My entity has a property which is allowed to be null. BUT, if it isn't null, then it must be unique. In other words, the column is unique but allows multiple nulls.
config.Property(p => p.ProductId).IsRequired(false);
I remember struggling to get this to work in pre-Core EF.
Is this possible? How do I configure the entity?
Yes, you can do that with EF Core, as a Unique index by default is created as a filtered index (WHERE ... IS NOT NULL)
config.Entity<Product>() .HasIndex(b => b.ProductId) .IsUnique();