Execute a Razor view server side in a MVC project. I'm returning the HTML in a JSON blob for this example

PreExecutingResult.cs

public class PreExecutingResult : JsonResult {
  private ViewResult viewResult;
  private object model;
  
  public PreExecutingResult(string viewName, object model) {
    viewResult = new ViewResult {
      ViewName = viewName
    };
    
    viewResult.ViewData.Model = model;
    this.model = model;
  }
  
  public override void ExecuteResult(ControllerContext context)
  {
      var builder = new StringBuilder();
      using (var stringWriter = new StringWriter(builder))
      {
          using (var htmlTextWriter = new HtmlTextWriter(stringWriter))
          {
              var controller = context.Controller;
              var currentContext = controller.ControllerContext.HttpContext;

              try
              {
                  var currentRequest = HttpContext.Current.ApplicationInstance.Context.Request;
                  var currentUser = HttpContext.Current.ApplicationInstance.Context.User;

                  var newResponse = new HttpResponse(htmlTextWriter);

                  var newContext = new HttpContextWrapper(
                              new HttpContext(currentRequest, newResponse)
                              {
                                  User = currentUser
                              });

                  controller.ControllerContext.HttpContext = newContext;

                  viewResult.ExecuteResult(controller.ControllerContext);

                  newResponse.Flush();
                  htmlTextWriter.Flush();
                  stringWriter.Flush();

              }
              finally
              {
                  // and no matter what happens, set the context back!
                  controller.ControllerContext.HttpContext = currentContext;
              }
          }
      }

      Data = new
                 {
                     html = builder.ToString(),
                     model = model
                 };

      base.ExecuteResult(context);
  }
}