New York City Mayor Bill de Blasio made a rare joint appearance with predecessor Michael Bloomberg to celebrate the groundbreaking of Cornell University's technology campus expansion on Roosevelt Island, which received a surprise $100 million donation by Bloomberg.

The 108th and 109th mayors made occasional conversation and posed for photos coordinated by aides, but were seated on opposite sides of the stage at Franklin D. Roosevelt Four Freedoms Park.

Still, de Blasio said he wanted to give "a true compliment" to Bloomberg, citing his "extraordinary work" and "extraordinary generosity" for shepherding Cornell's expansion during his mayoralty and making the nine-figure donation to name a building after his daughters, Emma and Georgina Bloomberg.

"Largely due to his will and his sense of vision, we are here today," de Blasio said.

Bloomberg, in turn, praised the role of de Blasio, whose team picked up where Bloomberg's left off on the project.

"Bill, you and your administration -- thank you for all the support that you have given," Bloomberg said. "As we all know none of these things ever end. They're going to need a lot of support from you and your successors down the road, so we're all in this together."

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The project dates to 2011, when Cornell Tech won a city contest that came with millions of dollars and city-owned land to jump-start the expansion, the first phase of which is set to open in the summer of 2017.

The comity Tuesday was a contrast with de Blasio's frequent criticism of Bloomberg on issues such as income inequality, real estate development and policing during de Blasio's campaign for mayor.

The men have rarely been seen together since speakers at de Blasio's January 2014 inauguration slammed Bloomberg's record, with him seated and stone-faced nearby.

Tuesday, the two men stood in separate groups during a pre-ceremony photo-op as a Cornell official described the project for reporters: de Blasio asked questions with City Council Speaker Melissa Mark-Viverito, as Bloomberg chatted in the background with two of his former deputy mayors, Robert K. Steel and Patricia Harris.

The de Blasio administration did not find out until a day before that Bloomberg was planning to make the donation.

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"That was a -- something they kept close to the vest," de Blasio said after.

Lt. Gov. Kathy Hochul, appearing on behalf of Gov. Andrew M. Cuomo, referenced the surprise.

"Mayor Bloomberg, I have to tell you," Hochul said, in a ceremony that lasted more than an hour. "You know how to steal the show."

Later, when asked by a reporter how it felt to share a stage with his successor, Bloomberg said only, "It felt great," and hurried away.