Factory-based middleware activation in ASP.NET Core

IMiddlewareFactory/IMiddleware is an extensibility point for middleware activation that offers the following benefits:

• Activation per client request (injection of scoped services)
• Strong typing of middleware

UseMiddleware extension methods check if a middleware's registered type implements IMiddleware. If it does, the IMiddlewareFactory instance registered in the container is used to resolve the IMiddleware implementation instead of using the convention-based middleware activation logic. The middleware is registered as a scoped or transient service in the app's service container.

IMiddleware is activated per client request (connection), so scoped services can be injected into the middleware's constructor.

IMiddleware

IMiddleware defines middleware for the app's request pipeline. The InvokeAsync(HttpContext, RequestDelegate) method handles requests and returns a Task that represents the execution of the middleware.

Middleware activated by convention:

C#

public class ConventionalMiddleware
{
    private readonly RequestDelegate _next;

    public ConventionalMiddleware(RequestDelegate next)
        => _next = next;

    public async Task InvokeAsync(HttpContext context, SampleDbContext dbContext)
    {
        var keyValue = context.Request.Query["key"];

        if (!string.IsNullOrWhiteSpace(keyValue))
        {
            dbContext.Requests.Add(new Request("Conventional", keyValue));

            await dbContext.SaveChangesAsync();
        }

        await _next(context);
    }
}

Middleware activated by MiddlewareFactory:

C#

public class FactoryActivatedMiddleware : IMiddleware
{
    private readonly SampleDbContext _dbContext;

    public FactoryActivatedMiddleware(SampleDbContext dbContext)
        => _dbContext = dbContext;

    public async Task InvokeAsync(HttpContext context, RequestDelegate next)
    {
        var keyValue = context.Request.Query["key"];

        if (!string.IsNullOrWhiteSpace(keyValue))
        {
            _dbContext.Requests.Add(new Request("Factory", keyValue));

            await _dbContext.SaveChangesAsync();
        }

        await next(context);
    }
}

Extensions are created for the middleware:

C#

public static class MiddlewareExtensions
{
    public static IApplicationBuilder UseConventionalMiddleware(
        this IApplicationBuilder app)
        => app.UseMiddleware<ConventionalMiddleware>();

    public static IApplicationBuilder UseFactoryActivatedMiddleware(
        this IApplicationBuilder app)
        => app.UseMiddleware<FactoryActivatedMiddleware>();
}

It isn't possible to pass objects to the factory-activated middleware with UseMiddleware:

C#

public static IApplicationBuilder UseFactoryActivatedMiddleware(
    this IApplicationBuilder app, bool option)
{
    // Passing 'option' as an argument throws a NotSupportedException at runtime.
    return app.UseMiddleware<FactoryActivatedMiddleware>(option);
}

The factory-activated middleware is added to the built-in container in Program.cs:

C#

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddDbContext<SampleDbContext>
    (options => options.UseInMemoryDatabase("SampleDb"));

builder.Services.AddTransient<FactoryActivatedMiddleware>();

Both middleware are registered in the request processing pipeline, also in Program.cs:

C#

var app = builder.Build();

app.UseConventionalMiddleware();
app.UseFactoryActivatedMiddleware();

IMiddlewareFactory

IMiddlewareFactory provides methods to create middleware. The middleware factory implementation is registered in the container as a scoped service.

The default IMiddlewareFactory implementation, MiddlewareFactory, is found in the Microsoft.AspNetCore.Http package.

Additional resources

• View or download sample code (how to download)
• ASP.NET Core Middleware
• Middleware activation with a third-party container in ASP.NET Core