The plus operator in enum definition

Question

I stumbled upon the usage of the plus (+) operator in an enum definition today, I was surprised to see the accompanying tests pass. Anyone have any idea where this may be documented?

public enum ApprovalItemState
{
    Enqueued = 1,
    Approved = 2,
    Denied = 4,
    Acknowledged = 8,
    ApprovalAcknowledged = ApprovalItemState.Approved + ApprovalItemState.Acknowledged,
    DenialAcknowledged = ApprovalItemState.Denied + ApprovalItemState.Acknowledged
}


[TestClass]
public class ApprovalItemStateTests
{
    [TestMethod]
    public void AreFlagsDeniedAndAcknowledged()
    {
        Assert.AreEqual(ApprovalItemState.DenialAcknowledged, ApprovalItemState.Denied | ApprovalItemState.Acknowledged);
    }

    [TestMethod]
    public void IsDenialAcknowledged()
    {
        Assert.IsTrue(Enum.IsDefined(typeof(ApprovalItemState), ApprovalItemState.Denied | ApprovalItemState.Acknowledged));
        Assert.AreEqual(ApprovalItemState.Denied | ApprovalItemState.Acknowledged, (ApprovalItemState)Enum.Parse(typeof(ApprovalItemState), "DenialAcknowledged"));
    }


    [TestMethod]
    public void IsNotDeniedAndApproved()
    {
        Assert.IsFalse(Enum.IsDefined(typeof(ApprovalItemState), ApprovalItemState.Approved | ApprovalItemState.Denied));
    }
}

Answer 1

Reed's answer is of course correct. I just thought I'd add an interesting bit of trivia. First off, when you are inside the enum, all the members of the enum are in scope. This is the only situation in C# in which you can use an enum member via its unqualified name!

public enum ApprovalItemState 
{
    Enqueued = 1,
    Approved = 2,
    Denied = 4,
    Acknowledged = 8,
    ApprovalAcknowledged = Approved | Acknowledged,
    DenialAcknowledged =  Denied | Acknowledged 
} 

The second trivia point is that the C# compiler actually allows enum arithmetic involving other enums inside an enum!

enum E
{
    E1
}
enum F
{
    F1
}
enum G
{
    G1 = E.E1 + F.F1
}

Normally that would not be at all legal; you cannot add two dissimilar enums together and you cannot assign the result. The compiler relaxes those rules inside an enum initializer so that you can do things like:

enum MyFlags
{
    MyReadOnly = FileFlags.ReadOnly,
    ...

Answer 2

The C# Language Spec, in 14.5, states:

The following operators can be used on values of enum types: ==, !=, <, >, <=, >= (§7.10.5), binary + (§7.8.4), binary ‑ (§7.8.5), ^, &, | (§7.11.2), ~ (§7.7.4), ++ and -- (§7.6.9 and §7.7.5).

Basically, since the enum is internally stored as an Int32 (that's the default, unless you specify a different storage type), you can use addition like this.

However, it's far more common to use | instead of + to define masks. Also, it would be common to include [Flags] if you're going to use this as a flags enumeration.