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Derived classes and Entity Framework Mapping

If you wanted to add extra (unmapped) data to an Entity Framework model, you might think to create a derived class to hold the extra fields, but this is a mistake and will cause more problems.

The usual way to add unmapped data to an EF class is to stick the [NotMapped] attribute on the new field, but for some reason on this occassion I did it differently, creating a derived class. Say I originally had a Ship class, and then wanted to link some crew but didn’t want to map that, I made a new class ShipWithCrew : Ship.

The first problem you’ll encounter is “Invalid column name ‘Discriminator’.” as soon as your code touches the derived class. The solution is to decorate the new ShipWithCrew class (the whole thing) as [NotMapped], which will get you started. But then when you try to cast back to the base class to write to DB, you’ll have a new problem:

var dbShip = (Ship)shipWithCrew;

This gave me the error: “Mapping and metadata information could not be found for EntityType ShipWithCrew”. Weird. I cast it… isnt’t it just writing a Ship to the ShipRepository? What’s the problem? Well, check out this surprising C# fact:

var shipWithCrew = new ShipWithCrew();

//Strictly cast back to the base class
Ship usingCast = (Ship)shipWithCrew;
Ship usingAs = shipWithCrew as Ship;

//Check the type is now Ship:

//Both output 'ShipWithCrew'!!

Even if you cast into an explicitly base-class-typed variable, the type will still be the derived class. And that’s why EF won’t write it to the database.

So the best approach is to go back to basics, suck it up, and just add a new [NotMapped] property to the original model class!