Adding a css class to select using @Html.DropDownList()

Looking at the controller, and learing a bit more about how MVC actually works, I was able to make sense of this.

My view was one of the auto-generated ones, and contained this line of code:

@Html.DropDownList("PriorityID", string.Empty)

To add html attributes, I needed to do something like this:

@Html.DropDownList("PriorityID", (IEnumerable<SelectListItem>)ViewBag.PriorityID, new { @class="dropdown" })

Thanks again to @Laurent for your help, I realise the question wasn't as clear as it could have been...

UPDATE:

A better way of doing this would be to use DropDownListFor where possible, that way you don't rely on a magic string for the name attribute

@Html.DropDownListFor(x => x.PriorityID, (IEnumerable<SelectListItem>)ViewBag.PriorityID, new { @class = "dropdown" })

As the signature from the error message implies, the second argument must be an IEnumerable, more specifically, an IEnumerable of SelectListItem. It is the list of choices. You can use the SelectList type, which is a IEnumerable of SelectListItem. For a list with no choices:

@Html.DropDownList("PriorityID", new List<SelectListItem>(), new {@class="textbox"} )

For a list with a few choices:

@Html.DropDownList(
    "PriorityID", 
    new List<SelectListItem> 
    { 
        new SelectListItem { Text = "High", Value = 1 }, 
        new SelectListItem { Text = "Low",  Value = 0 },
    }, 
    new {@class="textbox"})

Maybe this tutorial can be of help: How to create a DropDownList with ASP.NET MVC