Shifting the sign bit in .NET

Question

I'm reading bits from a monochrome bitmap. I'm storing every 16 bits in a short in the reverse order. If the bit in the bitmap is black, store a 1. If white, store a 0.

E.g.: for bitmap: bbbw bbbw bbbw wwww
my short is: 0000 0111 0111 0111

The 1st way I tried to do this was:

short m;
// ...
Color c = bmp.GetPixel(j, i);
if (c.R == Color.Black)
    m |= short.MinValue;
m >>= 1;
// ...

After one assignment and shift, I got the expected -32768 (1000 0000 0000 0000).
After the 2nd time I got -16384 (1100 0000 0000 0000).

I changed my code to use ushort and changed the if line to s |= (ushort)Math.Pow(2, 15); and now it works.

My question is: why will the sign bit not shift in .NET? Is there a way to shift the sign bit?

Answer 1

In C#, shifts are arithmetic shifts (in contrast to logical shifts). In a right arithmetic shift, the sign bit is shifted in on the left, so the sign of the number is preserved. A right shift is equivalent to dividing by 2:

If you want a logical shift (no sign extension), use unsigned numbers:

Answer 2

http://msdn.microsoft.com/en-us/library/k2ay192e.aspx

"The >> operator shifts the bits of expression1 right by the number of bits specified in expression2. The sign bit of expression1 is used to fill the digits from the left. Digits shifted off to the right are discarded. The data type of expression1 determines the data type returned by this operator."