How to pass a dynamic variable into the authorize attribute class in asp.net mvc?

For Example I have this piece of code, how can I pass a variable like userRoles variable into the Authorize attribute class?

```
private  string userRoles;
private string getuserRoles() 
{
     //Write your code to get userRoles
     userRoles = "admin";
     return "admin";
 }

[Authorize(Roles = object.getuserRoles())]
public ActionResult Admin()
{
       ViewBag.Message = "Your contact page.";

        return View();

}

```

My code issues this error

> 
> Error 1 An attribute argument must be a constant expression, typeof
>  expression or array creation expression of an attribute parameter
>  type C:\Users\Nashat\Downloads\New folder
>  (3)\MvcPWy\Controllers\HomeController.cs 39 28 MvcPWy
> 
> 
> 

So please could anyone help me to resolve this error.