public static MvcHtmlString TextBoxFor(this HtmlHelper html, ModelProperty prop, object htmlAttributes)
{
    prop.ControlHtmlAttributes = HtmlHelper.AnonymousObjectToHtmlAttributes(htmlAttributes);
    return PartialExtensions.Partial(html, "MyView", prop);
}

Html.TextArea(Model.ControlName, Model.Value, Model.ControlHtmlAttributes);

public static object ToAnonymousObject(this IDictionary<string, object> @this)
    {
        var expandoObject = new ExpandoObject();
        var expandoDictionary = (IDictionary<string, object>) expandoObject;

        foreach (var keyValuePair in @this)
        {
            expandoDictionary.Add(keyValuePair);
        }
        return expandoObject;
    }

Html.TextArea(Model.ControlName, Model.AsString(), Model.ControlHtmlAttributes);