = Environment.MachineName,
            Timestamp = DateTimeOffset.UtcNow
        };
        await context.Response.WriteAsJsonAsync(data);
    });
});

---

Pattern 4: Short-Circuit Logic

Middleware can short-circuit the pipeline by not calling next().

Request Validation

public sealed class ApiKeyMiddleware
{
    private readonly RequestDelegate _next;
    private readonly string _expectedKey;

    public ApiKeyMiddleware(RequestDelegate next, IConfiguration config)
    {
        _next = next;
        _expectedKey = config["ApiKey"]
            ?? throw new InvalidOperationException("ApiKey configuration is required");
    }

    public async Task InvokeAsync(HttpContext context)
    {
        if (!context.Request.Headers.TryGetValue("X-Api-Key", out var providedKey)
            || !string.Equals(providedKey, _expectedKey, StringComparison.Ordinal))
        {
            context.Response.StatusCode = StatusCodes.Status401Unauthorized;
            await context.Response.WriteAsJsonAsync(new
            {
                Error = "Invalid or missing API key"
            });
            return; // Short-circuit
        }

        await _next(context);
    }
}

Feature Flag Gate

app.UseWhen(
    context => context.Request.Path.StartsWithSegments("/beta"),
    betaApp =>
    {
        betaApp.Use(async (context, next) =>
        {
            var featureManager = context.RequestServices
                .GetRequiredService<IFeatureManager>();

            if (!await featureManager.IsEnabledAsync("BetaFeatures"))
            {
                context.Response.StatusCode = StatusCodes.Status404NotFound;
                return;
            }

            await next(context);
        });
    });

---

Pattern 5: Request and Response Manipulation

Reading the Request Body

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

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

    public async Task InvokeAsync(HttpContext context)
    {
        context.Request.EnableBuffering();

        if (context.Request.ContentLength > 0 && context.Request.ContentLength < 64_000)
        {
            context.Request.Body.Position = 0;
            using var reader = new StreamReader(context.Request.Body, leaveOpen: true);
            var body = await reader.ReadToEndAsync();
            _logger.LogDebug("Request body for {Path}: {Body}", context.Request.Path, body);
            context.Request.Body.Position = 0;
        }

        await _next(context);
    }
}

Modifying the Response

public async Task InvokeAsync(HttpContext context)
{
    var originalBodyStream = context.Response.Body;

    using var responseBody = new MemoryStream();
    context.Response.Body = responseBody;

    await _next(context);

    context.Response.Body.Seek(0, SeekOrigin.Begin);
    var responseText = await new StreamReader(context.Response.Body).ReadToEndAsync();
    context.Response.Body.Seek(0, SeekOrigin.Begin);

    await responseBody.CopyToAsync(originalBodyStream);
}

Caution: Response body replacement adds memory overhead. Use only for diagnostics.

---

Pattern 6: Exception Handling Middleware

Built-in Exception Handler

app.UseExceptionHandler(exceptionApp =>
{
    exceptionApp.Run(async context =>
    {
        context.Response.StatusCode = StatusCodes.Status500InternalServerError;
        context.Response.ContentType = "application/json";

        var exceptionFeature = context.Features.Get<IExceptionHandlerFeature>();

        var logger = context.RequestServices.GetRequiredService<ILogger<Program>>();
        logger.LogError(exceptionFeature?.Error, "Unhandled exception for {Path}", context.Request.Path);

        await context.Response.WriteAsJsonAsync(new
        {
            Error = "An internal error occurred",
            TraceId = context.TraceIdentifier
        });
    });
});

IExceptionHandler (.NET 8+)

Multiple handlers can be registered and are invoked in order:

public sealed class ValidationExceptionHandler : IExceptionHandler
{
    public async ValueTask<bool> TryHandleAsync(
        HttpContext context,
        Exception exception,
        CancellationToken ct)
    {
        if (exception is not ValidationException validationException)
            return false;

        context.Response.StatusCode = StatusCodes.Status400BadRequest;
        await context.Response.WriteAsJsonAsync(new
        {
            Error = "Validation failed",
            Details = validationException.Errors
        }, ct);

        return true;
    }
}

public sealed class GlobalExceptionHandler(ILogger<GlobalExceptionHandler> logger) : IExceptionHandler
{
    public async ValueTask<bool> TryHandleAsync(
        HttpContext context,
        Exception exception,
        CancellationToken ct)
    {
        logger.LogError(exception, "Unhandled exception");

        context.Response.StatusCode = StatusCodes.Status500InternalServerError;
        await context.Response.WriteAsJsonAsync(new
        {
            Error = "An internal error occurred",
            TraceId = context.TraceIdentifier
        }, ct);

        return true;
    }
}

builder.Services.AddExceptionHandler<ValidationExceptionHandler>();
builder.Services.AddExceptionHandler<GlobalExceptionHandler>();
builder.Services.AddProblemDetails();

app.UseExceptionHandler();

StatusCodePages for Non-Exception Errors

app.UseStatusCodePagesWithReExecute("/error/{0}");

app.UseStatusCodePages(async context =>
{
    context.HttpContext.Response.ContentType = "application/json";
    await context.HttpContext.Response.WriteAsJsonAsync(new
    {
        Error = $"HTTP {context.HttpContext.Response.StatusCode}",
        TraceId = context.HttpContext.TraceIdentifier
    });
});

---

Pattern 7: Conditional Middleware

UseWhen -- Conditional Branch (Rejoins Pipeline)

app.UseWhen(
    context => context.Request.Path.StartsWithSegments("/api"),
    apiApp =>
    {
        apiApp.UseRateLimiter();
    });

MapWhen -- Conditional Branch (Does Not Rejoin)

app.MapWhen(
    context => context.WebSockets.IsWebSocketRequest,
    wsApp =>
    {
        wsApp.Run(async context =>
        {
            using var ws = await context.WebSockets.AcceptWebSocketAsync();
        });
    });

Environment-Specific Middleware

if (app.Environment.IsDevelopment())
{
    app.UseDeveloperExceptionPage();
    app.UseSwagger();
    app.UseSwaggerUI();
}
else
{
    app.UseExceptionHandler("/error");
    app.UseHsts();
}

---

Pattern 8: Branching Middleware

Create completely separate pipelines for different route prefixes:

app.Map("/api", apiApp =>
{
    apiApp.UseExceptionHandler("/api/error");
    apiApp.UseHttpsRedirection();
    apiApp.UseAuthentication();
    apiApp.UseAuthorization();
    apiApp.UseRateLimiter();
    apiApp.MapControllers();
});

app.Map("/webhooks", webhookApp =>
{
    webhookApp.UseMiddleware<WebhookSignatureValidation>();
    webhookApp.UseMiddleware<WebhookIdempotency>();
    webhookApp.MapRazorPages();
});

app.UseExceptionHandler("/Error");
app.UseHttpsRedirection();
app.UseStaticFiles();
app.UseRouting();
app.UseAuthentication();
app.UseAuthorization();
app.MapRazorPages();

---

Pattern 9: Middleware with Options

public class RateLimitingMiddlewareOptions
{
    public int MaxRequestsPerSecond { get; set; } = 10;
    public int BurstSize { get; set; } = 20;
    public TimeSpan BlockDuration { get; set; } = TimeSpan.FromMinutes(1);
}

public class RateLimitingMiddleware(
    RequestDelegate next,
    IOptions<RateLimitingMiddlewareOptions> options,
    IMemoryCache cache)
{
    private readonly RateLimitingMiddlewareOptions _options = options.Value;

    public async Task Invoke(HttpContext context)
    {
        var clientId = GetClientIdentifier(context);
        var cacheKey = $"ratelimit:{clientId}";

        if (!await TryAcquireTokenAsync(cacheKey))
        {
            context.Response.StatusCode = StatusCodes.Status429TooManyRequests;
            context.Response.Headers.RetryAfter = _options.BlockDuration.TotalSeconds.ToString();
            await context.Response.WriteAsync("Rate limit exceeded");
            return;
        }

        await next(context);
    }

    private string GetClientIdentifier(HttpContext context)
    {
        return context.User.Identity?.Name ??
               context.Connection.RemoteIpAddress?.ToString() ??
               "anonymous";
    }

    private async Task<bool> TryAcquireTokenAsync(string cacheKey) => true;
}

builder.Services.Configure<RateLimitingMiddlewareOptions>(options =>
{
    options.MaxRequestsPerSecond = 5;
    options.BurstSize = 10;
});

app.UseMiddleware<RateLimitingMiddleware>();

---

Pattern 10: Middleware Testing

public class MiddlewareTests
{
    [Fact]
    public async Task SecurityHeadersMiddleware_AddsRequiredHeaders()
    {
        var middleware = new SecurityHeadersMiddleware(async (context) =>
        {
            await Task.CompletedTask;
        });

        var context = new DefaultHttpContext();

        await middleware.Invoke(context);

        Assert.Equal("nosniff", context.Response.Headers["X-Content-Type-Options"].ToString());
        Assert.Equal("DENY", context.Response.Headers["X-Frame-Options"].ToString());
    }

    [Fact]
    public async Task ApiKeyMiddleware_Returns401_WhenKeyMissing()
    {
        var config = new ConfigurationBuilder()
            .AddInMemoryCollection(new[] { new KeyValuePair<string, string?>("ApiKey", "test-key") })
            .Build();

        var middleware = new ApiKeyMiddleware(async (context) =>
        {
            await Task.CompletedTask;
        }, config);

        var context = new DefaultHttpContext();
        context.Response.Body = new MemoryStream();

        await middleware.Invoke(context);

        Assert.Equal(401, context.Response.StatusCode);
    }
}

---

Anti-Patterns

Calling Next After Response Started

// BAD: Calling next after response has started
public async Task Invoke(HttpContext context)
{
    await context.Response.WriteAsync("Before");
    await next(context); // May fail
    await context.Response.WriteAsync("After"); // Won't work
}

// GOOD: Only modify response before calling next
public async Task Invoke(HttpContext context)
{
    var originalBody = context.Response.Body;
    context.Response.Body = new MemoryStream();

    await next(context);

    context.Response.Body.Position = 0;
    await context.Response.Body.CopyToAsync(originalBody);
}

Not Restoring Context

// BAD: Not restoring HttpContext state
public async Task Invoke(HttpContext context)
{
    var originalUser = context.User;
    context.User = new ClaimsPrincipal();
    await next(context);
    // Missing: context.User = originalUser;
}

// GOOD: Always restore state
public async Task Invoke(HttpContext context)
{
    var originalUser = context.User;
    try
    {
        context.User = new ClaimsPrincipal();
        await next(context);
    }
    finally
    {
        context.User = originalUser;
    }
}

---

Key Principles

• Order is everything -- middleware executes top-to-bottom for requests and bottom-to-top for responses
• Exception handler goes first -- UseExceptionHandler must be outermost
• Prefer classes over inline for reusable middleware -- testable, composable, single-responsibility
• Use IMiddleware for scoped dependencies -- convention-based is singleton
• Short-circuit intentionally -- always document why a middleware does not call next()
• Avoid response body manipulation in hot paths -- doubles memory usage per request

---

Agent Gotchas

1. Do not place UseAuthorization() before UseRouting() -- authorization requires endpoint metadata.
2. Do not place UseCors() after UseAuthorization() -- CORS preflight requests lack auth tokens.
3. Do not forget to call next() in pass-through middleware -- silently short-circuits the pipeline.
4. Do not read Request.Body without EnableBuffering() -- the body is forward-only by default.
5. Do not register IMiddleware without DI registration -- requires explicit services.AddScoped<T>().
6. Do not write to Response.Body after calling next() if downstream has started response -- check context.Response.HasStarted.

---

References

• ASP.NET Core middleware
• Write custom ASP.NET Core middleware
• Factory-based middleware activation
• Handle errors in ASP.NET Core
• IExceptionHandler in .NET 8
• Exploring ASP.NET Core (Andrew Lock)