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>
 /// 提供对未指定数据类型的特定数据源的查询进行计算的功能。
 /// </summary>
 /// <filterpriority>2</filterpriority>
 public interface IQueryable : IEnumerable
 {
 /// <summary>
 /// 获取与 <see cref="T:System.Linq.IQueryable"/> 的实例关联的表达式目录树。
 /// </summary>
 /// 
 /// <returns>
 /// 与 <see cref="T:System.Linq.IQueryable"/> 的此实例关联的 <see cref="T:System.Linq.Expressions.Expression"/>。
 /// </returns>
 Expression Expression { get; }
 /// <summary>
 /// 获取在执行与 <see cref="T:System.Linq.IQueryable"/> 的此实例关联的表达式目录树时返回的元素的类型。
 /// </summary>
 /// 
 /// <returns>
 /// 一个 <see cref="T:System.Type"/>，表示在执行与之关联的表达式目录树时返回的元素的类型。
 /// </returns>
 Type ElementType { get; }
 /// <summary>
 /// 获取与此数据源关联的查询提供程序。
 /// </summary>
 /// 
 /// <returns>
 /// 与此数据源关联的 <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>
 /// 为参数验证提供有用的方法
 /// </summary>
 public static class ArgumentValidator
 {
  /// <summary>
  /// 如果argumentToValidate为空，则抛出一个ArgumentNullException异常
  /// </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为空，则抛出一个ArgumentException异常
  /// </summary>
  public static void ThrowIfNullOrEmpty(string argumentToValidate, string argumentName)
  {
   ThrowIfNull(argumentToValidate, argumentName);
   if (argumentToValidate == string.Empty)
   {
    throw new ArgumentException(argumentName);
   }
  }
  /// <summary>
  /// 如果condition为真，则抛出ArgumentException异常
  /// </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>
  /// 如果指定目录存在该文件则抛出FileNotFoundException异常
  /// </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">不变的文化</see>.
  /// </summary>
  /// <remarks>
  /// <para>这应该是用于显示给用户的任何字符串时使用的“B”>“B”>“”。它意味着日志
  ///消息，异常消息，和其他类型的信息，不使其进入用户界面，或不会
  ///无论如何，对用户都有意义；）.</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>
  /// 如果时间不为DateTimeKind.Utc，则抛出ArgumentException异常
  /// </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>
  /// 获取名字
  /// </summary>
  /// <param name="e"></param>
  /// <returns></returns>
  public static string GetName(this Enum e)
  {
   return Enum.GetName(e.GetType(), e);
  }
  /// <summary>
  /// 获取名字和值
  /// </summary>
  /// <param name="enumType">枚举</param>
  /// <param name="lowerFirstLetter">是否转化为小写</param>
  /// <returns></returns>
  public static Dictionary<string, int> GetNamesAndValues( this Type enumType, bool lowerFirstLetter)
  {
   //由于扩展方法实际是对一个静态方法的调用，所以CLR不会生成代码对调用方法的表达式的值进行null值检查
   ArgumentValidator.ThrowIfNull(enumType, "enumType");
   //获取枚举名称数组
   var names = Enum.GetNames(enumType);
   //获取枚举值数组
   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>
  /// 转换为小写
  /// </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);
  }
 }