Why does this implicit conversion from int to uint work?

Question

Using Casting null doesn't compile as inspiration, and from Eric Lippert's comment:

That demonstrates an interesting case. "uint x = (int)0;" would succeed even though int is not implicitly convertible to uint.

We know this doesn't work, because object can't be assigned to string:

string x = (object)null;

But this does, although intuitively it shouldn't:

uint x = (int)0;

Why does the compiler allow this case, when int isn't implicitly convertible to uint?

Answer 1

Integer constant conversions are treated as very special by the C# language; here's section 6.1.9 of the specification:

A constant expression of type int can be converted to type sbyte, byte, short, ushort, uint, or ulong, provided the value of the constant-expression is within the range of the destination type. A constant expression of type long can be converted to type ulong, provided the value of the constant expression is not negative.

This permits you to do things like:

byte x = 64;

which would otherwise require an ugly explicit conversion:

byte x = (byte)64; // gross