Fundamentals - Middleware - Write middleware

In this article

Middleware class

using System.Globalization;

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

app.UseHttpsRedirection();

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(async (context) =>
{
    await context.Response.WriteAsync(
        $"CurrentCulture.DisplayName: {CultureInfo.CurrentCulture.DisplayName}");
});

app.Run();

• One takes a HttpContext and a Func<Task>. Invoke the Func<Task> without any parameters.

• The other takes a HttpContext and a RequestDelegate. Invoke the RequestDelegate by passing the HttpContext.

using System.Globalization;

namespace Middleware.Example;

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);
    }
}

• A public constructor with a parameter of type RequestDelegate.

• A public method named Invoke or InvokeAsync. This method must:

  • Return a Task.

  • Accept a first parameter of type HttpContext.

using System.Globalization;

namespace Middleware.Example;

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);
    }
}

public static class RequestCultureMiddlewareExtensions
{
    public static IApplicationBuilder UseRequestCulture(
        this IApplicationBuilder builder)
    {
        return builder.UseMiddleware<RequestCultureMiddleware>();
    }
}

using Middleware.Example;
using System.Globalization;

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

app.UseHttpsRedirection();

app.UseRequestCulture();

app.Run(async (context) =>
{
    await context.Response.WriteAsync(
        $"CurrentCulture.DisplayName: {CultureInfo.CurrentCulture.DisplayName}");
});

app.Run();

Middleware dependencies

Per-request middleware dependencies

namespace Middleware.Example;

public class MyCustomMiddleware
{
    private readonly RequestDelegate _next;

    public MyCustomMiddleware(RequestDelegate next)
    {
        _next = next;
    }

    // IMessageWriter is injected into InvokeAsync
    public async Task InvokeAsync(HttpContext httpContext, IMessageWriter svc)
    {
        svc.Write(DateTime.Now.Ticks.ToString());
        await _next(httpContext);
    }
}

public static class MyCustomMiddlewareExtensions
{
    public static IApplicationBuilder UseMyCustomMiddleware(
        this IApplicationBuilder builder)
    {
        return builder.UseMiddleware<MyCustomMiddleware>();
    }
}

using Middleware.Example;
var builder = WebApplication.CreateBuilder(args);

builder.Services.AddScoped<IMessageWriter, LoggingMessageWriter>();

var app = builder.Build();

app.UseHttpsRedirection();

app.UseMyCustomMiddleware();

app.MapGet("/", () => "Hello World!");

app.Run();

namespace Middleware.Example;

public interface IMessageWriter
{
    void Write(string message);
}

public class LoggingMessageWriter : IMessageWriter
{

    private readonly ILogger<LoggingMessageWriter> _logger;

    public LoggingMessageWriter(ILogger<LoggingMessageWriter> logger) =>
        _logger = logger;

    public void Write(string message) =>
        _logger.LogInformation(message);
}

Additional resources

• Sample code used in this article

• UseExtensions source on GitHub

• Lifetime and registration options contains a complete sample of middleware with scoped, transient, and singleton lifetime services.

• DEEP DIVE: HOW IS THE ASP.NET CORE MIDDLEWARE PIPELINE BUILT

• ASP.NET Core Middleware

• Test ASP.NET Core middleware

• Migrate HTTP handlers and modules to ASP.NET Core middleware

• App startup in ASP.NET Core

• Request Features in ASP.NET Core

• Factory-based middleware activation in ASP.NET Core

• Middleware activation with a third-party container in ASP.NET Core

Ref: Write custom ASP.NET Core middleware