Use / not use a css class with a condition

I am working on an MVC project,
what I need is to have custom css classes to use one when there is a user logged in to the site, and the other when accessing the site as a guest.
I thought about using a function in the controller that checks if there is a logged in account and a ViewBag to pass a true / false value to the view.
What I ask myself is:

How do I create a condition so that taking this ViewBag with the true / false value can enable a css class and disable another?

>Solution :

You can use the following code to apply different css using C# variable.
Assuming you have user and guest css class created, and you want to check is user logged in through ViewBag.UserLoggedIn:

@{
    var cssName = "";
    if (ViewBag.UserLoggedIn)
    {
        cssName = "user";
    }
    else
    {
        cssName = "guest";
    }
}


<div class="otherclass @cssName"> Hello </div>