Support value conversions from two properties into one column

[bcallaghan-et]

I have a several classes that contain property pairs like the following

class MyEntity
{
    public int MyValue { get; set; }
    public bool MyValueSpecified { get; set; }
}

I would like EF to ignore MyValue and store null in its place if MyValueSpecified is false. Otherwise, MyValue should be mapped to the database as normal. The column behind this pairing should be nullable, but otherwise match the type of MyValue. To do this, I propose a new method on the model builder that allows multiple properties mapped to the same column. For example:

modelBuilder.Entity<MyEntity>()
    .Property(entity => new { entity.MyValue, entity.MyValueSpecified })
    .HasValueConversion(
        x => x.MyValueSpecified ? x.MyValue : null,
        x => x is null ? (false, default) : (true, x)
    );

I realize that this looks a lot like nullable value types, and I would rather use int? in my entities, but these classes are auto-generated by WCF and are outside of my control.

Implementing this feature probably depends on #13850.

[ajcvickers]

Similar to #13947. They should probably both be implemented together,

[AndriySvyryd]

Duplicate of #31234