Gets a Picasa user, hence a google user, using an email address.

GetPicasaUserByEmail.cs

using System;
using System.Collections.Generic;
using System.IO;;
using System.Net;
using System.Xml.Serialization;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {            
            Console.Write(GetPicasaUserByEmail([YOUR EMAIL HERE]));
            Console.Read();
        }

        public static GoogleUser GetPicasaUserByEmail(string email)
        {
            if (string.IsNullOrWhiteSpace(email))
            {
                throw new ArgumentException("Parameter email cannot be null nor empty");
            }
            var endpoint = $"http://picasaweb.google.com/data/entry/api/user/{email}";
            var client = new WebClient();
            byte[] buffer = client.DownloadData(endpoint);

            GoogleUser googleUser = null;
            using (var ms = new MemoryStream(buffer))
            {
                XmlSerializer serializer = new XmlSerializer(typeof(GoogleUser));
                googleUser = (GoogleUser)serializer.Deserialize(ms);
            }

            return googleUser;
        }
    }

    [XmlRoot("entry", Namespace = "http://www.w3.org/2005/Atom")]
    public class GoogleUser
    {
        [XmlElement("id")]
        public string Id { get; set; }

        [XmlElement("published")]
        public DateTime PublishedUtcDate { get; set; }

        [XmlElement("updated")]
        public DateTime UpdatedUtcDate { get; set; }

        [XmlElement("category")]
        public string Category { get; set; }

        [XmlElement("title")]
        public string Title { get; set; }

        [XmlElement("summary")]
        public string Summary { get; set; }

        [XmlElement("link")]
        public List<Link> Links { get; set; }

        [XmlElement("author")]
        public Author Author { get; set; }

        [XmlElement("user", Namespace = "http://schemas.google.com/photos/2007")]
        public string UserId { get; set; }

        [XmlElement("nickname", Namespace = "http://schemas.google.com/photos/2007")]
        public string NickName { get; set; }

        [XmlElement("thumbnail", Namespace = "http://schemas.google.com/photos/2007")]
        public string ThumbnailUrl { get; set; }
    }

    public class Author
    {
        [XmlElement("name")]
        public string Name { get; set; }

        [XmlElement("uri")]
        public string PicasaUrl { get; set; }
    }

    public class Link
    {
        [XmlAttribute("rel")]
        public string Relationship { get; set; }

        [XmlAttribute("type")]
        public string Type { get; set; }

        [XmlAttribute("href")]
        public string Url { get; set; }
    }
}