I’m struggling with an interesting problem with WCF REST since last night and I haven’t been able to track this down. I have a WCF REST Service set up and when accessing the .SVC file it crashes with a version mismatch for System.ServiceModel:

Server Error in '/AspNetClient' Application.

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Could not load type 'System.ServiceModel.Activation.HttpHandler' 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.HttpHandler' from assembly 'System.ServiceModel, Version=3.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'.
Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:

[TypeLoadException: Could not load type 'System.ServiceModel.Activation.HttpHandler' from assembly 'System.ServiceModel, Version=3.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'.]
   System.RuntimeTypeHandle.GetTypeByName(String name, Boolean throwOnError, Boolean ignoreCase, Boolean reflectionOnly, StackCrawlMarkHandle stackMark, Boolean loadTypeFromPartialName, ObjectHandleOnStack type) +0
   System.RuntimeTypeHandle.GetTypeByName(String name, Boolean throwOnError, Boolean ignoreCase, Boolean reflectionOnly, StackCrawlMark& stackMark, Boolean loadTypeFromPartialName) +95
   System.RuntimeType.GetType(String typeName, Boolean throwOnError, Boolean ignoreCase, Boolean reflectionOnly, StackCrawlMark& stackMark) +54
   System.Type.GetType(String typeName, Boolean throwOnError, Boolean ignoreCase) +65
   System.Web.Compilation.BuildManager.GetType(String typeName, Boolean throwOnError, Boolean ignoreCase) +69
   System.Web.Configuration.HandlerFactoryCache.GetTypeWithAssert(String type) +38
   System.Web.Configuration.HandlerFactoryCache.GetHandlerType(String type) +13
   System.Web.Configuration.HandlerFactoryCache..ctor(String type) +19
   System.Web.HttpApplication.GetFactory(String type) +81
   System.Web.MaterializeHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +223
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +184

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Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.0.30319.1

What’s really odd about this is that it crashes only if it runs inside of IIS (it works fine in Cassini) and only if ASP.NET Compatibility is enabled in web.config:

<serviceHostingEnvironment aspNetCompatibilityEnabled="true" multipleSiteBindingsEnabled="true" />

Arrrgh!!!!!

After some experimenting and some help from Glenn Block and his team mates I was able to track down the problem in ApplicationHost.config. Specifically the problem was that there were multiple *.svc mappings in the ApplicationHost.Config file and the older 2.0 runtime specific versions weren’t marked for the proper runtime. Because these handlers show up at the top of the list they execute first resulting in assembly load errors for the wrong version assembly.

To fix this problem I ended up making a couple changes in applicationhost.config. On the machine level root’s Handler mappings I had an entry that looked like this:

<add name="svc-Integrated" path="*.svc" verb="*" 
     type="System.ServiceModel.Activation.HttpHandler, System.ServiceModel, Version=3.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" 
     preCondition="integratedMode" />

and it needs to be changed to this:

<add name="svc-Integrated" path="*.svc" verb="*" 
     type="System.ServiceModel.Activation.HttpHandler, System.ServiceModel, Version=3.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" 
     preCondition="integratedMode,runtimeVersionv2.0" />

Notice the explicit runtime version assignment in the preCondition attribute which is key to keep ASP.NET 4.0 from executing that handler. The key here is that the runtime version needs to be set explicitly so that the various *.svc handlers don’t fire only in the order defined which in case of a .NET 4.0 app with the original setting would result in an incompatible version of System.ComponentModel to load.

What was really hard to track this down is that even when looking in the debugger when launching the Web app, the AppDomain assembly loads showed System.ServiceModel V4.0 starting up just fine. Apparently the ASP.NET runtime load occurs at a different point and that’s when things break.

So how did this break? According to the Microsoft folks it’s some older tools that got installed that change the default service handlers.

There’s a blog entry that points at this problem with more detail:

http://blogs.iis.net/webtopics/archive/2010/04/28/system-typeloadexception-for-system-servicemodel-activation-httpmodule-in-asp-net-4.aspx

Note that I tried running aspnet_regiis and that did not fix the problem for me. I had to manually change the entries in applicationhost.config.