add watermark to powerpoint

I want 开发者_高级运维to add watermark to powerpoint file from c# by using powerpoint object library but I could not find any example/documentation which explains how to do it.

Is it possible at all? Any hints or suggestions?

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You need to make add Shape, set TextFrame.TextRange.Text to some string, set TextFrame.TextRange.Font to something big, rotate the shape, and send it to back.