C# nullable refs and virtual vs abstract properties

Consider:

public class InitialStep2Sqlite : ScriptMigration
{
    protected override string UpPath => "Sqlite2.Up.sql";
    protected override string DownPath => "Sqlite2.Down.sql";
}

public abstract class ScriptMigration : Migration
{
  protected virtual string UpPath {get;}
  protected virtual string DownPath {get;}

  protected override void Up(MigrationBuilder mb) 
    => mb.RunSqlFile(UpPath);
  protected override void Down(MigrationBuilder mb) 
    => mb.RunSqlFile(DownPath);
}

which, compiled under nullable enable, says “[CS8618] Non-nullable property 'UpPath' must contain a non-null value when exiting constructor. Consider declaring the property as nullable.”

Changing the property's virtual to abstact removes the warning.

The lesson: If your non-null proof of a property depends crucially on the subclass making it non-null, then making the parent property abstract not virtual is the correct statement of intent.

Generally, if a parent Member depends on the subclass for correctness – if the parent can't provide a correct implementation itself – then, logically, its correctness requires it to be abstract not virtual.