C# numeric promotions

Promotion rules are in some situations quite tricky to understand. Here is the list of applied rules so the situations where an explicit cast is needed get clarified.

From ECMA-334

14.2.6.1 Unary numeric promotions

Unary numeric promotion occurs for the operands of the predefined +, --, and ~ unary operators. Unary numeric promotion simply consists of converting operands of type sbyte, byte, short, ushort, or char to type int. Additionally, for the unary -- operator, unary numeric promotion converts operands of typeuint to type long.

14.2.6.2 Binary numeric promotions

Binary numeric promotion occurs for the operands of the predefined +, --, *, /, %, &, |, ^, ==, !=, >, <, >=, and <= binary operators. Binary numeric promotion implicitly converts both operands to a common type which, in case of the non-relational operators, also becomes the result type of the operation. Binary numeric promotion consists of applying the following rules, in the order they appear here:

• If either operand is of type decimal, the other operand is converted to type decimal, or a compile-time error occurs if the other operand is of type floator double.

• Otherwise, if either operand is of type double, the other operand is converted to type double.

• Otherwise, if either operand is of type float, the other operand is converted to type float.

• Otherwise, if either operand is of type ulong, the other operand is converted to type ulong, or a compile-time error occurs if the other operand is of type sbyte, short, int, or long.

• Otherwise, if either operand is of type long, the other operand is converted to type long.

• Otherwise, if either operand is of type uint and the other operand is of type sbyte, short, or int, both operands are converted to type long.

• Otherwise, if either operand is of type uint, the other operand is converted to type uint.

• Otherwise, both operands are converted to type int.

[Note: The first rule disallows any operations that mix the decimal type with the double and float types. The rule follows from the fact that there are no implicit conversions between thedecimal type and the double and float types. end note]

[Note: Also note that it is not possible for an operand to be of type ulong when the other operand is of a signed integral type. The reason is that no integral type exists that can represent the full range of ulong as well as the signed integral types. end note]

In both of the above cases, a cast expression can be used to explicitly convert one operand to a type that is compatible with the other operand.