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

Question

What I\'m doing:

• Opening Visual Studio Community 2015
• File -> New -> Project
• Under Visual C#: Web -> ASP.NET Web Application
• Web App

Answer 1

I won't pretend to fully understand what MS bug creates this problem, but here is another potential solution:

In the .vs/config/applicationHost.config file, find the section for <system.applicationHost><applicationPools>. Under the pools, ensure that the managedRuntimeVersion attribute value matches the value which is in the IIS config for the system (and/or the version of the .NET framework which is installed).

For example, you may find (as I did) that the generated file has:

    <add name="Clr4IntegratedAppPool" managedRuntimeVersion="v4.0" managedPipelineMode="Integrated" CLRConfigFile="%IIS_USER_HOME%\config\aspnet.config" autoStart="true" />

In my case, you would replace this with:

    <add name="Clr4IntegratedAppPool" managedRuntimeVersion="v4.0.30319" managedPipelineMode="Integrated" CLRConfigFile="%IIS_USER_HOME%\config\aspnet.config" autoStart="true" />

Note the replacement from "v4.0" to "v4.0.30319". This resolved the issue.

---

What appears to be going on:

I believe that VS is generating an applicationHost.config file with "default" versions for the .NET framework, which may not match the specific version which is installed/configured on the system. You can debug/observe this issue by tracing the execution in Process Monitor, and finding the command line for iisexpress.exe. Running this command with /trace:error added yields a more informative message about a failure to preload the CLR with version v4.0. To wit:

Starting IIS Express ...
Failed while trying to preload CLR version v4.0. hr = 80131700
Failed to initalize the W3WP_HOST hr = 80131700
Process Model Shutdown called
Unable to start iisexpress.

---

Anyway, figured this might be helpful to someone else, since it's common enough to have multiple references online with bad information, and I've personally hit it a few times now.

Answer 2

My solution (for .net core 2.0) was that i had forgot to add the port number in the applicationUrl, under iisExpress in launchSettings.json

"iisExpress": {
  "applicationUrl": "https://localhost:50770",
  "sslPort": 50770
}

Answer 3

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).

Answer from: https://stackoverflow.com/a/28650554/134761

Answer 4

TO CLARIFY

Really a lot of answers here are the same and say something like "Restart and it magically works again".

Well, 9 out of 10 times people have this issue like the OP it is because THE IP-ADDRESS IS ALREADY IN USE.

ANSWER

There could be 2 ip-addresses that are in use. Both of them you can find by:

1) Right-clicking on the start-up project

2) Click on "Properties"

3) Click on the "Debug" tab

Here you see your "App URL" and your "SSL URL".

• If your "App URL" is in use, just change it there and save it and it should work again.

• If your "SSL URL" is in use, close down VS, delete the "applicationhost.config" file in the hidden .vs folder of your project and open VS up again.

Answer 5

In my case that was some other application listening on the same port which IIS Express trying to attach to. I have to run netstat -ao to see PID of process which is use same port and shutdown application. In my case application was Viber.

Answer 6

Just close the visual studio and reopen and execute. It worked for me.