Initialize collection in interceptor

I'm trying out something like this

public class DefaultInterceptor : EmptyInterceptor {
    public override bool OnLoad(object entity, object id, object[] state, string[] propertyNames, IType[] types) {
        var details = ((ContentItem) entity).Details;
        var properties = entity.GetType().GetProperties();
        foreach (PropertyInfo propertyInfo in properties) {
            if(details.ContainsKey(propertyInfo.Name)) {
                propertyInfo.SetValue(entity,details[propertyInfo.Name].Value,null);
            }
        }
        return base.OnLoad(entity, id, state, propertyNames, types);
    }
}

but my details collection is always empty here, is there any way of initialize this collection so i can do operations on it?