MVC :: Clean Way Chain Html Attributes In View?

I would like to add html attributes to form inputs, specifically disabled="disabled" in addition to others. I have conditional logic in the Controller that determines whether to add this html attribute or not that sets a bool IsDisabled flag in my ViewModel e.g.

class ViewModel
{
bool IsDisabled {get;set;}
}

So in my View, I want to do something like:

<%= Html.TextBoxFor(x => x.Data, new {maxlength = 30, IsDisabled ? disabled = "disabled": null}) %>

or rather something like

<%= Html.TextBoxFor(x => x.Data, new Dictionary<string, object>().Add("maxlength", "30").AddConditional("disabled", "disabled", IsDisabled) %>

Except that Add returns void, and there is no such thing as AddConditional. Is there something like an MVC HtmlAttributeBuilder class, or should I just extend Dictionary with these two extension methods?