return result as PartialViewResult, and translation from PartialViewResult to string.

sample.cs

public ViewResultBase Render(HtmlHelper helper)
{
    return new PartialViewResult
    {
        ViewName = "AnyPartialViewResult",
        ViewData = new ViewDataDictionary(helper.ViewData)
        {
            Model = new AnyPartialViewModel()
        },
        TempData = helper.ViewContext.TempData
    };
}

public static IHtmlString ToHtml(this ViewResultBase result, HtmlHelper html)
{
    using (var writer = new StringWriter())
    {
        var context = html.ViewContext.Controller.ControllerContext;
        var viewEngineResult = result
            .ViewEngineCollection
            .FindPartialView(context, result.ViewName);
        result.View = viewEngineResult.View;
        var viewContext = new ViewContext(
            context,
            result.View,
            result.ViewData,
            result.TempData,
            writer);
        result.View.Render(viewContext, writer);
        viewEngineResult.ViewEngine.ReleaseView(context, result.View);
        return new MvcHtmlString(writer.ToString());
    }
}