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

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.