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 . 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:

In my case, you would replace this with:

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

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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.

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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.