Merging PDFs programatically while maintaining the "Combine files…" bookmark structure?

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C#2.0

C#

PDF

I have to merge a set of many PDF files into a single file. I currently have code that will combine pdf files, but it does not keep the bookmark structure the same way that "Combine supported files in Acrobat" does.
In other words, say I have three files called "A.pdf", "B.pdf", and "C.pdf", and each file contains two bookmarks called "Bkmrk 1" and "Bkmrk 2". I want to programatically combine these three files into a single file that has 9 bookmarks that look like the structure below:

A
    Bkmrk 1
    Bkmrk 2
B
    Bkmrk 1
    Bkmrk 2
C
    Bkmrk 1
    Bkmrk 2

I used pdfsharp to insert bookmark but there is no option to get bookmark (outline) of existing PDF files also want to get same bookmark heirarchy to create the bookmark structure on new merged PDF.

Any idea or suggestions would be greatly appreciated!