Conditionally disable Html.DropDownList

There is no need to add helper methods, you can just use

<%= Html.DropDownList("Quantity", new SelectList(...), IsEditable == true ? new { @disabled = "disabled" } as object : new {} as object %>

If you were to remove the as object entries this wouldn't work because by default new {} is a dynamic object compiled at runtime, therefore the two possible objects must have the same properties. But the Html attributes parameter is actually just an object, so these dynamics can be cast as objects to get around this.

This solution even allows you to use multiple HTML attributes where one is optional and another is not, i.e class='whatever' is not optional but disabled is so you put class='whatever' in both the objects, but the optional one only in the first. Dimitrov's answer does not support any custom attributes other than disabled.

Please don't write spaghetti code. Html helpers are there for this purpose:

public static MvcHtmlString DropDownList(this HtmlHelper html, string name, SelectList values, bool canEdit)
{
    if (canEdit)
    {
        return html.DropDownList(name, values);
    }
    return html.DropDownList(name, values, new { disabled = "disabled" });
}

And then:

<%= Html.DropDownList("Quantity", new SelectList(...), Model.CanEdit) %>

Or maybe you could come up with something even better (if the model contains the options):

<%= Html.DropDownList("Quantity", Model) %>

You will also get the bonus of having more unit testable code.