Unable to utilize UrlHelper

Question

I'm currently trying to do something that was dead simple and straight forward in ASP.NET 4 however this ins't the case now in ASP.NET 5.

Previously to use the UrlHelper it was dead simple:

var urlHelper = new UrlHelper(HttpContext.Current.Request.RequestContext);

However I can't for the life of me wrap my head around how to use the new UrlHelper. I'm looking at the test cases and either I'm completely daft or I'm missing something and I can't seem to figure it out. Any help here in clearing up this would be great.

Answer 1

Update - Post RC2

As @deebo mentioned, you no longer can get an IUrlHelper directly from DI. Instead you need to inject an IUrlHelperFactory and an IActionContextAccessor into your class and use them to get the IUrlHelper instance as in:

public MyClass(IUrlHelperFactory urlHelperFactory, IActionContextAccessor actionAccessor)
{
    this.urlHelperFactory = urlHelperFactory;
    this.actionAccessor = actionAccessor;
}

public void SomeMethod()
{
    var urlHelper = this.urlHelperFactory.GetUrlHelper(this.actionAccessor.ActionContext);
}

You need to also register the in your startup class (IUrlHelperFactory is already registered by default):

services.AddSingleton<IActionContextAccessor, ActionContextAccessor>();

Bear in mind this will only work as long as the code where you get the actionContext is running after the MVC/routing middleware! (Otherwise actionAccessor.ActionContext would be null)

---

I have retrieved the IUrlHelper using the IServiceProvider in HttpContext.RequestServices.

Usually you will have an HttpContext property at hand:

• In a controller action method you can do:

  var urlHelper = this.Context.RequestServices.GetRequiredService<IUrlHelper>();
  ViewBag.Url = urlHelper.Action("Contact", "Home", new { foo = 1 });
  

• In a filter you can do:

  public void OnActionExecuted(ActionExecutedContext context)
  {
      var urlHelper = context.HttpContext.RequestServices.GetRequiredService<IUrlHelper>();
      var actionUrl = urlHelper.Action("Contact", "Home", new { foo = 1 });
      //use actionUrl ...
  }
  

Another option would be taking advantage of the built-in dependency injection, for example your controller could have a constructor like the following one and at runtime an IUrlHelper instance will be provided:

private IUrlHelper _urlHelper;
public HomeController(IUrlHelper urlHelper)
{
    _urlHelper = urlHelper;
}

Answer 2

In Startup.cs

   services.AddSingleton<IActionContextAccessor, ActionContextAccessor>();
   services.AddSingleton<IUrlHelperFactory, UrlHelperFactory>();
   services.AddScoped(it => it.GetService<IUrlHelperFactory>()
                    .GetUrlHelper(it.GetService<IActionContextAccessor>().ActionContext));

Alternatively

PM> Install-Package AspNetCore.IServiceCollection.AddIUrlHelper 

In Startup.cs

services.AddUrlHelper();

Answer 3

A shorter version without constructing special Factory class

services.AddSingleton<IActionContextAccessor, ActionContextAccessor>()
            .AddScoped<IUrlHelper>(sp => new UrlHelper(sp.GetRequiredService<IActionContextAccessor>().ActionContext));

Answer 4

If you just need the UrlHelper.Link method like I did, you don't even need the UrlHelper any more, just use Url.Link

Answer 5

Thought I would share for the upcoming RC2 since the current answer won't work anymore then.

From RC 2 you will need to explicitly register IActionContextAccessor and IUrlHelperFactory

services.AddSingleton<IActionContextAccessor, ActionContextAccessor>();
services.AddSingleton<IUrlHelperFactory, UrlHelperFactory>();

Then use the DI/service locator:

public EmailTagHelper(IUrlHelperFactory urlHelperFactory, IActionContextAccessor actionContextAccessor)
{
  _urlHelper = urlHelperFactory.GetUrlHelper(actionContextAccessor.ActionContext);
}

I blogged about it here with regard to TagHelpers: http://devonburriss.me/asp-net-5-tips-urlhelper