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NHibernate–Using record types

At one of my customers, we have been using NHibernate for a really long time. And although I think that with the latest Entity Framework Core, the feature set is almost on par, we are still using NHibernate today.

Of course, we want to use the newest C# features including Record types.

Let’s see if (and how) we can use record types in combination with NHibernate.

Here is a simple record type using positional parameters:

And the mapping code using Fluent NHibernate:

If we try to use this type in our application, NHibernate starts to complain with the following error message:

   ---- NHibernate.InvalidProxyTypeException : The following types may not be used as proxies:

    Tests.Domain.Category: type should have a visible (public or protected) no-argument constructor

    Tests.Domain.Category: method Deconstruct should be 'public/protected virtual' or 'protected internal virtual'

    Tests.Domain.Category: method get_Id should be 'public/protected virtual' or 'protected internal virtual'

    Tests.Domain.Category: method set_Id should be 'public/protected virtual' or 'protected internal virtual'

    Tests.Domain.Category: method get_CategoryName should be 'public/protected virtual' or 'protected internal virtual'

    Tests.Domain.Category: method set_CategoryName should be 'public/protected virtual' or 'protected internal virtual'

    Tests.Domain.Category: method get_Description should be 'public/protected virtual' or 'protected internal virtual'

    Tests.Domain.Category: method set_Description should be 'public/protected virtual' or 'protected internal virtual'

This makes sense as in NHibernate by default lazy loading is enabled. To use lazy loading a proxy class need to be created and this requires that all properties are declared virtual.

In this case I don’t need lazy loading for this class, so let’s disable this in our mapping file:

This solves a part of the errors above. If we try to run our application again, we still got an error:

FluentNHibernate.Cfg.FluentConfigurationException : An invalid or incomplete configuration was used while creating a SessionFactory. Check PotentialReasons collection, and InnerException for more detail.

    ---- NHibernate.InstantiationException : Object class NHibernate.Tests.Domain.Category must declare a default (no-argument) constructor

NHibernate needs a (private) default constructor without arguments to be able to create the type. As we are using a positional Record type, only a constructor with the 3 properties as argument is available. Although some ‘hacks’ exist to get rid of this error in NHibernate, the easiest solution is to switch to the standard property syntax and introduce an extra constructor:

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