Do you need to mask a number before retrieving the value stored at a certain byte?

Question

I want to know if one needs to mask a number before retrieving the value stored at a certain byte when doing bit shifting.

Take, for example, this code:

short b1 = 1;
short b2 = 2;
short b0 = (short)((b1 << 8) | b2);         //store two values in one variable

Console.WriteLine(b0);                      //b1 and b2 combined
Console.WriteLine((b0 & (255 << 8)) >> 8);  //gets the value of b1

As far as I am concerned, doing a right shift drops all bits that are less than the number of bits you've shifted. Therefore, right shifting b0 by 8 bits will drop the 8 bits of b2, leaving just b1.

Console.WriteLine(b0 >> 8);  //this also gets b1!!

I want to find out, is there any need to mask b0 with 255 << 8 before shifting to get the value of b1?

NB:
The only need I can think of for masking before retrieving a value is if there is something else stored at a higher byte, such as trying to get back the value of b2, in which this code would be used:

Console.WriteLine(b0 & 255);  //gets the value of b2

This would be very obvious if you would make b1 and b2 bytes, and b0 a ushort. Right shifting ushort does not propagate the sign bit. — Michael Graczyk

David Schwartz David Schwartz · Accepted Answer · 2012-08-08T01:16:53

I want to find out, is there any need to mask b0 with 255 << 8 before shifting to get the value of b1?

No, there's no need. So the compiler will omit the masking. Some people think it makes the code easier to understand or protects them against some imagined failure scenario. It's completely harmless.

Do you need to mask a number before retrieving the value stored at a certain byte?

1 Answers