They had finally started work on Patronuses, which everybody had been very keen to practice, though, as Harry kept reminding them, producing a Patronus in the middle of a brightly lit classroom when they were not under threat was very different from producing it when confronted by something like a Dementor.

-- OotP Chapter 27: The Centaur and the Sneak

A hundred dementors were advancing, gliding toward them, sucking their way closer to Harry's despair, which was like a promise of a feast. . . . He saw Ron's silver terrier burst into the air, flicker feebly, and expire; he saw Hermione's otter twist in midair and fade, and his own wand trembled in his hand, and he almost welcomed the oncoming oblivion, the promise of nothing, of no feeling. . . . And then a silver hare, a boar, and fox soared past Harry, Ron, and Hermione's heads: the dementors fell back before the creatures’ approach. Three more people had arrived out of the darkness to stand beside them, their wands outstretched, continuing to cast Patronuses: Luna, Ernie, and Seamus.

-- DH Chapter 32: The Elder Wand