AttributeCache.cs

    public static class AttributeCache<TAttribute> where TAttribute : Attribute
    {
        private static readonly ConcurrentDictionary<MethodInfo, ReadOnlyCollection<TAttribute>> Lookup =
            new ConcurrentDictionary<MethodInfo, ReadOnlyCollection<TAttribute>>();

        public static IReadOnlyCollection<TAttribute> GetOrSet(MethodInfo methodInfo) =>
            Lookup.GetOrAdd(methodInfo, CachedDelegates.GetCustomAttributes);

        private static class CachedDelegates
        {
            internal static readonly Func<ICustomAttributeProvider, ReadOnlyCollection<TAttribute>> GetCustomAttributes
                = memberInfo => new ReadOnlyCollection<TAttribute>(
                    (TAttribute[]) memberInfo.GetCustomAttributes(typeof(TAttribute), inherit: true));
        }
    }