Understanding Middleware in ASP.NET Core
Middleware are components that process an HTTP request and response pipeline. Each piece of middleware can perform the following actions:
- Execute any code before the next middleware.
- Enque or deque work to be done after the next middleware has completed.
- Call the next middleware in the pipeline.
- Short-circuit the request pipeline by not calling the next middleware.
The ASP.NET Core web platform's request processing model is built around an ordered pipeline of middleware components. When a request comes in, it flows through the middleware components in order, and the response flows back through them in reverse order. This allows for a highly modular and customizable request handling process.
Key Concepts
The Request Pipeline
The request pipeline is a series of delegates, known as middleware components, that are invoked sequentially as the request is processed. Think of it as a chain reaction where each link performs a specific task.
Middleware Components
Each middleware component has access to the incoming request and the outgoing response. It can inspect, modify, or even terminate the request/response flow.
The `next` Delegate
A crucial part of middleware is the `next` delegate. This delegate represents the remainder of the request pipeline. A middleware component can choose to call `next` to pass the request further down the pipeline, or it can choose not to call `next` to short-circuit the pipeline (e.g., if it handles the request completely, like returning a specific error or static file).
Common Middleware Examples
- Static Files Middleware: Serves static files (HTML, CSS, JavaScript, images) directly.
- Authentication Middleware: Handles user authentication.
- Authorization Middleware: Checks if the authenticated user is authorized to access a resource.
- Routing Middleware: Matches the incoming request URL to an endpoint.
- Error Handling Middleware: Catches exceptions and returns appropriate error responses.
- CORS Middleware: Configures Cross-Origin Resource Sharing.
Writing Custom Middleware
You can create your own middleware to implement custom logic. A common pattern for creating middleware is using a class with a constructor that accepts a `RequestDelegate` (the `next` delegate) and an `InvokeAsync` method that contains the middleware's logic.
Here's a simplified example of a custom middleware that logs the request path:
public class LoggingMiddleware
{
private readonly RequestDelegate _next;
public LoggingMiddleware(RequestDelegate next)
{
_next = next;
}
public async Task InvokeAsync(HttpContext context)
{
Console.WriteLine($"Incoming request: {context.Request.Method} {context.Request.Path}");
// Call the next middleware in the pipeline
await _next(context);
Console.WriteLine($"Outgoing response: {context.Response.StatusCode}");
}
}
To use this middleware, you would register it in your application's startup configuration:
// In Startup.cs (or Program.cs in .NET 6+)
app.UseMiddleware<LoggingMiddleware>();
Middleware in the `Program.cs` / `Startup.cs` File
In modern ASP.NET Core applications, middleware is configured using the Use... extension methods on the IApplicationBuilder interface. The order of these calls defines the pipeline.
var builder = WebApplication.CreateBuilder(args);
// Add services to the container.
builder.Services.AddRazorPages(); // Example service
var app = builder.Build();
// Configure the HTTP request pipeline.
if (!app.Environment.IsDevelopment())
{
app.UseExceptionHandler("/Error");
// The default HSTS value is 30 days. You may want to change this for production scenarios, see https://aka.ms/aspnetcore-hsts.
app.UseHsts();
}
app.UseHttpsRedirection();
app.UseStaticFiles(); // Serves static files
app.UseRouting(); // Enables endpoint routing
app.UseAuthentication(); // Handles authentication
app.UseAuthorization(); // Handles authorization
// Custom middleware example
app.UseMiddleware<LoggingMiddleware>();
app.MapRazorPages(); // Maps Razor Pages endpoints
app.Run();