Value cannot be null. Parameter name: String in asp.net mvc C#

Assuming the default route is configured in Global.asax ({controller}/{action}/{id}) you could have your controller action take an id parameter and the default model binder will automatically set its value:

public ActionResult Foo(string id)
{
    ...
}

If you want to fetch this id value from some other portion of your code that does have access to an HttpContext you need to fetch it from the RouteData:

var id = HttpContext.Request.RequestContext.RouteData["id"];

RouteData is available in all standard MVC locations. In your example you have used the static HttpContext.Current property which is something that you should never use. I suspect that you are trying to fetch this id from a portion of your code where you are not supposed to have access to the HttpContext. So you'd better fetch this id using standard techniques and then pass it as parameter to other parts of your code.

If Item is your controller, and ItemSpec is action, you can get the Id just by

public ActionResult ItemSpec(int id) { }

You routing have to be setup to:

context.MapRoute(
    "Default",
    "{controller}/{action}/{id}",
    new { controller = "Home", action = "Index"  }
);

If you haven't changed your routing so it's still defined as it was when you created Asp.net MVC Web project then this should be one of your controllers:

public class ItemController : ControllerBase
{
    ...

    public ActionResult ItemSpec(int id, int dep, int cat, int tab)
    {
        // implementation that uses all four values
    }

    ...
}

This is of course just one of the actions in it. There may be others as well. Most likely the Index one that's generated by default and is also used by default routing...