ActionContext gone in Microsoft.AspNetCore.Mvc.Controller

In .NET core 3.1 at least, ControllerBase has a property Url that works as a UrlHelper so you don't have to make one yourself. https://stackoverflow.com/a/52537244/4172413

I replaced ActionContext with ControllerContext, and it works for me. I don't know if it's an official migration step, though.

You can inject IActionContextAccessor to your class. It provides access to the action context.

services.AddSingleton<IActionContextAccessor, ActionContextAccessor>();

Use it:

private readonly IActionContextAccessor actionContextAccessor

public FooController(IActionContextAccessor actionContextAccessor)
{
    this.actionContextAccessor = actionContextAccessor;
}

See this issue.