Take a look at this question (and answer) on stackoverflow, I think that's it:
OpenFileDialog: Select Multiple Files Using a Check Box[
^]
You need to build your own OpenFileDialog to achieve this. Create a Class and inherit from FileDialog, because OpenFileDialog is a sealed Class and can not be inherited or extended.
The
Customizing OpenFileDialog in .NET (OpenFileDialogEx) Project could give you information to implement a nice Solution. Take a look.