问题
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?
回答1:
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.
回答2:
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.
来源:https://stackoverflow.com/questions/55638528/why-does-c-sharp-issue-the-error-cannot-implicitly-convert-int-to-ushort-again