Event delegate contravariance

events.cs

private ImmutableArray<THandler> handlers = ImmutableArray<THandler>.Empty;

internal static void Add<THandler>(ref ImmutableArray<Action<THandler>> handlers, Action<THandler> handler) {
	if (handler == null)
		return;

	ImmutableArray<Action<THandler>> initial, computed;
	do {
		initial = InterlockedGet(ref handlers);
		computed = initial.Add(handler);
	}
	while (initial != ImmutableInterlocked.InterlockedCompareExchange(ref handlers, computed, initial));
}

internal static void Remove<THandler>(ref ImmutableArray<Action<THandler>> handlers, Action<THandler> handler) {
	if (handler == null)
		return;

	ImmutableArray<Action<THandler>> initial, computed;
	do {
		initial = InterlockedGet(ref handlers);
		computed = RemoveLast(initial, handler);
	}
	while (initial != ImmutableInterlocked.InterlockedCompareExchange(ref handlers, computed, initial));
}

private static void Raise<THandler>(ref ImmutableArray<Action<THandler>> handlers, THandler entry) {
	var latestHandlers = InterlockedGet(ref handlers);
	for (var i = 0; i < latestHandlers.Length; i++)
		latestHandlers[i].Invoke(entry);
}

private static ImmutableArray<Action<THandler>> InterlockedGet<THandler>(ref ImmutableArray<Action<THandler>> handlers) {
	return ImmutableInterlocked.InterlockedCompareExchange(ref handlers, ImmutableArray<Action<THandler>>.Empty, ImmutableArray<Action<THandler>>.Empty);
}

private static ImmutableArray<TDelegate> RemoveLast(ImmutableArray<TDelegate> source, TDelegate value) {
	var list = source;
	var index = list.LastIndexOf(value);
	if (index >= 0)
		return list.RemoveAt(index);
	return list;
}