Allow properties to use a different backing field on a derived type

[anranruye]

EF Core version: 5.0.2
Database provider: Microsoft.EntityFrameworkCore.SqlServer
Target framework: .NET 5.0
Operating system: Windows 7
IDE: Visual Studio 2019 16.8

Consider we have the following entities:

    public class BaseEntity
    {
        public int Id { get; set; }

        public virtual string MyProperty { get; set; }
    }

    public class DerivedEntity : BaseEntity
    {
        private int myProperty ;

        [BackingField(nameof(myProperty))]
        public override string MyProperty 
        { 
            get => MyDecoder.Decode(myProperty) ; 
            set => myProperty = MyEncoder.Encode(value); 
        }
    }

When we query for DerivedEntity, we will lose MyProperty property values. This is because the backing field of the base class is used, rather than the backing field of the derived class. Fluent api also does't work:

    modelBuilder.Entity<DerivedEntity>().Property(x => x.MyProperty).HasField("myProperty");

A workaround is to use same backing field for both base and derived classes. However, it's not always possible to change the base class.

I don't know whether this is a by-design/known limitation. I don't find this in the pages:
https://docs.microsoft.com/en-us/ef/core/modeling/inheritance
https://docs.microsoft.com/en-us/ef/core/modeling/backing-field.

[ajcvickers]

@anranruye Thanks for reporting this. I'm putting this on the backlog for now, but this will be non-trivial to implement and so we are only likely to do so we significantly more feedback indicating that this is important. You can instead access the property through its navigation which will produce the correct results.

[anranruye]

@ajcvickers Fine.

I found this just by accident and don't really need this feature.