Turn on leftmost bit of a Short.

Question

Original question changed.

I want to bitwise turn off the left most bit of a Short value (&H8000) and leave the other bits as they are.

Dim x = BitConverter.GetBytes(Short.MaxValue Or &H8000)
Dim z = BitConverter.ToInt16(x, 0)

Isn't there any shorter way with bitwise operators?

When I do

Dim a = Short.MaxValue Or &H8000

I get a compiler error, cuz it goes up, instead of negating it.

Answer 1

That's because .NET uses two's complement, not signed magnitude. To negate, you have to flip all the bits and then add one.

Please just use the unary minus. Please...Pretty please.

EDIT: If for some strange reason you had to do this with bitwise operations, you would have to do this:

Dim somewhatCorrect = (Short.MaxValue xor Short.MinValue) + 1;

which of course is still not bitwise because two's complement negation cannot be done efficiently with bitwise operators.

EDIT2: And here's an unary minus:

Dim correct = -Short.MaxValue;

EDIT3: In response to edited question:

Dim x As Short = 42
Dim xWithHighBitOn = x Or Short.MinValue

Answer 2

Two's complement

Dim testV As Short = &HD555S 'a test value

Dim twosC As Short 'twos comp

twosC = (testV Xor &HFFFFS) + 1S 'reverse the bits, add 1

Change most significant bit

twosC = twosC Xor &H8000S