Enum addition vs subtraction and casting

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 🙂