c#: Asp.net WebApi default route to index.html

Just add a route to your WebApiConfig file for index page. Your method should look like this:

    public static class WebApiConfig
    {
        public static void Register(HttpConfiguration config)
        {
            // Attribute routing
            config.MapHttpAttributeRoutes();

            // Route to index.html
            config.Routes.MapHttpRoute(
                name: "Index",
                routeTemplate: "{id}.html",
                defaults: new {id = "index"});

            // Default route
            config.Routes.MapHttpRoute(
                name: "DefaultApi",
                routeTemplate: "api/{controller}/{id}",
                defaults: new { id = RouteParameter.Optional }
            );
        }
    }

The solution was to create an empty DefaultRoute.

// I think this passes control through before trying to use a Controller
routes.MapHttpRoute(
    "DefaultRoute", "");

routes.MapHttpRoute(
    "DefaultAPI",
    "{controller}/{id}",
    new { Controller = "Home", Id = RouteParameter.Optional });

Another solution was to prefix my WebApi with 'api', which is not entirely what I wanted, but is a suitable solution. This was shown in Herman Guzman's answer, but is not his answer.

// Default route
config.Routes.MapHttpRoute(
    name: "DefaultApi",
    routeTemplate: "api/{controller}/{id}",
    defaults: new { id = RouteParameter.Optional }

For ASP.NET Core users

As mentioned in the comments, you can use app.UseDefaultFiles() to achieve this. The method adds a rewriter that rewrites requests to the default route "/" to /index.html. For this to work, it is important that you place this line before .UseStaticFiles(). Example:

// ...
app.UseDefaultFiles();
app.UseStaticFiles();

app.MapControllers();

app.Run();

There is a good blog post that describes how this works in detail: Blog Post