Why does C# issue the error "cannot implicitly convert int to ushort" against modulo arithmetic on ushorts?

Question

In another thread, someone asked about why adding two ushort values raised errors in C#. e.g.

ushort x = 4;
ushort y = 23;
ushort z = x+y;  // ERROR cannot implicitly convert int to ushort 

On that thread, people argued that the plus + operater takes two ints by default, and this is a language feature to help avoid arithmetic overflows. But I get the same kind of error in the following function:

public RGB(ushort red, ushort green, ushort blue)
{
    // this class RGB has three ushort fields: r, g, and b
    r = red % ((ushort)256);
    g = green % ((ushort)256);
    b = blue % ((ushort)256);
}

where the compiler errors and says "Cannot implicitly convert type 'int' to 'ushort'. An explicit conversion exists...". But here the argument that the modulo % operator is guarding against overflow doesn't make any sense at all: if x and y are ushort values, then x%y < max(x,y), so there is no risk of overflowing into ints. So why am I getting this error?

Answer 1

It's because the % operator is not defined for integer types smaller than int. The C# spec lists all overloads defined for the modulo operator on integer types:

int operator %(int x, int y);
uint operator %(uint x, uint y);
long operator %(long x, long y);
ulong operator %(ulong x, ulong y);

https://github.com/dotnet/csharplang/blob/master/spec/expressions.md#remainder-operator

Using %on ushorts then defaults to the first overload from the list above, which returns an int that can't be cast to ushort implicitly.

---

If you ask why it's not defined, you probably would have to ask the creators of the C# specification.

Answer 2

The % operator that's being used, even with shorts or ushorts, has a signature of int %(int a, int b). So your shorts are being lifted up into integers, and your result is an integer you are attempting to assign to a ushort, which is a lossy cast so you are required to be explicit.

Consider this:

ushort x = 5;
ushort y = 6;
var res = x % y;
Console.WriteLine(res.GetType()); // System.Int32
ushort z = res; // cast error, explicit conversion exists
ushort zz = (ushort)res; // Fine, we cast down.