Always add existing route parameter in Url.Action

Question

I'd like to be able to force Url.Action to always add one of my route values to the URL. Basically, I need to be able to add a customer ID in the URL. My routes look like this:

app.MapControllerRoute(
     name: "Areas",
     pattern: "{customerid:int}/{area:exists}/{controller}/{action=Index}");

app.MapControllerRoute(
     name: "default",
     pattern: "{customerid:int}/{controller=Home}/{action=Index}/{id?}");

I'm not concerned with exposing the Customer ID since it will always be protected against the authenticated user. These routes will allow a user to view/manage different customers across multiple tabs.

If i navigate to /99/Admin/User, I can correctly read RouteData.Values["customerid"]. If I use @Url.Action("Index", "Role") in the view, it correctly renders the URL "/99/Admin/Role". Since I'm not jumping across areas, it's smart enough to pass along the customerid route value. However, what if I want to leave to an empty area? For example, using @Url.Action("Index", "Home", new { area = "" }). Of course, this technically correctly renders /Home/Index, but is it possible to have it "remember" and use the customerid route value across areas to render /99/Home/Index?

I know I can use @Url.Action("Index", "Home", new { area = "", customerid = ViewContext.RouteData.Values["customerid"] }) to get the proper path. Or even just create an extension/overload to UrlHelper to sniff the customerid route value. I'm fine with either. Unfortunately, this is an improvement to a large existing application, so making sure to hit every single Url.Action or Html.ActionLink is going to be a daunting task. Albeit, one I think I might have to endure. I was just hoping somebody may know of a way to create a "sticky" route of {domain}/{customerid}/....

Answer 1

You can implement this feature by using custom IActionFilter.

Test Result

Here is the sample code for you.

AppendCustomerIdFilter

using Microsoft.AspNetCore.Mvc.Filters;

namespace WebApplication4
{
    public class AppendCustomerIdFilter : IActionFilter
    {
        public void OnActionExecuted(ActionExecutedContext context)
        {
            //throw new NotImplementedException();
        }

        public void OnActionExecuting(ActionExecutingContext context)
        {
            var routeValues = context.RouteData.Values;

            if (!routeValues.ContainsKey("customerid"))
            {
                var customerid = context.HttpContext.Request.RouteValues["customerid"];
                if (customerid != null)
                {
                    context.RouteData.Values["customerid"] = customerid;
                }
            }
        }
    }
}

Register it

using WebApplication4;

var builder = WebApplication.CreateBuilder(args);

// Add services to the container.
// Register it
builder.Services.AddControllersWithViews(options =>
{
    options.Filters.Add<AppendCustomerIdFilter>();
});

var app = builder.Build();

// Configure the HTTP request pipeline.
if (!app.Environment.IsDevelopment())
{
    app.UseExceptionHandler("/Home/Error");
    // The default HSTS value is 30 days. You may want to change this for production scenarios, see https://aka.ms/aspnetcore-hsts.
    app.UseHsts();
}

app.UseHttpsRedirection();
app.UseStaticFiles();

app.UseRouting();

app.UseAuthorization();

//app.MapControllerRoute(
//    name: "default",
//    pattern: "{controller=Home}/{action=Index}/{id?}");
// Change code like below
app.MapControllerRoute(
    name: "default",
    pattern: "{customerid:int}/{controller=Home}/{action=Index}/{id?}");

app.Run();

My HomeController.cs

using Microsoft.AspNetCore.Mvc;
using System.Diagnostics;
using WebApplication4.Models;

namespace WebApplication4.Controllers
{
    public class HomeController : Controller
    {
        private readonly ILogger<HomeController> _logger;

        public HomeController(ILogger<HomeController> logger)
        {
            _logger = logger;
        }

        public IActionResult Index()
        {
            ViewBag.CustomerId = RouteData.Values["customerid"];
            return View();
        }

        public IActionResult Privacy()
        {
            return View();
        }

        [ResponseCache(Duration = 0, Location = ResponseCacheLocation.None, NoStore = true)]
        public IActionResult Error()
        {
            return View(new ErrorViewModel { RequestId = Activity.Current?.Id ?? HttpContext.TraceIdentifier });
        }
    }
}

Privacy .cshtml

@{
    ViewData["Title"] = "Privacy Page";
}

<h2>Privacy Page</h2>
<a href="@Url.Action("Index", "Home")">Back to Home</a>

Index.cshtml

@{
    ViewData["Title"] = "Home Page";
}

<h2>Customer ID: @ViewBag.CustomerId</h2>
<a href="@Url.Action("Privacy", "Home")">Privacy Page</a>