JZUtil.cs

using System.IO;
using System.Web.Mvc;

namespace JZLib
{
  public static class JZUtil
  {
    public static string RenderViewToString(ControllerContext controllerContext, string viewName, object viewData = null)
    {
      using (var writer = new StringWriter())
      {
        var razorViewEngine = new RazorViewEngine();
        var razorViewResult = razorViewEngine.FindView(controllerContext, viewName, "", false);

        var viewContext = new ViewContext(controllerContext, razorViewResult.View, new ViewDataDictionary(viewData), new TempDataDictionary(), writer);
        razorViewResult.View.Render(viewContext, writer);
        return writer.ToString();
      }
    }
  }
}