Casting an int to a short gives unexpected results

Question

I'm a little confused about loss information on numeric types in C#.

When i do this:

int x = 32780;
short y = (short)x;

I have the result: -32756 for y and not the expected 32767. Why? How this is calculated?

Range of short: -32768 to 32767 Range of int: -2,147,483,648 to 2,147,483,647

Answer 1

You seem to be expecting a “rounding down” effect rather than what is actually happening, which is a bitwise re-interpretation of the data.

In binary, x is equal to 00000000000000001000000000001100, which is a 32-bit number with only 16 significant bits. A short is a 16-bit signed integer which is represented using two’s complement notation.

When you convert, the last 16 bits of your x are being copied into y, giving 1000000000001100. Importantly here, the first digit is a 1. In two’s-complement notation, this is -32756. Your number hasn't been rounded—it’s been read as though it were 16-bit.