[BsonDiscriminator(RootClass = true)]
    [BsonKnownTypes(typeof(Apple))]
    [BsonIgnoreExtraElements]
    public abstract class Fruit {
       [BsonIgnore]        
       public abstract string Color { get; }    
    }

    [BsonIgnoreExtraElements]
    public class Apple : Fruit 
    {
        [BsonConstructor]
        public Apple( int seeds ) {
            Seeds = seeds;
        }

        public int Seeds { get; }
        
        [BsonIgnore] 
        public override string Color => "Red";
    }

 [BsonConstructor] 
 public Apple(int seeds) : base ( color: "Red" )
 {
    Seeds = seeds; 
 }