Unable to resolve service for type 'Microsoft.AspNetCore.Mvc.IUrlHelper' while attempting to activate

For Object reference not set to an instance of an object, it is caused by that you did not register IActionContextAccessor.

Try follow steps below:

1. UserService

   public interface IUserService
   {
       void RegisterUser();
   }
   public class UserService : IUserService
   {
       private IUrlHelper _urlHelper;
       private HttpRequest _request;
   
       public UserService(IUrlHelper urlHelper, IHttpContextAccessor httpContextAccessor)
       {
           _urlHelper = urlHelper;
           _request = httpContextAccessor.HttpContext.Request;
       }
       public void RegisterUser()
       {
           var callbackUrl = _urlHelper.EmailConfirmationLink("user.Email", "token", _request.Scheme);
           //throw new NotImplementedException();
       }
   }
   

2. Register

   services.TryAddSingleton<IHttpContextAccessor, HttpContextAccessor>();
   services.AddSingleton<IActionContextAccessor, ActionContextAccessor>();
   services.AddScoped<IUrlHelper>(x => {
       var actionContext = x.GetRequiredService<IActionContextAccessor>().ActionContext;
       var factory = x.GetRequiredService<IUrlHelperFactory>();
       return factory.GetUrlHelper(actionContext);
   });
   
   services.AddScoped<IUserService, UserService>();
   

Register IUrlHelper in your DI Container like this :

services.AddScoped<IUrlHelper>(factory =>
{
    var actionContext = factory.GetService<IActionContextAccessor>()
                               .ActionContext;
    return new UrlHelper(actionContext);
});

And to use HttpContext in your services, you must use IHttpContextAccessor:

public class MyService
{
    private IHttpContextAccessor _httpContextAccessor;
    public MyService(IHttpContextAccessor httpContextAccessor)
    {
        _httpContextAccessor = httpContextAccessor;
    }

    public void MyMethod()
    {
        // Use HttpContext like this
        var username = _httpContextAccessor.HttpContext.User.Identity.Name;
    }
}

Also, don't forget to register IHttpContextAccessor in your DI Container :

services.AddHttpContextAccessor();