Evans Gambit variation The generally very useful and interesting book Bologan's Black Weapons suggests the following line as a practical way to meet the Evans Gambit.

[FEN ""]
[Title "Evans Gambit"]
[Startply "12"]

1. e4 e5 2. Nf3 Nc6 3. Bc4 Bc5 4. b4 Bxb4 5. c3 Ba5 6. d4 b5!? 7. Bxb5 Nxd4


The line continues 7. Bxb5 Nxd4 8. Nxd4 (8. Nxe5 is less good, see analysis in the book) exd4 9. Qxd4 Qf6 with reasonable play for black.

However, the book doesn't mention the move 9. O-O !?. 

[FEN ""]
[Title "Evans Gambit"]
[Startply "17"]

1. e4 e5 2. Nf3 Nc6 3. Bc4 Bc5 4. b4 Bxb4 5. c3 Ba5 6. d4 b5!? 7. Bxb5 Nxd4 8. Nxd4 exd4 9. O-O!? (9. Qxd4 Qf6) dxc3 10. Bc4!


In a blitz game I played as black, I thought I should prevent white from building a big center with cxd4, so I played 9... dxc3. But then after 10. Bc4! I had a very bad position.

So my question is, what should black play after 9. O-O !??
After 9.0-0, 9....dxc3 is indeed too dangerous. After 10.Bc4! black cannot finish his development as white's threats Qd5/Qh5/Bxf7+ are too strong.

However, 9....c6 seems to be a good move. White's strongest reply is probably 10.Bc4, as both 10.Ba4 Ba6 and 10.Be2 dxc3 are fine for black.
After 10.Bc4, black can play 10....Ne7 and continue his development with 0-0 and d5. For instance, 11.cxd4 0-0 and black shouldn't be afraid to sacrifice a pawn after 12.Ba3 d5! 13.exd5 cxd5 14.Bxe7?! Qxe7 15.Bxd5 Rb8 because black's bishop pair and development are more important than the weak pawn on d4. Instead, white should play 14.Bd3 and after 14....Re8, the position is more or less equal.

  [StartPly "17"]

  [FEN ""]
  1.e4 e5 2.Nf3 Nc6 3.Bc4 Bc5 4.b4 Bxb4 5.c3 Ba5 6.d4 b5 7.Bxb5 Nxd4 8.Nxd4 exd4 9.O-O c6 (9...dxc3 10.Bc4!) 10.Bc4 (10.Ba4 Ba6) (10.Be2 dxc3) 10...Ne7 11.cxd4 O-O 12.Ba3 d5! 13.exd5 cxd5 14.Bd3 (14.Bxe7?! Qxe7 15.Bxd5 Rb8) 14...Re8