Convert 15 Char Salesforce Id to 18 Char Salesforce Id

gistfile1.cs

// Based on algorithm given at http://wp.me/pWWz3-14
static string Convert15CharTo18CharId(string id)
{
    if (string.IsNullOrEmpty(id)) throw new ArgumentNullException("id");
    if (id.Length == 18) return id;
    if (id.Length != 15) throw new ArgumentException("Illegal argument length. 15 char string expected.", "id");

    var triplet = new List<string> {id.Substring(0, 5), id.Substring(5, 5), id.Substring(10, 5)};
    StringBuilder str = new StringBuilder(5);
    string suffix = string.Empty;
    foreach (var value in triplet)
    {
        str.Clear();
        var reverse = value.Reverse().ToList();
        reverse.ForEach(c => str.Append(Char.IsUpper(c) ? "1" : "0"));
        suffix += BinaryIdLookup[str.ToString()];
    }
    return id + suffix;
}

static readonly Dictionary<string, char> BinaryIdLookup = new Dictionary<string, char>
{
    {"00000", 'A'}, {"00001", 'B'}, {"00010", 'C'}, {"00011", 'D'}, {"00100", 'E'}, {"00101", 'F'}, {"00110", 'G'}, {"00111", 'H'},
    {"01000", 'I'}, {"01001", 'J'}, {"01010", 'K'}, {"01011", 'L'}, {"01100", 'M'}, {"01101", 'N'}, {"01110", 'O'}, {"01111", 'P'},
    {"10000", 'Q'}, {"10001", 'R'}, {"10010", 'S'}, {"10011", 'T'}, {"10100", 'U'}, {"10101", 'V'}, {"10110", 'W'}, {"10111", 'X'},
    {"11000", 'Y'}, {"11001", 'Z'}, {"11010", '0'}, {"11011", '1'}, {"11100", '2'}, {"11101", '3'}, {"11110", '4'}, {"11111", '5'}
};

Based on the blog post here. Also see the related answer on SF.StackExchange.