Centralize event declaration in C#

Question

I have a piece of code that follows the Observer pattern. Each publisher creates events and invokes them accordingly for subscribers to consume. I thought of creating an EventRegistry where all events are registered (as dictionaries <Enum, event>). That way both subscribers and publishers have a central place to know what events are available, instead of having to import each file where the event was created.

I tried the code below but it doesn't work. I implemented the registry as a singleton (following this implementation), so events are created only once. I'm not too familiar with C#, so I'm not sure what's wrong. The error I get:

A field initializer cannot reference the non-static field, method, or property 'EventRegistry.Event1' [Assembly-CSharp]

The code:

using System;
using System.Collections.Generic;
using MyNamespace;

namespace MyNamespace
{
    class EventRegistry
    {
        private EventRegistry() { }

        private static EventRegistry _instance;

        private static readonly object _lock = new object();

        public static EventRegistry GetInstance()
        {
            if (_instance == null)
            {
                lock (_lock)
                {
                    if (_instance == null)
                    {
                        _instance = new EventRegistry();
                    }
                }
            }
            return _instance;
        }

        private enum Publisher1 {
            Event1,
        }
        private event EventHandler Event1;

        public Dictionary<Publisher1, EventHandler> Publisher1Events {get;} = new Dictionary<Publisher1, EventHandler> {
            {Publisher1.Event1, Event1}
        };
    }
}

I'm aware that Singletons are evil. I think this is a good use case for it though, but if there's a better way feel free to suggest otherwise!

score 1 · Accepted Answer · answered Jul 03 '23 at 04:23

You have to initialize Publisher1Events in your private constructor, like:

private EventRegistry()
{
    Publisher1Events = new Dictionary<Publisher1, EventHandler> {
        {Publisher1.Event1, Event1}
    };
}

And you have to make the Publisher1 enum public.

The reason you have to initialize in the constructor is because field initializers require constant or static values.

Centralize event declaration in C#

1 Answers1