ASP.NET Core -Automatically add middleware through IStartupFilter

While browsing through the ASP.NET Core documentation, I accidently stumbled over the IStartupFilter interface. I had no idea what it did but it triggered my interest. Let's find out together in this post what this interface does and when it can be useful.

What is IStartupFilter?

IStartupFilter is an interface that allows you to intercept and modify how the application's request pipeline is built. It's particularly useful when you need to:

• Ensure certain middleware runs before any other middleware
• Add middleware consistently across multiple applications
• Create reusable middleware configurations
• Modify the order of middleware registration

Here's the interface definition:

	public interface IStartupFilter
	{
	Action<IApplicationBuilder> Configure(Action<IApplicationBuilder> next);
	}

view raw IStartupFilter.cs hosted with ❤ by GitHub

How IStartupFilter Works

When ASP.NET Core builds the middleware pipeline, it executes all registered IStartupFilter instances in the order they were registered. Each filter can add middleware before and after calling the next delegate, creating a wrapper around the existing pipeline configuration.

Here's a practical example:

	public class RequestTimingStartupFilter : IStartupFilter
	{
	public Action<IApplicationBuilder> Configure(Action<IApplicationBuilder> next)
	{
	return builder =>
	{
	// Add middleware before the existing pipeline
	builder.Use(async (context, nextMiddleware) =>
	{
	var sw = Stopwatch.StartNew();
	// Register a callback to add the timing header before the response starts
	context.Response.OnStarting(() =>
	{
	sw.Stop();
	context.Response.Headers["X-Request-Time-Ms"] = sw.ElapsedMilliseconds.ToString();
	return Task.CompletedTask;
	});
	// Call the next middleware
	await nextMiddleware();
	});
	// Call the existing pipeline configuration
	next(builder);
	};
	}
	}

view raw RequestTimingStartupFilter.cs hosted with ❤ by GitHub

To use our IStartupFilter, we need register it in our Program.cs:

	var builder = WebApplication.CreateBuilder(args);
	// Add services to the container.
	builder.Services.AddControllers();
	builder.Services.AddOpenApi();
	// Register the RequestTimingStartupFilter
	builder.Services.AddTransient<IStartupFilter, RequestTimingStartupFilter>();
	var app = builder.Build();

view raw Program.cs hosted with ❤ by GitHub

Remark: Multiple IStartupFilter implementations might interact with the same objects. If ordering is important, order their IStartupFilter service registrations to match the order that their middlewares should run.

When should I use it?

I don’t see that many use cases where you need the IStartupFilter. I think this is mostly useful for library authors that want to ensure that their middleware runs at the beginning (or end) of the middleware pipeline. For example,  you’ve create a custom error handling middleware. In that case you want a filter that ensures error handling middleware is always first in the pipeline across all your applications.

If you are using the IStartupFilter in some smart ways, please let me know. Always interested to learn from your experiences.

More information

Exploring IStartupFilter in ASP.NET Core

App startup in ASP.NET Core | Microsoft Learn