NullReferenceException from NHibernate in a WCF Service

Don’t you hate it when you get an NullReferenceException? NHibernate 2.1.2 throws one if you have an WCF Service that accesses the NHibernate context. If you get one from NHibernate.Context.WebSessionContext.GetMap(), read on…

It’s really simple but easy to forget – NHibernate’s web session models requires you to have HttpContext available. That particular mentioned method calls HttpContext.Current.Items[…], causing a NullReferenceException when the HttpContext.Current returns null. Quite logical, but since you rarely call WebSessionContext directly, it’s also easy to miss.

The fix is a very straightforward one: Make your service run in ASP.NET Compatibility mode. As described in MSDN article on WCF and ASP.NET coexistence, WCF services have their own processing pipeline, and also have a null HttpContext. However, you can coerce your WCF requests into the ASP.NET Model by enabling compatibility mode. You do this by tagging your service class with an attribute:

[AspNetCompatibilityRequirements(RequirementsMode = AspNetCompatibilityRequirementsMode.Required)]
public class MyService : DataService<MyDataSource>

Once done, your service should execute fine. However, you might also get an InvalidOperationException with the following message:

The service cannot be activated because it requires ASP.NET compatibility. ASP.NET compatibility is not enabled for this application. Either enable ASP.NET compatibility in web.config or set the AspNetCompatibilityRequirementsAttribute.AspNetCompatibilityRequirementsMode property to a value other than Required.

This would indicate a need to add the following to your web.config:

<system.serviceModel>
    <serviceHostingEnvironment aspNetCompatibilityEnabled="false" />
</system.serviceModel>

Hopefully this will then finally get you back on the track. Smile

December 21, 2010 · Jouni Heikniemi · 4 Comments
Tags: , ,  · Posted in: .NET

4 Responses

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