[6.0] Mark internal the ValueConverter constructors that allow conversion of nulls

[ajcvickers]

Resolves #26230.

Description

In 6.0 we implemented Allow HasConversion/ValueConverters to convert nulls. However, this has proved to be very problematic in practice with many pitfalls. For example:

• Value conversion to null in the store generates bad queries
• Value conversion from null in the store generates bad queries
• Value converters do not handle cases where the database column has multiple different values that convert to the same value
• Allow value converters to change nullability of columns

These are not trivial issues to fix and for the query issues they are not easy to detect and will result in unexpected/bad data.

This makes the feature somewhat a pit-of-failure. However, the issue has 43 votes on GitHub, putting it in the top 25 and meaning that people are likely already using it. Therefore, we are proposing to mark these APIs as internal for 6.0. This still allows people to use them, but behind an analyzer warning and documentation. (This is similar to marking them as experimental--see EntityFrameworkExperimentalAttribute) We can then make them fully public in a future release after gathering more feedback and possibly making improvements.

Customer impact

Customers will get a warning when trying to use this feature. If they proceed (suppressing or ignoring the warning), it will still work as it does now, but they will hopefully be drawn to the caveats outlined above.

How found

Validating and documenting new features in EF Core 6.0.

Regression

No.

Testing

N/A

Risk

Low. The change marks the constructors that enable this as internal, but doesn't change them. We also make sure that none of our built-in converters use this feature.

[ajcvickers]

@Pilchie For 6.0. However, I think it makes sense to send this one through Tactics.

[Pilchie]

Agreed - I think a Tactics discussion makes sense for this one.