Prefixing all MVC6 / Web API routes with a configured value

AutoPrefixingRouteConvention.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNet.Mvc.ApplicationModels;
using Microsoft.AspNet.Mvc;

namespace MrSheep.Shared
{
    public class AutoPrefixingRouteConvention : IApplicationModelConvention
    {
        private string m_baseUrl = "";
        public AutoPrefixingRouteConvention(string baseUrl)
        {
            this.m_baseUrl = baseUrl;
        }
        public void Apply(ApplicationModel application)
        {
            var centralPrefix = new AttributeRouteModel(new RouteAttribute(m_baseUrl));
            foreach (var controller in application.Controllers)
            {
                if (controller.AttributeRoutes.Any())
                {
                    for (var i = 0; i < controller.AttributeRoutes.Count; i++)
                    {
                        controller.AttributeRoutes[i] = AttributeRouteModel.CombineAttributeRouteModel(
                            centralPrefix, controller.AttributeRoutes[i]);
                    }
                }
                else
                {
                    controller.AttributeRoutes.Add(centralPrefix);
                }
            }
        }
    }
}

config.json

{
  "MrSheepWeb": {
    "BaseUrl": "/base/"
  }
}

Startup.cs

using System;
using Microsoft.AspNet.Builder;
using Microsoft.Framework.DependencyInjection;
using Microsoft.AspNet.Hosting;
using Microsoft.Framework.Configuration;
using Microsoft.Dnx.Runtime;
using Microsoft.AspNet.Http.Features;
using Microsoft.Net.Http.Server;
using Microsoft.Framework.OptionsModel;
using MrSheep.Shared;

namespace MrSheep.WebExample
{
    public class Startup
    {
        public IConfiguration Configuration;

        public Startup(IHostingEnvironment env, IApplicationEnvironment appEnv)
        {
            Configuration = new ConfigurationBuilder()
                .SetBasePath(appEnv.ApplicationBasePath)
                .AddJsonFile("config.json")
                .AddJsonFile($"config.{env.EnvironmentName}.json", optional: true)
                .AddEnvironmentVariables()
                .Build();
        }

        public void ConfigureServices(IServiceCollection services)
        {
            // Configuration
            services.Configure<MrSheepWebConfig>(Configuration.GetSection("MrSheepWeb"));

            // Calculate the api base URL - need to trim the leading slash if present.
            var apiBaseUrl = Configuration["MrSheepWeb:BaseUrl"];
            if (apiBaseUrl.StartsWith("/"))
            {
                if (apiBaseUrl.Length > 1) apiBaseUrl = apiBaseUrl.Substring(1);
                else apiBaseUrl = "";
            }

            // Add an auto-prefixing convention to add the api base URL to any attribute routes
            services.AddMvc(opt =>
            {
                if (apiBaseUrl != "")
                    opt.Conventions.Insert(0, new AutoPrefixingRouteConvention(apiBaseUrl));
            });
        }

        public void Configure(IApplicationBuilder app, IOptions<MrSheepWebConfig> config)
        {
            app.UseIISPlatformHandler();

            // Serve images etc - need to use the base URL without a trailing /
            var baseUrl = config.Value.BaseUrl;
            if (baseUrl.EndsWith("/")) baseUrl = baseUrl.Substring(0, baseUrl.Length - 1);
            app.UseStaticFiles(baseUrl);

            // Serve views and the API.
            app.UseMvc();
        }
    }
}