C# language specification, version 5, section 6.1.9:
An implicit constant expression conversion permits the following conversions:
- A constant-expression (§7.19) 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.
Your first example is a constant expression, because it can be evaluated at compile time. But see section 7.19 for more details:
Only the following constructs are permitted in constant expressions:
- Literals (including the null literal).
[…]
- The predefined +, –, *, /, %, <<, >>, &, |, ^, &&, ||, ==, !=, <, >, <=, and >= binary operators, provided each operand is of a type listed above.
- The ?: conditional operator.