XOR drawing doesn't work right in Bitmap

Question

I'm writing an Image processing program. To make the selection rectangle, I use R2_XORPEN from GDI, when I draw it on Form, the rubber band rectangle work well. The problem is when I draw it on a Bitmap, the XOR operation seems not work. Imagine : I draw a rectangle, then draw the 2nd one with the same coordinate but it doesn't clear the 1st one (it's expected to do this because of XOR operation) I think this's because XOR operation doesn't work on Bitmap. Anyone ever see this problem ??