Get a specific bit from byte

Question

I have a byte, specifically one byte from a byte array which came in via UDP sent from another device. This byte stores the on/off state of 8 relays in the device.

How do I get the value of a specific bit in said byte? Ideally an extension method would look the most elegant and returning a bool would make the most sense to me.

public static bool GetBit(this byte b, int bitNumber) {     //black magic goes here } 

Answer 1

Easy. Use a bitwise AND to compare your number with the value 2^bitNumber, which can be cheaply calculated by bit-shifting.

//your black magic var bit = (b & (1 << bitNumber-1)) != 0; 

EDIT: To add a little more detail because there are a lot of similar answers with no explanation:

A bitwise AND compares each number, bit-by-bit, using an AND join to produce a number that is the combination of bits where both the first bit and second bit in that place were set. Here's the logic matrix of AND logic in a "nibble" that shows the operation of a bitwise AND:

  0101 & 0011   ----   0001 //Only the last bit is set, because only the last bit of both summands were set 

In your case, we compare the number you passed with a number that has only the bit you want to look for set. Let's say you're looking for the fourth bit:

  11010010 & 00001000   --------   00000000 //== 0, so the bit is not set    11011010 & 00001000   --------   00001000 //!= 0, so the bit is set 

Bit-shifting, to produce the number we want to compare against, is exactly what it sounds like: take the number, represented as a set of bits, and shift those bits left or right by a certain number of places. Because these are binary numbers and so each bit is one greater power-of-two than the one to its right, bit-shifting to the left is equivalent to doubling the number once for each place that is shifted, equivalent to multiplying the number by 2^x. In your example, looking for the fourth bit, we perform:

       1 (2^0) << (4-1) ==        8 (2^3) 00000001       << (4-1) == 00001000 

Now you know how it's done, what's going on at the low level, and why it works.