Why is a cast required for byte subtraction in C#?

Question

I have to following code in VS2008 .net 3.5 using WinForms:

byte percent = 70;
byte zero = 0;

Bitmap copy = (Bitmap)image1.Clone();
...

Color oColor = copy.GetPixel(x, y);
byte oR = (byte)(oColor.R - percent < zero ? zero : oColor.R - percent);

When I leave the "(byte)" off the last line of code, I get a compiler error saying it "Cannot implicitly convert type 'int' to 'byte'." If everything is of type byte and byte is an integer type... then why do I need to have the cast?

Solution 1

Because subtraction is coercing up to an integer. As I recall, byte is an unsigned type in C#, so subtraction can take you out of the domain of bytes.

Solution 2

That's because the result of a byte subtraction doesn't fit in a byte:

byte - byte = (0..255) - (0..255) = -255..255

Solution 3

Arithmetic on bytes results in an int value by default.

Solution 4

Because arithmetic on bytes returns integers by default so of the two possible assignments the narrower type of zero (byte) is promoted to int (that of oColor.r - percent). Thus the type of the operation is an int. The compiler will not, without a cast, allow you to assign a wider type to a narrower type, because it's a lossy operation. Hence you get the error, unless you explicitly say "I know I'm losing some data, it's fine" with the cast.

Solution 5

This is because byte subtraction returns an int. Actually any binary arithmetic operations on bytes will return an int, so the cast is required.

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Comments

[Charlie Martin]

I think that's subtraction, actually....

[Jhonny D. Cano -Leftware-]

What a nice graphic explanation !!!

[Adam Frisby]

Neither does any integer type except floats or decimals. int is of finite range too, so (min_int - max_int) wont fit, but it returns an int.

[Andrew Hare]

Is there a reason for the downvote?

[Charlie Martin]

Wasn't me, at least. I think we're having a random downvoter, I got one too.

[dhara tcrails]

Subtraction can take you out of the domain of every number type. That doesn't really explain why there is no byte-specific subtraction operator.

[Charlie Martin]

Well, then, the answer is "because the developers of C# decided they didn't want one."

[chtenb]

Link is dead...

[Rafael Herscovici]

@Timothy Cartner, Link only answers are not recommended on SO, For the reason it stands now for a year, with a broken link. Could you provide some context?