ASP.NET 5 MVC: unable to connect to web server 'IIS Express' – Dev

The best answers to the question “ASP.NET 5 MVC: unable to connect to web server 'IIS Express'” in the category Dev.


What I’m doing:

  • Opening Visual Studio Community 2015
  • File -> New -> Project
  • Under Visual C#: Web -> ASP.NET Web Application
  • Web Application
  • And press f5 for the popup error “unable to connect to web server ‘IIS Express’.”

Deleting applicationhost.config, located in Documents\IISExpress\config, doesn’t change the error message. (There’s also an IISExpress folder in program files and program files (x86).)

Something I noticed, and I don’t know if it’s a problem:

Referenced file ‘lib/jquery-validation/jquery.validate.js’ not found.

I got a dump with rawcap but I don’t notice much in there. Some of what was there:

“Framework”:{“FrameworkName”:”DNXCore,Version=v5.0″,”FriendlyName”:”DNX Core 5.0″,”ShortName”:”dnxcore50″,”RedistListPath”:null}

I don’t notice a problem, but I have the network data if that can help figure out why I cannot connect to the web server. I get a RST,ACK immediately so I’m guessing the port is closed and whatever this web server is, isn’t being setup.

More on this problem: 800700c1 error from /trace:error

I’ve tried:

  • deleting applicationhost.config (and changing port number)
  • running visual studio as administrator
  • deleting IISExpress folder in Documents (changes error message until the folder is reinstalled)
  • toggling ssl off and on, copying url to launch box. (note: I’m not using ssl)
  • clearing all sfc /scannow errors
  • starting iisexpress with x86 version and 64-bit version


Exit VS and delete the (project)\.vs\applicationhost.config file. Restart VS. It should start working.


After installing Update 2 for Visual Studio 2015 I started getting the same error. I tried everything above with no luck. However, I found a solution that works for me:

  1. Delete YourSolutionFolder\\.vs\config\applicationhost.config file (note: .vs is a hidden folder)
  2. Open Visual Studio, right-click on web site > Properties > Debug tab > Web Server Settings > App URL – change port number.

If you have IIS configured to use the same port, (stop the application / use different port) and try again.


I had this issue on .net core 2.1, visual studio version 15.9.6. When i deleted the .vs folder, i reopened visual studio and the problem was still there. The solution that worked for me below:

  1. Delete .vs folder (it is a hidden folder).
  2. Restart Computer.

Other Solutions:

  • Close Visual Studio, delete bin and obj folders from the project folder. Open Project then rebuild solution.
  • Try running Visual Studio as Administrator.
  • Clean Solution then rebuild
    1. Clean Solution. 2) Exit Visual Studio. 3) delete .vs folder 4) Re-open Visual Studio
  • Restart machine (if you can afford to)


Try using a different port. I just set up a new Core 1.0 RC1 project and got the same error.

  • Right click web project
  • Debug tab
  • Toggle Enable SSL off and on again, it should generate a new random port
  • Copy the SSL URL and paste into Launch URL box
  • Run the project

Worked for me (TM).

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