在 EF Core 3.1 中定义可为空的拥有类型时，为什么会出现 NullReferenceException？

Question

I'm connecting to a Cosmos DB instance using Entity Framework Core 3.1.4.
I have a Broadcast class that contains a property of owned type Availability .
I don't want Availability to be mapped as an Entity. Availabilities don't have Ids, nor should they be stored in their own containers. Availabilities are nullable . The classes look like:

public class Broadcast
{
    public string id {get; set;}
    public string Name { get; set; }
    public Availability Availability { get; set; }
}

[Owned]
public class Availability
{
    public DateTime? Start { get; set; }
    public DateTime? End { get; set; }
}

public class BroadcastContext : DbContext
{
    public DbSet<Broadcast> Broadcast{ get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Broadcast>().OwnsOne(e => e.Availability).WithOwner();
    }
}

So an item inside the Broadcast container in the DB should look like:

{
   id: "GUID",
   Name: "Foobar Broadcast",
   Availability: {
     Start: FooDate,
     End: BarDate
   }
}

When trying to Add a new Broadcast object with "Availability" set to null, this line: modelBuilder.Entity<Channel>().OwnsOne(e => e.Availabilities).WithOwner(); throws the following exception:

System.NullReferenceException: 'Object reference not set to an instance of an object.'

I understand that EF Core 3.0 supports nullable Owned Types.
Am I missing something? How can I sort this out?.

Answer 1

When using navigation properties in EF, you want to use the virtual keyword. This will allow EF to perform an override on the property.

Try changing Broadcast to the following:

public class Broadcast
{
    public string id {get; set;}
    public string Name { get; set; }
    public virtual Availability Availability { get; set; } //added virtual keyword.
}