How to self register a service with Consul

Startup.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Options;

namespace WebApplication1
{
    public class Startup
    {
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        public IConfiguration Configuration { get; }

        // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddConsulConfig(Configuration);
            services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_2);
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IHostingEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }
            app.UseConsul();
            app.UseMvc();
        }
    }
}

AppExtensions.cs

using Consul;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Logging;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace WebApplication1
{
    public static class AppExtensions
    {
        public static IServiceCollection AddConsulConfig(this IServiceCollection services, IConfiguration configuration)
        {
            services.AddSingleton<IConsulClient, ConsulClient>(p => new ConsulClient(consulConfig =>
            {
                var address = configuration.GetValue<string>("http://localhost:8500");
                consulConfig.Address = new Uri(address);
            }));
            return services;
        }

        public static IApplicationBuilder UseConsul(this IApplicationBuilder app)
        {
            var consulClient = app.ApplicationServices.GetRequiredService<IConsulClient>();
            var logger = app.ApplicationServices.GetRequiredService<ILoggerFactory>().CreateLogger("AppExtensions");
            var lifetime = app.ApplicationServices.GetRequiredService<IApplicationLifetime>();


            // var uri = new Uri(address);
            // Better approach should be used to set the below settings. I hard coded just to explain.
            var registration = new AgentServiceRegistration()
            {
                ID = $"MyService",
                Name = "WebApplication1",
                Address = "localhost", //$"{uri.Host}",
                Port = 57084  // uri.Port
            };

            logger.LogInformation("Registering with Consul");
            consulClient.Agent.ServiceDeregister(registration.ID).ConfigureAwait(true);
            consulClient.Agent.ServiceRegister(registration).ConfigureAwait(true);

            lifetime.ApplicationStopping.Register(() =>
            {
                logger.LogInformation("Unregistering from Consul");
                consulClient.Agent.ServiceDeregister(registration.ID).ConfigureAwait(true);
            });

            return app;
        }
    }
}

Github repo

First of all I recommend to use Consul.NET to interact with Consul. Using it, a service registration may look like:

var registration = new AgentServiceRegistration
{
    Name = "foo",
    Port = 4242,
    Address = "http://bar"
};

using (var client = new ConsulClient())
{
    await client.Agent.ServiceRegister(registration);
}

Now let's integrate this code into ASP.NET Core startup process with help of DI and loose coupling. Read your json file into ConsulOptions instance (DTO without any logic):

public void ConfigureServices(IServiceCollection services)
{
    services.AddOptions();
    services.Configure<ConsulOptions>(Configuration);
}

Encapsulate all Consul-related logic in class ConsulService accepting ConsulOptions as a dependency:

public class ConsulService : IDisposable
{
    public ConsulService(IOptions<ConsulOptions> optAccessor) { }

    public void Register() 
    {
        //possible implementation of synchronous API
        client.Agent.ServiceRegister(registration).GetAwaiter().GetResult();
    }
}

Add the class itself to the DI container:

services.AddTransient<ConsulService>();

Then create an extention method of IApplicationBuilder and call it:

public void Configure(IApplicationBuilder app)
{
    app.ConsulRegister();
}

In ConsulRegister implementation we add our hooks on application start/stop:

public static class ApplicationBuilderExtensions
{
    public static ConsulService Service { get; set; }

    public static IApplicationBuilder ConsulRegister(this IApplicationBuilder app)
    {
        //design ConsulService class as long-lived or store ApplicationServices instead
        Service = app.ApplicationServices.GetService<ConsulService>();

        var life = app.ApplicationServices.GetService<IApplicationLifetime>();

        life.ApplicationStarted.Register(OnStarted);
        life.ApplicationStopping.Register(OnStopping);

        return app;
    }

    private static void OnStarted()
    {
        Service.Register(); //finally, register the API in Consul
    }
}

Locking absence and static fields are OK because the Startup class is executed exactly once on application start. Don't forget to de-register the API in OnStopping method!