Use .net core middleware with .Map configured to a specific path

Question

IApplicationBuilder has a .Map method that all the examples online show you how to use when you are inlining your middleware configuration and using .Run or .Use directly.

https://docs.microsoft.com/en-us/aspnet/core/fundamentals/middleware/?view=aspnetcore-2.2#use-run-and-map

Most of the examples then go on to say how this is a bad idea (rightly so for maintenance reasons alone) and show you how to make a middle ware component which looks something like this:

 public class CustomMiddleware: IMiddleware
    {
        public async Task InvokeAsync(HttpContext context, RequestDelegate next)
        {
            ... do stuff.
            await next(context);
        }
    }

    // Extension method used to add the middleware to the HTTP request pipeline.
    public static class CustomMiddlewareExtensions
    {
        public static IApplicationBuilder UseCustomMiddleware(this IApplicationBuilder builder)
        {
            return builder.UseMiddleware<CustomMiddleware>();
        }
    }

What I can't see is how to combine the two. One thing I tried was this:

public static class CustomMiddlewareExtensions
    {
        public static IApplicationBuilder UseCustomMiddleware(this IApplicationBuilder builder)
        {
            return builder.Map("/CustomPath", app => { 
                app.UseMiddleware<CustomMiddleware>(); });
        }
    }

But this is apparently not working as it is calling InvokeAsync for everything.

I could easily look at the HttpContext.Request.Path and skip my do stuff if it does not match but wanted to know if it was possible to use .Map before doing so.

rhytonix rhytonix · Accepted Answer · 2019-08-13T00:32:45

The code you provided in working as it should and yielding the result you desire; InvokeAsync is called only when the current request path starts with the one provided to Map -I'm not sure if this small detail is what might be causing the issue i.e. /CustomPath/anything will match and cause Map to invoke the different pipeline-.

For example using the following code in a new template:

Map will match /home, /home/privacy, /home/values etc.
Map won't match /anything_not_starting_with_home, and InvokeAsync won't be called.

public static class CustomMiddlewareExtensions {
    public static IApplicationBuilder UseCustomMiddleware(this IApplicationBuilder appBuilder) {
        appBuilder.Map("/home", b => {
            b.UseMiddleware<CustomMiddleware1>();
            b.UseMiddleware<CustomMiddleware2>();
        });
        return appBuilder;
    }

    public class CustomMiddleware1 {
        private readonly RequestDelegate _next;

        public CustomMiddleware1(RequestDelegate next) {
            _next = next;
        }

        public async Task InvokeAsync(HttpContext context) {
            Debug.WriteLine($" ======== Response handled by {nameof(CustomMiddleware1)} ======== ");
            await _next.Invoke(context);
        }
    }

    public class CustomMiddleware2 {
        private readonly RequestDelegate _next;

        public CustomMiddleware2(RequestDelegate next) {
            _next = next;
        }

        public async Task InvokeAsync(HttpContext context) {
            Debug.WriteLine($" ======== Response handled by {nameof(CustomMiddleware2)} ======== ");
            await context.Response.WriteAsync("Custom Middleware used");
        }
    }
}

Use .net core middleware with .Map configured to a specific path

1 Answers