Custom attribute access from FastMember Member class

gistfile1.cs

    /// <summary>
    /// Represents an abstracted view of an individual member defined for a type
    /// </summary>
    public sealed class Member
    {
        private readonly MemberInfo member;

        private readonly static ConcurrentDictionary<MemberInfo, object> attributeLookup = new ConcurrentDictionary<MemberInfo, object>(); 
        
        /// <summary>
        /// Provides an access to the custom attribute of Member
        /// </summary>
        public object GetCustomAttribute(Type attributeType)
        {
            if(IsDefined(attributeType))
            {
                return attributeLookup.GetOrAdd(member, m => Attribute.GetCustomAttribute(m, attributeType));
            }
            throw new InvalidOperationException("Attribute is not defined");
        }
    }