interrupting a lengthy sequence

I have a lengthy sequence of operations that are executed and reported on in a status window in a Windows Form application. Some work is done by background threads but other work is not. I am wondering how to recognize if the user presses an Escape (or even just a Shift key if that is simpler) so I may then abort the remaining foreground operations. To recognize a shift key I tried:
if ((Control.ModifierKeys & Keys.Shift) == Keys.Shift) { ... }
My code included a Thread.Sleep(100) every so often to allow the user keystrokes to be recognized, or so I thought. But neither keystrokes nor button clicks are being recognized. What am I missing?

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