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;  // E         


        

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.

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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.