C# - Check for attribute's existence on enum's element

Question

I've got a situation like the following:

enum Header
{
    Sync,
    [OldProtocol] Keepalive,
    Ping,
    [OldProtocol] Auth,
    [OldProtocol] LoginData
    //...
}

I need to obtain an array of elements on which the OldProtocolAttribute is defined. I've noticed that the Attribute.IsDefined() method and its overloads apparently don't support this kind of situation.

My question is:

• Is there a way to solve the problem without using in any part of the solution typeof(Header).GetField()?
• If not, what's the most optimal way to solve it?

Eyeleen

Answer 1

As far as I'm aware, you have to get the attribute from the field. You'd use:

var field = typeof(Header).GetField(value.ToString());
var old = field.IsDefined(typeof(OldProtocolAttribute), false);

Or to get a whole array:

var attributeType = typeof(OldProtocolAttribute);
var array = typeof(Header).GetFields(BindingFlags.Public |
                                     BindingFlags.Static)
                          .Where(field => field.IsDefined(attributeType, false))
                          .Select(field => (Header) field.GetValue(null))
                          .ToArray();

Obviously if you need this often, you may well want to cache the results.

Catkin

@Jon Skeet, u miss ")" after ".Where(field => field.IsDefined(attributeType, false)" - this code will not compile. – Tresa

Actually it will cache internally. – Vulgarism

@AridaneÁlamo: IsDefined may cache, but if the OP needs the whole array of fields frequently, it would be worth caching that. – Catkin

Answer 2

Reflection is pretty much your only tool available for this. The query is not too bad though:

var oldFields = typeof(Header).GetFields(BindingFlags.Static | BindingFlags.Public).Select(field => Attribute.IsDefined(field, typeof(OldProtocolAttribute)));

Grayback