ASP.NET Core Middleware

An ASP.NET Core application's workflow consist of accepting request and serving response to client. With this workflow, an application has different features / functionality like authentication, error handling or logging etc. ASP.NET Core has provided concept of middleware to implement application's features. A middleware is nothing but a C# class which is placed inside ASP.NET Core's request processing pipeline.

A middleware can perform below tasks :

Decide whether to pass the HTTP request to the next middleware in request processing pipeline.
Can modify the request / response before / after passing it to next middleware in pipeline.

Configuring the middleware

Middleware components are configured in Configure method of Startup.cs file. ASP.NET Core has provided some built-in middleware. Middleware can also be added via Nuget package manager or custom middleware can be added.

Below is the Startup.cs file's Configure method. All the middleware shown in this file are built-in middleware

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
    }
    else
    {
        app.UseExceptionHandler("/Home/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();
    app.UseRouting();
    app.UseAuthorization();
    app.UseEndpoints(endpoints =>
    {
        endpoints.MapControllerRoute(
            name: "default",
            pattern: "{controller=Home}/{action=Index}/{id?}");
    });
}

Middleware ordering

Middleware components are executed in the order they are added to the pipeline and care should be taken to add the middleware in the right order otherwise the application may not function as expected. This ordering is critical for security, performance, and functionality.

Adding inline middleware using app.Use and app.Run method

Inline middleware can be added using app.use or app.Run method.

Middleware defined using app.Use may call next middleware component in the pipeline. Middlware defined using app.Run will never call subsequent middleware.

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    app.Use(async (context, next) =>
    {
        await context.Response.WriteAsync("Middleware 1 using app.Use()");
        await next();
    });

    app.Use(async (context, next) =>
    {
        await context.Response.WriteAsync("Middleware 2 using app.Use()");
        await next();
    });

    app.Run(async context =>
    {
        await context.Response.WriteAsync("Middleware 3 using app.Run()");
        // Short circuiting the pipeline
    });

    // This will never be called
    app.Use(async (context, next) =>
    {
        await context.Response.WriteAsync("Middleware 4 using app.Use()");
        await next();
    });
}

Adding custom middleware

below is an example where a middleware is created to log the incoming request's http method

public class LogRequestMethodMiddleware
{
    private readonly RequestDelegate _next;
    private readonly ILogger<LogRequestMethodMiddleware> _logger;
    public LogRequestMethodMiddleware(RequestDelegate next, 
            ILogger<LogRequestMethodMiddleware> logger)
    {
        _next = next;
        _logger = logger;
    }

    public async Task InvokeAsync (HttpContext context)
    {
        _logger.LogInformation(context.Request.Method);
        await _next(context);
    }
}

Inside Startup.cs file

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    app.UseMiddleware<LogRequestMethodMiddleware>();
    app.Run(async context =>
    {
        await context.Response.WriteAsync("\n Adding custom middleware");
    });
}

Built-in Middleware

ASP.NET Core has some built-in middleware for common application scenario :

Exception/error handling
HTTPS Redirection Middleware (UseHttpsRedirection) redirects HTTP requests to HTTPS.
Static File Middleware (UseStaticFiles) returns static files and short-circuits further request processing.
Cookie Policy Middleware (UseCookiePolicy) conforms the app to the EU General Data Protection Regulation (GDPR) regulations.
Routing Middleware (UseRouting) to route requests.
Authentication Middleware (UseAuthentication) attempts to authenticate the user before they're allowed access to secure resources.
Authorization Middleware (UseAuthorization) authorizes a user to access secure resources.
Session Middleware (UseSession) establishes and maintains session state. If the app uses session state, call Session Middleware after Cookie Policy Middleware and before MVC Middleware.
Endpoint Routing Middleware (UseEndpoints with MapRazorPages) to add Razor Pages endpoints to the request pipeline.

ASP.NET Core Middleware

Comments

ASP.NET Core Advance Concept

Filters in ASP.NET Core

More from this blog

ASP.NET Core Configuration Guide

How to Modernize Legacy Delphi Applications with a Hybrid Approach

Working with EF Core pipeline

Options pattern in ASP.NET Core: Advance scenario

Options pattern in ASP.NET Core: Introduction

Configuring the middleware

Middleware ordering

Adding inline middleware using app.Use and app.Run method

Adding custom middleware

Built-in Middleware

Command Palette

Comments

ASP.NET Core Advance Concept

Filters in ASP.NET Core

More from this blog

Configuring the middleware

Middleware ordering

Adding inline middleware using app.Use and app.Run method

Adding custom middleware

Built-in Middleware