public static IEnumerable<int> Range(int start, int count) { 
   long max = ((long)start) + count - 1;
   if (count < 0 || max > Int32.MaxValue) throw Error.ArgumentOutOfRange("count"); 
   return RangeIterator(start, count);
  }
  static IEnumerable<int> RangeIterator(int start, int count) { 
   for (int i = 0; i < count; i++) yield return start + i;
  }

 public static IEnumerable<TSource> Where<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate) {
   if (source == null) throw Error.ArgumentNull("source"); 
   if (predicate == null) throw Error.ArgumentNull("predicate"); 
   if (source is Iterator<TSource>) return ((Iterator<TSource>)source).Where(predicate);
   if (source is TSource[]) return new WhereArrayIterator<TSource>((TSource[])source, predicate); 
   if (source is List<TSource>) return new WhereListIterator<TSource>((List<TSource>)source, predicate);
   return new WhereEnumerableIterator<TSource>(source, predicate);
  }
  public WhereEnumerableIterator(IEnumerable<TSource> source, Func<TSource, bool> predicate) { 
    this.source = source;
    this.predicate = predicate; 
   }

 /// <summary>
 /// 提供對(duì)未指定數(shù)據(jù)類(lèi)型的特定數(shù)據(jù)源的查詢(xún)進(jìn)行計(jì)算的功能。
 /// </summary>
 /// <filterpriority>2</filterpriority>
 public interface IQueryable : IEnumerable
 {
 /// <summary>
 /// 獲取與 <see cref="T:System.Linq.IQueryable"/> 的實(shí)例關(guān)聯(lián)的表達(dá)式目錄樹(shù)。
 /// </summary>
 /// 
 /// <returns>
 /// 與 <see cref="T:System.Linq.IQueryable"/> 的此實(shí)例關(guān)聯(lián)的 <see cref="T:System.Linq.Expressions.Expression"/>。
 /// </returns>
 Expression Expression { get; }
 /// <summary>
 /// 獲取在執(zhí)行與 <see cref="T:System.Linq.IQueryable"/> 的此實(shí)例關(guān)聯(lián)的表達(dá)式目錄樹(shù)時(shí)返回的元素的類(lèi)型。
 /// </summary>
 /// 
 /// <returns>
 /// 一個(gè) <see cref="T:System.Type"/>，表示在執(zhí)行與之關(guān)聯(lián)的表達(dá)式目錄樹(shù)時(shí)返回的元素的類(lèi)型。
 /// </returns>
 Type ElementType { get; }
 /// <summary>
 /// 獲取與此數(shù)據(jù)源關(guān)聯(lián)的查詢(xún)提供程序。
 /// </summary>
 /// 
 /// <returns>
 /// 與此數(shù)據(jù)源關(guān)聯(lián)的 <see cref="T:System.Linq.IQueryProvider"/>。
 /// </returns>
 IQueryProvider Provider { get; }
 }

 public static IQueryable<TSource> Where<TSource>(this IQueryable<TSource> source, Expression<Func<TSource, bool>> predicate) { 
   if (source == null)
    throw Error.ArgumentNull("source"); 
   if (predicate == null)
    throw Error.ArgumentNull("predicate");
   return source.Provider.CreateQuery<TSource>(
    Expression.Call( 
     null,
     ((MethodInfo)MethodBase.GetCurrentMethod()).MakeGenericMethod(typeof(TSource)), 
     new Expression[] { source.Expression, Expression.Quote(predicate) } 
     ));
  }

  public static IQueryable<TResult> Select<TSource,TResult>(this IQueryable<TSource> source, Expression<Func<TSource, TResult>> selector) {
   if (source == null)
    throw Error.ArgumentNull("source");
   if (selector == null) 
    throw Error.ArgumentNull("selector");
   return source.Provider.CreateQuery<TResult>( 
    Expression.Call( 
     null,
     ((MethodInfo)MethodBase.GetCurrentMethod()).MakeGenericMethod(typeof(TSource), typeof(TResult)), 
     new Expression[] { source.Expression, Expression.Quote(selector) }
     ));
  }

 /// <summary>
 /// 為參數(shù)驗(yàn)證提供有用的方法
 /// </summary>
 public static class ArgumentValidator
 {
  /// <summary>
  /// 如果argumentToValidate為空，則拋出一個(gè)ArgumentNullException異常
  /// </summary>
  public static void ThrowIfNull(object argumentToValidate, string argumentName)
  {
   if (null == argumentName)
   {
    throw new ArgumentNullException("argumentName");
   }
   if (null == argumentToValidate)
   {
    throw new ArgumentNullException(argumentName);
   }
  }
  /// <summary>
  /// 如果argumentToValidate為空，則拋出一個(gè)ArgumentException異常
  /// </summary>
  public static void ThrowIfNullOrEmpty(string argumentToValidate, string argumentName)
  {
   ThrowIfNull(argumentToValidate, argumentName);
   if (argumentToValidate == string.Empty)
   {
    throw new ArgumentException(argumentName);
   }
  }
  /// <summary>
  /// 如果condition為真，則拋出ArgumentException異常
  /// </summary>
  /// <param name="condition"></param>
  /// <param name="msg"></param>
  public static void ThrowIfTrue(bool condition, string msg)
  {
   ThrowIfNullOrEmpty(msg, "msg");
   if (condition)
   {
    throw new ArgumentException(msg);
   }
  }
  /// <summary>
  /// 如果指定目錄存在該文件則拋出FileNotFoundException異常
  /// </summary>
  /// <param name="fileSytemObject"></param>
  /// <param name="argumentName"></param>
  public static void ThrowIfDoesNotExist(FileSystemInfo fileSytemObject, String argumentName)
  {
   ThrowIfNull(fileSytemObject, "fileSytemObject");
   ThrowIfNullOrEmpty(argumentName, "argumentName");
   if (!fileSytemObject.Exists)
   {
    throw new FileNotFoundException("'{0}' not found".Fi(fileSytemObject.FullName));
   }
  }
  public static string Fi(this string format, params object[] args)
  {
   return FormatInvariant(format, args);
  }
  /// <summary>
  /// 格式化字符串和使用<see cref="CultureInfo.InvariantCulture">不變的文化</see>.
  /// </summary>
  /// <remarks>
  /// <para>這應(yīng)該是用于顯示給用戶(hù)的任何字符串時(shí)使用的“B”>“B”>“”。它意味著日志
  ///消息，異常消息，和其他類(lèi)型的信息，不使其進(jìn)入用戶(hù)界面，或不會(huì)
  ///無(wú)論如何，對(duì)用戶(hù)都有意義；）.</para>
  /// </remarks>
  public static string FormatInvariant(this string format, params object[] args)
  {
   ThrowIfNull(format, "format");
   return 0 == args.Length ? format : string.Format(CultureInfo.InvariantCulture, format, args);
  }
  /// <summary>
  /// 如果時(shí)間不為DateTimeKind.Utc，則拋出ArgumentException異常
  /// </summary>
  /// <param name="argumentToValidate"></param>
  /// <param name="argumentName"></param>
  public static void ThrowIfNotUtc(DateTime argumentToValidate, String argumentName)
  {
   ThrowIfNullOrEmpty(argumentName, "argumentName");
   if (argumentToValidate.Kind != DateTimeKind.Utc)
   {
    throw new ArgumentException("You must pass an UTC DateTime value", argumentName);
   }
  }
 }

 public static class EnumExtensions
 {
  /// <summary>
  /// 獲取名字
  /// </summary>
  /// <param name="e"></param>
  /// <returns></returns>
  public static string GetName(this Enum e)
  {
   return Enum.GetName(e.GetType(), e);
  }
  /// <summary>
  /// 獲取名字和值
  /// </summary>
  /// <param name="enumType">枚舉</param>
  /// <param name="lowerFirstLetter">是否轉(zhuǎn)化為小寫(xiě)</param>
  /// <returns></returns>
  public static Dictionary<string, int> GetNamesAndValues( this Type enumType, bool lowerFirstLetter)
  {
   //由于擴(kuò)展方法實(shí)際是對(duì)一個(gè)靜態(tài)方法的調(diào)用，所以CLR不會(huì)生成代碼對(duì)調(diào)用方法的表達(dá)式的值進(jìn)行null值檢查
   ArgumentValidator.ThrowIfNull(enumType, "enumType");
   //獲取枚舉名稱(chēng)數(shù)組
   var names = Enum.GetNames(enumType);
   //獲取枚舉值數(shù)組
   var values = Enum.GetValues(enumType);
   var d = new Dictionary<string, int>(names.Length);
   for (var i = 0; i < names.Length; i++)
   {
    var name = lowerFirstLetter ? names[i].LowerFirstLetter() : names[i];
    d[name] = Convert.ToInt32(values.GetValue(i));
   }
   return d;
  }
  /// <summary>
  /// 轉(zhuǎn)換為小寫(xiě)
  /// </summary>
  /// <param name="s"></param>
  /// <returns></returns>
  public static string LowerFirstLetter(this string s)
  {
   ArgumentValidator.ThrowIfNull(s, "s");
   return char.ToLowerInvariant(s[0]) + s.Substring(1);
  }
 }