Using null Model

Question

I have two classes:
Gallery with some attributes
Location with some attributes and one gallery.
I would like to have or not a gallery in location, so I did like this:

public Gallery? Gallery { get; set; }

The problem is that I'm getting this error:

Error   3   The type 'Gallery' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method 'System.Nullable<T>'

I've been looking for this error, but I could't find any solution (at least that I understood). My question is how can I fix this situation?

Gallery is (presumably) a class, which is naturally nullable and can't be used with ?, which is a decoration for a struct. If Gallery is optional on your Location model, indicate that with a nullable id property as the foreign key. For example, public int? GalleryId { get; set; } — Anthony Pegram
– Anthony Pegram, Commented Jun 7, 2015 at 22:36
Why do you use "Gallery?" while Gallery is a class, thus nullable type? — Alexander Bell
– Alexander Bell, Commented Jun 7, 2015 at 22:37
@AnthonyPegram: you could just define Gallery as optional in the ORM layer, no? That way you can keep the navigational property. — Jeroen Vannevel
– Jeroen Vannevel, Commented Jun 7, 2015 at 22:39
@JeroenVannevel, the Id property is in addition to the navigation property. You would define both. — Anthony Pegram
– Anthony Pegram, Commented Jun 7, 2015 at 22:48

Jon Hanna · Accepted Answer · 2015-06-07 23:00:53Z

2

public Gallery? Gallery { get; set; }

This means:

Gallery is a type that cannot be null.
The Gallery property can either be a Gallery or null.

The first of these isn't true. Therefore either:

Change Gallery so that it can't be null by changing class Gallery to struct Gallery.
Change the property to public Gallery Gallery { get; set; } to use the fact that Gallery as currently defined can already be null.

answered Jun 7, 2015 at 23:00

Jon Hanna

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