Harry: "Can't house-elves speak their minds about their masters, then?"
Dobby: "Oh no, sir, no. 'Tis part of the house-elf's enslavement, sir. We keeps their secrets and our silence, sir. We upholds the family's honor, and we never speaks ill of them - though Professor Dumbledore told Dobby he does not insist upon this. Professor Dumbledore said we is free to - to... He said we is free to call him a - a barmy old codger if we likes, sir!"