Q:

How to use Url.Action() in a class file?

How can I use Url.Action() in a class file of MVC project?
Like:
namespace _3harf
{
    public class myFunction
    {
        public static void CheckUserAdminPanelPermissionToAccess()
        {
            if (ReferenceEquals(HttpContext.Current.Session["Loged"], "true") &&
                myFunction.GetPermission.AdminPermissionToLoginAdminPanel(
                    Convert.ToInt32(HttpContext.Current.Session["UID"])))
            {
                HttpContext.Current.Response.Redirect(Url.Action("MainPage", "Index"));
            }
        }
    }
}

A:

You will need to manually create the UrlHelper class and pass the appropriate RequestContext. It could be done with something like:
var requestContext = HttpContext.Current.Request.RequestContext;
new UrlHelper(requestContext).Action("Index", "MainPage");

However, you are trying to achieve redirection based on authentication. I suggest you look at implementing a custom AuthorizeAttribute filter to achieve this kind of behavior to be more in line with the framework

A:

Pass the RequestContext to your custom class from the controller. I would add a Constructor to your custom class to handle this.
using System.Web.Mvc;
public class MyCustomClass
{
    private UrlHelper _urlHelper;
    public MyCustomClass(UrlHelper urlHelper)
    {
        _urlHelper = urlHelper;
    }
    public string GetThatURL()
    {         
      string url=_urlHelper.Action("Index", "Invoices"); 
      //do something with url or return it
      return url;
    }
}

You need to import System.Web.Mvc namespace to this class to use the UrlHelper class.
Now in your controller, create an object of MyCustomClass and pass the controller context in the constructor,
UrlHelper uHelp = new UrlHelper(this.ControllerContext.RequestContext);
var myCustom= new MyCustomClass(uHelp );    
//Now call the method to get the Paging markup.
string thatUrl= myCustom.GetThatURL();

