There are several c# project about QR code. Among them,
http://code.google.com/p/zxing/
looks one of the best as of now.
Tip
1. When reading QR code from jpg file, the image size should be relatively small. I thought that big image of QR code is better to read, but it was wrong. Small QR code image without background is the best. Relatively blur image(QR code that was taken by iphone) also works well with this library