Error: CS1617: Invalid option 6 for langversion

Error: CS1617: Invalid option 6 for langversion
00:39 by Anjan Kant


I was upgrading my MVC app from MVC 4 to MVC 5 in visual studio 2013 then eventually got message CS1617: Invalid option '6' for /langversion. I looked into issue very closely and then finally come up with the right solution. I am sharing the resolution with you all as given below. In my earlier article, I discussed also How to implement custom error pages using MVC application using Visual Studio.
Error CS1617 Invalid option 6 for langversion

Error Description

Error: CS1617: Invalid option '6' for /langversion; must be ISO-1, ISO-2, 3, 4, 5 or Default


I have just added langversion:5 in my application web.config, it was earlier langversion:6 and it resolved my issue. You just need to lookup into system.codedom and modify compilerOptions attribute from langversion:6 to langversion:5 and modify it accordingly. I have provided web.config code part needed to change as below:



While we are working on real time world application then we need to upgrade/change our existing application code to meet up real time challenge, sometimes changes can direct into new challenges. In the same way, today here, I have provided a resolution of error CS1617: Invalid option 6 for langversion.

Relevant Reading


MVC Compression: How to use GZIP Compression in MVC

MVC Compression: How to use GZIP Compression in MVC
09:01 by Anjan Kant


GZIP compression can help to our website to reduce its bandwidth usage. In this article, we can incorporate GZIP feature in our MVC app. We can enable GZIP header through our website, alternatively we can also enable GZIP through our website IIS. Here describing, how to use GZIP compression in our MVC app.

Already Discussed on ASP.Net MVC Security & Performance

Main benefits of GZIP compression are under below:

  • It helps to reduce sizes of our website pages.
  • It helps to increase speed of web pages.
  • It mounts to cost-benefit ratio high.


We need to add two namespaces before to write code of GZIP compression.

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

Code of GZIP Compress under FilterConfig.cs

Write this code under FilterConfig.cs in App_Start folder of your MVC App.

public class CompressAttribute : ActionFilterAttribute
        public override void OnActionExecuting(ActionExecutingContext filterContext)

            var _encodingsAccepted = filterContext.HttpContext.Request.Headers["Accept-Encoding"];
            if (string.IsNullOrEmpty(_encodingsAccepted)) return;

            _encodingsAccepted = _encodingsAccepted.ToLowerInvariant();
            var _response = filterContext.HttpContext.Response;

            if (_encodingsAccepted.Contains("deflate"))
                _response.AppendHeader("Content-encoding", "deflate");
                _response.Filter = new DeflateStream(_response.Filter, CompressionMode.Compress);
            else if (_encodingsAccepted.Contains("gzip"))
                _response.AppendHeader("Content-encoding", "gzip");
                _response.Filter = new GZipStream(_response.Filter, CompressionMode.Compress);

Call Compress Attribute in MVC Controller

Now call Compress attribute on your controller action to get done this functionality in your MVC application.

Working Sample:

Here, I am showing statistics of my web page difference between zipped and unzipped webpage.

Unzip Webpage:

unzipped Webpage

Unzip Webpage

Requests 34
Size124 KB
DOContentLoaded617 ms
Load689 ms

Gzip Webpage:

Gzip Webpage
Gzip Webpage

Requests 34
Size123 KB
DOContentLoaded523 ms
Load582 ms

Finally, you can find difference between unzip and zip web pages. It is very required that your web pages should be Gzip enabled through IIS or visual studio project so that it will increase performance of your web pages in web browser (Chrome, IE, FF, Opera etc.). Here, I have explained how to use GZIP compression in MVC app.

public ActionResult Index()
    /// write here your logic to pull data from database
    return View();


GZIP compression helps to our site pages and other resource files before sending them over to the program. This radically decreases exchange time since the documents are considerably littler. Here, I have explained how to use GZIP compression in MVC.