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Error message after you install the .NET Framework 4.0: "Could not load type 'System.ServiceModel.Activation.HttpModule'"

Symptoms

Consider the following scenario. You install the .NET Framework 4.0. Then, you install an earlier version of the .NET Framework, or you enable .NET 3.0 WCF HTTP Activation. In this scenario, you may receive the following error message when you when run applications that are hosted on Internet Information Services (IIS):

Could not load type 'System.ServiceModel.Activation.HttpModule' from assembly 'System.ServiceModel, Version=3.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.TypeLoadException: Could not load type 'System.ServiceModel.Activation.HttpModule' from assembly 'System.ServiceModel, Version=3.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'.

Cause

This issue occurs because the Applicationhost.config file for Windows Process Activation Service (WAS) has the following section defined, and this section is incompatible with the .NET Framework 4.0:

<add name="ServiceModel" type="System.ServiceModel.Activation.HttpModule, System.ServiceModel, Version=3.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" preCondition="managedHandler" />

This section should be defined as follows:

<add name="ServiceModel" type="System.ServiceModel.Activation.HttpModule, System.ServiceModel, Version=3.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" preCondition="managedHandler,runtimeVersionv2.0" />

Note You can find the Applicationhost.config file in the following location:

%windir%\system32\inetsrv\config

Resolution

To resolve this issue, run the following command line: 

aspnet_regiis.exe /iru

The Aspnet_regiis.exe file can be found in one of the following locations:

%windir%\Microsoft.NET\Framework\v4.0.30319

%windir%\Microsoft.NET\Framework64\v4.0.30319 (on a 64-bit computer)