TestViewExtension.cs

public static class TestViewExtension
    {
        public static string RenderViewUnderTest(this HtmlHelper helper, string controller, string action, object model)
        {
            var ctx = new ViewDataDictionary(model);
            helper.ViewContext.RouteData.Values["Controller"] = controller;
            helper.ViewContext.RouteData.Values["Action"] = action;

            var sw = new StringWriter();
            var viewContext = new ViewContext(helper.ViewContext, helper.ViewContext.View,ctx,helper.ViewContext.TempData, sw);

            var viewResult = ViewEngines.Engines.FindView(viewContext, action, string.Empty);
            
            viewResult.View.Render(viewContext, viewContext.Writer);
            sw.Close();
            var sb = sw.GetStringBuilder();
            var text = sb.ToString();
            text = stripDocType(text);
            text = stringHtmlTag(text);
            text = StringBodyTag(text);
            
            return text;
        }

        static string StringBodyTag(string text)
        {
            var body_begin = text.IndexOf("<body");
            if(body_begin >=0)
            {
                var body_end = text.IndexOf(">", body_begin);
                text = text.Remove(body_begin, body_end - body_begin + 1).Replace("</body>", "");    
            }
            
            return text;
        }

        static string stringHtmlTag(string text)
        {

            var html_begin = text.IndexOf("<html");
            if (html_begin >= 0)
            {
                var html_end = text.IndexOf(">", html_begin);
                text = text.Remove(html_begin, html_end - html_begin + 1).Replace("</html>", "");
            }
            return text;
        }

        static string stripDocType(string text)
        {
            var doctype_begin = text.IndexOf("<!DOCTYPE");
            if(doctype_begin >=0)
            {
                var doctype_end = text.IndexOf(">", doctype_begin);
                text = text.Remove(doctype_begin, doctype_end - doctype_begin + 1);
            }
            return text;
        }
    }