-1 * int.MinValue == int.MinValue?? Is this a bug?

Question

In C# I see that

-1 * int.MinValue == int.MinValue

Is this a bug? It really screwed me up when I was trying to implement a search tree. I ended up using (int.MinValue + 1) so that I could properly negate it.

Answer 1

This is not a bug.

int.MinValue * -1 is 1 greater than int.MaxValue can hold. Thus, the number wraps around back to int.MinValue.

This is basically caused by an integer overflow.

Int32.MinValue:

The value of this constant is -2,147,483,648

Int32.MaxValue:

The value of this constant is 2,147,483,647

So, -2,147,483,648 * -1 = 2,147,483,648 which is 1 greater than Int32.MaxValue.

Answer 2

It's not a bug, it's an overflow.

In two's complement representation, the space of representable numbers is not symmetric. The opposite of the smallest integer cannot be represented. Computing it overflows and gives you the same number again.

Answer 3

int i = -1 * int.MinValue;

This doesn't even compile unless you disable checking:

error CS0220: The operation overflows at compile time in checked mode

Answer 4

No, it isn't a bug. It is the nature of twos complement integer arithmetic.

For example, let us take a signed byte value which goes between -128 and 127.

127(0x7f)+1 = 128(0x80). However, 0x80 is in fact the binary representation of -128.

Thus, for a byte, 128(0x80) = -128(0x80)

So -128(0x80) * -1 = 128(0x80) = -128(0x80)