using System.Collections.Generic;
using System.Linq;

namespace NumberToWordsLib
{
 /// <summary>
 /// Number to word languages
 /// </summary>
 public enum Language
 {
  /// <summary>
  /// English Language
  /// </summary>
  English,

  /// <summary>
  /// Persian Language
  /// </summary>
  Persian
 }

 /// <summary>
 /// Digit's groups
 /// </summary>
 public enum DigitGroup
 {
  /// <summary>
  /// Ones group
  /// </summary>
  Ones,

  /// <summary>
  /// Teens group
  /// </summary>
  Teens,

  /// <summary>
  /// Tens group
  /// </summary>
  Tens,

  /// <summary>
  /// Hundreds group
  /// </summary>
  Hundreds,

  /// <summary>
  /// Thousands group
  /// </summary>
  Thousands
 }

 /// <summary>
 /// Equivalent names of a group 
 /// </summary>
 public class NumberWord
 {
  /// <summary>
  /// Digit's group
  /// </summary>
  public DigitGroup Group { set; get; }

  /// <summary>
  /// Number to word language
  /// </summary>
  public Language Language { set; get; }

  /// <summary>
  /// Equivalent names
  /// </summary>
  public IList<string> Names { set; get; }
 }

 /// <summary>
 /// Convert a number into words
 /// </summary>
 public static class HumanReadableInteger
 {
  #region Fields (4)

  private static readonly IDictionary<Language, string> And = new Dictionary<Language, string>
  {
   { Language.English, " " },
   { Language.Persian, " و " } 
  };
  private static readonly IList<NumberWord> NumberWords = new List<NumberWord>
  {
   new NumberWord { Group= DigitGroup.Ones, Language= Language.English, Names=
    new List<string> { string.Empty, "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine" }},
   new NumberWord { Group= DigitGroup.Ones, Language= Language.Persian, Names=
    new List<string> { string.Empty, "یک", "دو", "سه", "چهار", "پنج", "شش", "هفت", "هشت", "نه" }},

   new NumberWord { Group= DigitGroup.Teens, Language= Language.English, Names=
    new List<string> { "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen" }},
   new NumberWord { Group= DigitGroup.Teens, Language= Language.Persian, Names=
    new List<string> { "ده", "یازده", "دوازده", "سیزده", "چهارده", "پانزده", "شانزده", "هفده", "هجده", "نوزده" }},

   new NumberWord { Group= DigitGroup.Tens, Language= Language.English, Names=
    new List<string> { "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety" }},
   new NumberWord { Group= DigitGroup.Tens, Language= Language.Persian, Names=
    new List<string> { "بیست", "سی", "چهل", "پنجاه", "شصت", "هفتاد", "هشتاد", "نود" }},

   new NumberWord { Group= DigitGroup.Hundreds, Language= Language.English, Names=
    new List<string> {string.Empty, "One Hundred", "Two Hundred", "Three Hundred", "Four Hundred", 
     "Five Hundred", "Six Hundred", "Seven Hundred", "Eight Hundred", "Nine Hundred" }},
   new NumberWord { Group= DigitGroup.Hundreds, Language= Language.Persian, Names=
    new List<string> {string.Empty, "یکصد", "دویست", "سیصد", "چهارصد", "پانصد", "ششصد", "هفتصد", "هشتصد" , "نهصد" }},

   new NumberWord { Group= DigitGroup.Thousands, Language= Language.English, Names=
     new List<string> { string.Empty, " Thousand", " Million", " Billion"," Trillion", " Quadrillion", " Quintillion", " Sextillian",
   " Septillion", " Octillion", " Nonillion", " Decillion", " Undecillion", " Duodecillion", " Tredecillion",
   " Quattuordecillion", " Quindecillion", " Sexdecillion", " Septendecillion", " Octodecillion", " Novemdecillion",
   " Vigintillion", " Unvigintillion", " Duovigintillion", " 10^72", " 10^75", " 10^78", " 10^81", " 10^84", " 10^87",
   " Vigintinonillion", " 10^93", " 10^96", " Duotrigintillion", " Trestrigintillion" }},
   new NumberWord { Group= DigitGroup.Thousands, Language= Language.Persian, Names=
     new List<string> { string.Empty, " هزار", " میلیون", " میلیارد"," تریلیون", " Quadrillion", " Quintillion", " Sextillian",
   " Septillion", " Octillion", " Nonillion", " Decillion", " Undecillion", " Duodecillion", " Tredecillion",
   " Quattuordecillion", " Quindecillion", " Sexdecillion", " Septendecillion", " Octodecillion", " Novemdecillion",
   " Vigintillion", " Unvigintillion", " Duovigintillion", " 10^72", " 10^75", " 10^78", " 10^81", " 10^84", " 10^87",
   " Vigintinonillion", " 10^93", " 10^96", " Duotrigintillion", " Trestrigintillion" }},
  };
  private static readonly IDictionary<Language, string> Negative = new Dictionary<Language, string>
  {
   { Language.English, "Negative " },
   { Language.Persian, "منهای " } 
  };
  private static readonly IDictionary<Language, string> Zero = new Dictionary<Language, string>
  {
   { Language.English, "Zero" },
   { Language.Persian, "صفر" } 
  };

  #endregion Fields

  #region Methods (7)

  // Public Methods (5) 

  /// <summary>
  /// display a numeric value using the equivalent text
  /// </summary>
  /// <param name="number">input number</param>
  /// <param name="language">local language</param>
  /// <returns>the equivalent text</returns>
  public static string NumberToText(this int number, Language language)
  {
   return NumberToText((long)number, language);
  }


  /// <summary>
  /// display a numeric value using the equivalent text
  /// </summary>
  /// <param name="number">input number</param>
  /// <param name="language">local language</param>
  /// <returns>the equivalent text</returns>
  public static string NumberToText(this uint number, Language language)
  {
   return NumberToText((long)number, language);
  }

  /// <summary>
  /// display a numeric value using the equivalent text
  /// </summary>
  /// <param name="number">input number</param>
  /// <param name="language">local language</param>
  /// <returns>the equivalent text</returns>
  public static string NumberToText(this byte number, Language language)
  {
   return NumberToText((long)number, language);
  }

  /// <summary>
  /// display a numeric value using the equivalent text
  /// </summary>
  /// <param name="number">input number</param>
  /// <param name="language">local language</param>
  /// <returns>the equivalent text</returns>
  public static string NumberToText(this decimal number, Language language)
  {
   return NumberToText((long)number, language);
  }

  /// <summary>
  /// display a numeric value using the equivalent text
  /// </summary>
  /// <param name="number">input number</param>
  /// <param name="language">local language</param>
  /// <returns>the equivalent text</returns>
  public static string NumberToText(this double number, Language language)
  {
   return NumberToText((long)number, language);
  }

  /// <summary>
  /// display a numeric value using the equivalent text
  /// </summary>
  /// <param name="number">input number</param>
  /// <param name="language">local language</param>
  /// <returns>the equivalent text</returns>
  public static string NumberToText(this long number, Language language)
  {
   if (number == 0)
   {
    return Zero[language];
   }

   if (number < 0)
   {
    return Negative[language] + NumberToText(-number, language);
   }

   return wordify(number, language, string.Empty, 0);
  }
  // Private Methods (2) 

  private static string getName(int idx, Language language, DigitGroup group)
  {
   return NumberWords.Where(x => x.Group == group && x.Language == language).First().Names[idx];
  }

  private static string wordify(long number, Language language, string leftDigitsText, int thousands)
  {
   if (number == 0)
   {
    return leftDigitsText;
   }

   var wordValue = leftDigitsText;
   if (wordValue.Length > 0)
   {
    wordValue += And[language];
   }

   if (number < 10)
   {
    wordValue += getName((int)number, language, DigitGroup.Ones);
   }
   else if (number < 20)
   {
    wordValue += getName((int)(number - 10), language, DigitGroup.Teens);
   }
   else if (number < 100)
   {
    wordValue += wordify(number % 10, language, getName((int)(number / 10 - 2), language, DigitGroup.Tens), 0);
   }
   else if (number < 1000)
   {
    wordValue += wordify(number % 100, language, getName((int)(number / 100), language, DigitGroup.Hundreds), 0);
   }
   else
   {
    wordValue += wordify(number % 1000, language, wordify(number / 1000, language, string.Empty, thousands + 1), 0);
   }

   if (number % 1000 == 0) return wordValue;
   return wordValue + getName(thousands, language, DigitGroup.Thousands);
  }

  #endregion Methods
 }
}

public static TResult IfNotNull<TResult, TSource>(
    this TSource source,
    Func<TSource, TResult> onNotDefault)
    where TSource : class
{
    if (onNotDefault == null) throw new ArgumentNullException("onNotDefault");
    return source == null ? default(TResult) : onNotDefault(source);
}

var postCode =
    GetStore()
        .IfNotNull(x => x.Address)
        .IfNotNull(x => x.PostCode)
        .IfNotNull(x => x.ToString());

public static TResult IfNotDefault<TResult, TSource>(
    this TSource source,
    Func<TSource, TResult> onNotDefault,
    Predicate<TSource> isNotDefault = null)
{
    if (onNotDefault == null) throw new ArgumentNullException("onNotDefault");
    var isDefault = isNotDefault == null
        ? EqualityComparer<TSource>.Default.Equals(source, default(TSource))
        : !isNotDefault(source);
   return isDefault ? default(TResult) : onNotDefault(source);
}

return person
        . IfNotDefault(x => x.Name)
        . IfNotDefault(SomeOperation, x => !string.IsNullOrEmpty(x));

var avg = students
        .Where(IsNotAGraduate)
        .FirstOrDefault()
        .IfNotDefault(s => s.Grades) 
        .IfNotDefault(g => g.Average(), g => g != null && g.Length > 0);