IIS Express startup error “Failed while trying to preload CLR version v4.0″

Today, when I started my first ISAPI development and testing with Delphi for Habari Client message broker libraries, a fresh installation of Internet Information Server 7.5 (IIS Express) failed to start.

Launching with the debug parameter /trace:e gave this cryptic message:

Failed while trying to preload CLR version v4.0.  hr = 8007000d
Failed to initalize the W3WP_HOST  hr = 8007000d
Process Model Shutdown called
Failed to start 'HostedWASStart'.  Error = 28152744
HostableWebCore activation failed.

The “Failed while trying to preload CLR version v4.0” was the helpful hint. The reason of the startup error was simply a missing .NET 4.0 framework on the machine. To fix it, this line in C:\Users\Documents\IISExpress\config\applicationhost.config

<applicationDefaults applicationPool="Clr4IntegratedAppPool" />

had to be changed to

<applicationDefaults applicationPool="Clr2IntegratedAppPool" />

to switch to .NET framework 2.

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