In the previous post I looked into the F# Giraffe library which makes writing all sorts of web request/response code pretty simple. This in turn is built on top of the existing functionality within ASP.NET Core.

Let’s again look at the bare bones code from running up Kestrel adding code to the pipeline

using Microsoft.AspNetCore;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Http;

namespace KestrelTest
{
    public class Startup
    {
        public void Configure(
            IApplicationBuilder applicationBuilder,
            IHostingEnvironment hostingEnvironment)
        {
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            WebHost.CreateDefaultBuilder()
                .UseStartup<Startup>()
                .UseKestrel()
                .Build()
                .Run();
        }
    }
}

Within the Configure method we can start adding code to respond to different routes using the Map method, for example add the following to the Configure method

applicationBuilder.Map("/hello", app =>
{
   app.Run(async ctx =>
   {
      var request = 
         ctx.Request.Path.HasValue ? 
            ctx.Request.Path.Value.Substring(1) : 
            String.Empty;
       await ctx.Response.WriteAsync("Hello " + request);
   });
});
applicationBuilder.Map("/error", app =>
{
   app.Run(ctx => 
      Task.FromResult(
         ctx.Response.StatusCode = 404) 
   );
});

Note: The code is somewhat basic, obviously you would probably want to write some better code for extracting partial paths etc. from the routes/maps. However we’ll be covering a better alternative to this sort of code later in the post.

In the above we’ve created the equivalent of two “routes”. The first handles URL’s along the line of localhost:5000/hello/World where the return would become “Hello World”. The second route simply responds with a 404 for the URL localhost:5000/error.

We can also (as you’d expect) handle queries within our URL, so let’s say we expect this format URL, http://localhost:5000/hello?name=World, then we can amend our /hello code to the following

applicationBuilder.Map("/hello", app =>
{
   app.Run(async ctx =>
   {
      await ctx.Response.WriteAsync(
         "Hello " + ctx.Request.Query["name"]);
   });
});

These pieces of code write responses, but we can also insert code into the pipeline which doesn’t write to the response but instead might add debug/logging code. Other examples of usage might include (as per Writing Middleware) changes to the culture for the user response.

Here’s a simple example of such code, this is a little contrived as we’re going to (in essence) redirect calls to /hello path. Just place the following code before the applicationBuilder.Map method in the above

applicationBuilder.Use(async (ctx, next) =>
{
   if (ctx.Request.Path.Value == "/hello")
   {
      ctx.Request.Path = new PathString("/error");
   }
   await next.Invoke();
});

Note: there are redirect/URL rewriting capabilities already in ASP.NET Core, see URL Rewriting Middleware in ASP.NET Core.

Routing

In the previous example we demonstrated using the Map method to route our calls but ASP.NET Core libraries already supply a routing middleware which handles a lot of the standard routing type of functionality we’d expect. We can add the routing services by adding a method to the Startup class like this

public void ConfigureServices(
   IServiceCollection services)
{
   services.AddRouting();
}

Now the Configure method should look like this

public void Configure(
   IApplicationBuilder applicationBuilder,
   IHostingEnvironment hostingEnvironment)
{
   var routes = new RouteBuilder(applicationBuilder);
   routes.MapGet("hello/{name}", ctx =>
   {
      var name = ctx.GetRouteValue("name");
      return ctx.Response.WriteAsync($"Hello {name}");
   });

   applicationBuilder.UseRouter(routes.Build());
}

The RouteBuilder handles the routing in a more helpful/useful manner. Don’t forget to use the line applicationBuilder.UseRouter(routes.Build()); or you’ll find that the routes are not registered and hence your code will never get called.

References

ASP.NET Core MiddlewareRouting in ASP.NET Core