OR-ing bytes in C# gives int [duplicate]

Question

I have this code.

byte dup = 0;
Encoding.ASCII.GetString(new byte[] { (0x80 | dup) });

When I try to compile I get:

Cannot implicitly convert type 'int' to 'byte'. An explicit conversion exists (are you missing a cast?)

Why does this happen? Shouldn't | two bytes give a byte? Both of the following work, assuring that each item is a byte.

Encoding.ASCII.GetString(new byte[] { (dup) });
Encoding.ASCII.GetString(new byte[] { (0x80) });

Answer 1

It's that way by design in C#, and, in fact, dates back all the way to C/C++ - the latter also promotes operands to int, you just usually don't notice because int -> char conversion there is implicit, while it's not in C#. This doesn't just apply to | either, but to all arithmetic and bitwise operands - e.g. adding two bytes will give you an int as well. I'll quote the relevant part of the spec here:

Binary numeric promotion occurs for the operands of the predefined +, –, *, /, %, &, |, ^, ==, !=, >, <, >=, and <= binary operators. Binary numeric promotion implicitly converts both operands to a common type which, in case of the non-relational operators, also becomes the result type of the operation. Binary numeric promotion consists of applying the following rules, in the order they appear here:

• If either operand is of type decimal, the other operand is converted to type decimal, or a compile-time error occurs if the other operand is of type float or double.

• Otherwise, if either operand is of type double, the other operand is converted to type double.

• Otherwise, if either operand is of type float, the other operand is converted to type float.

• Otherwise, if either operand is of type ulong, the other operand is converted to type ulong, or a compile-time error occurs if the other operand is of type sbyte, short, int, or long.

• Otherwise, if either operand is of type long, the other operand is converted to type long.

• Otherwise, if either operand is of type uint and the other operand is of type sbyte, short, or int, both operands are converted to type long.

• Otherwise, if either operand is of type uint, the other operand is converted to type uint.

• Otherwise, both operands are converted to type int.

I don't know the exact rationale for this, but I can think about one. For arithmetic operators especially, it might be a bit surprising for people to get (byte)200 + (byte)100 suddenly equal to 44, even if it makes some sense when one carefully considers the types involved. On the other hand, int is generally considered a type that's "good enough" for arithmetic on most typical numbers, so by promoting both arguments to int, you get a kind of "just works" behavior for most common cases.

As to why this logic was also applied to bitwise operators - I imagine this is so mostly for consistency. It results in a single simple rule that is common for all non-boolean binary types.

But this is all mostly guessing. Eric Lippert would probably be the one to ask about the real motives behind this decision for C# at least (though it would be a bit boring if the answer is simply "it's how it's done in C/C++ and Java, and it's a good enough rule as it is, so we saw no reason to change it").