c# - Enum addition vs subtraction and casting

Question

Why does addition require a cast but subtraction works without a cast? See the code below to understand what I am asking

public enum Stuff
{
    A = 1,
    B = 2,
    C = 3
}

var resultSub = Stuff.A - Stuff.B; // Compiles
var resultAdd = Stuff.A + Stuff.B; // Does not compile
var resultAdd2 = (int)Stuff.A + Stuff.B; // Compiles     

note: For both addition and subtraction it does not matter whether result is out of range (of the enum) or not in all three examples above.

Answer 1

Good question - I was surprised that the first and third lines worked.

However, they are supported in the C# language specification - in section 7.8.4, it talks about enumeration addition:

Every enumeration type implicitly provides the following pre-defined operators, where E is the enum type and U is the underlying type of E:

E operator +(E x, U y)
E operator +(U x, E y)

At runtime, these operators are ealuated exactly as (E)((U)x + (U)y)

And in section 7.8.5:

Every enumeration type implicitly provides the following predefined operator, where E is the enum type and U is the underlying type of E:

U operator -(E x, E y)

This operator is evaluated exactly as (U)((U)x - (U)y)). In other words, the operator computes the difference between the ordinal values of x and y, and the type of the result is the underlying type of the enumeration.

E operator -(E x, U y);

This operator is evaluated exactly as (E)((U)x - y). In other words, the operator subtracts a value from the underlying type of the enumeration, yielding a value of the enumeration.

So that's why the compiler behaves like that - because it's what the C# spec says to do :)

I wasn't aware that any of these operators exist, and I've never knowingly seen them used. I suspect the reasons for their existence are buried somewhere in the language design meeting notes that Eric Lippert occasionally dives into - but I also wouldn't be surprised if they were regretted as adding features for little benefit. Then again, maybe they're really useful in some situations :)