ASP.NET Core–Factory based vs Convention based middleware

Today I had to create my own ASP.NET Core middleware. I forgot how to exactly do this but I noticed that an IMiddleware interface existed. I wasn't aware of this interface which brought me to the discovery that there in fact multiple ways to define and create middleware in ASP.NET Core.

Let’s explain each one of them and discuss the differences.

Inline middleware

The simplest solution is to use an inline request delegate:

	var builder = WebApplication.CreateBuilder(args);
	var app = builder.Build();
	app.Use(async (context, next) =>
	{
	var cultureQuery = context.Request.Query["culture"];
	if (!string.IsNullOrWhiteSpace(cultureQuery))
	{
	var culture = new CultureInfo(cultureQuery);
	CultureInfo.CurrentCulture = culture;
	CultureInfo.CurrentUICulture = culture;
	}
	// Call the next delegate/middleware in the pipeline.
	await next(context);
	});
	app.Run();

view raw Inline.cs hosted with ❤ by GitHub

This is a great way for simple use cases but once your middleware gets more complex it is time to move to one of the 2 other approaches.

Convention based middleware

The convention based middleware is the one I was aware of that it existed and that I had used before. To use the convention based middleware you create a separate class that follows these rules:

• It has a public constructor with a parameter of type RequestDelegate.
• It contains a public method named Invoke or InvokeAsync. This method must:
  • Return a Task.
  • Accept a first parameter of type HttpContext.

	public class RequestCultureMiddleware
	{
	private readonly RequestDelegate _next;
	public RequestCultureMiddleware(RequestDelegate next)
	{
	_next = next;
	}
	public async Task InvokeAsync(HttpContext context)
	{
	var cultureQuery = context.Request.Query["culture"];
	if (!string.IsNullOrWhiteSpace(cultureQuery))
	{
	var culture = new CultureInfo(cultureQuery);
	CultureInfo.CurrentCulture = culture;
	CultureInfo.CurrentUICulture = culture;
	}
	// Call the next delegate/middleware in the pipeline.
	await _next(context);
	}
	}

view raw Convention1.cs hosted with ❤ by GitHub

Remark: If you want to inject extra dependencies, you can use constructor injection for the dependencies with a singleton lifetime and add extra parameters to the InvokeAsync() method for your transient and scoped dependencies:

	public async Task InvokeAsync(HttpContext context, ILogger logger)
	{
	}

view raw Convention2.cs hosted with ❤ by GitHub

To add this middleware to the request pipeline, you need to use the UseMiddleware() method:

	using System.Globalization;
	var builder = WebApplication.CreateBuilder(args);
	var app = builder.Build();
	app.UseMiddleware<RequestCultureMiddleware>();
	app.Run();

view raw Convention3.cs hosted with ❤ by GitHub

Factory based middleware

The last option is the factory based middleware. To use this approach you need to implement the IMiddleware interface I was mentioning in the introduction.

The advantage is that there is no longer conventional magic and that dependencies are injected through the constructor. As the factory based middleware is activated per client request (connection), scoped services can be injected into the middleware's constructor without issues.

	public class RequestCultureMiddleware: IMiddleware
	{
	public async Task InvokeAsync(HttpContext context, RequestDelegate next)
	{
	var cultureQuery = context.Request.Query["culture"];
	if (!string.IsNullOrWhiteSpace(cultureQuery))
	{
	var culture = new CultureInfo(cultureQuery);
	CultureInfo.CurrentCulture = culture;
	CultureInfo.CurrentUICulture = culture;
	}
	// Call the next delegate/middleware in the pipeline.
	await next(context);
	}
	}

view raw Factory.cs hosted with ❤ by GitHub

Adding this middleware can be done in the same way as with the previous approach:

	using System.Globalization;
	var builder = WebApplication.CreateBuilder(args);
	var app = builder.Build();
	app.UseMiddleware<RequestCultureMiddleware>();
	app.Run();

view raw Convention3.cs hosted with ❤ by GitHub

The only thing you may not forget is that you have to register the middleware class in your IoC container:

	var builder = WebApplication.CreateBuilder(args);
	builder.Services.AddTransient<RequestCultureMiddleware>();

view raw Factory2.cs hosted with ❤ by GitHub

More information:

• ASP.NET Core Middleware | Microsoft Learn
• Write custom ASP.NET Core middleware | Microsoft Learn
• Factory-based middleware activation in ASP.NET Core | Microsoft Learn