Linq 扩展：MinOrDefault，MaxOrDefault

namespace ExtensionMethods
{
    using System;
    using System.Collections.Generic;
    using System.Linq;

    public static class IEnumerableExtensions
    {
      /// <summary>
      /// Invokes a transform function on each element of a sequence and returns the minimum Double value  
      /// if the sequence is not empty; otherwise returns the specified default value.  
      /// </summary>
      /// <typeparam name="TSource">The type of the elements of source.</typeparam>
      /// <param name="source">A sequence of values to determine the minimum value of.</param>
      /// <param name="selector">A transform function to apply to each element.</param>
      /// <param name="defaultValue">The default value.</param>
      /// <returns>The minimum value in the sequence or default value if sequence is empty.</returns>
      public static double MinOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, double> selector, double defaultValue)
      {
          if (source.Any<TSource>())
              return source.Min<TSource>(selector);  

          return defaultValue;
      }

      /// <summary>
      /// Invokes a transform function on each element of a sequence and returns the maximum Double value  
      /// if the sequence is not empty; otherwise returns the specified default value.  
      /// </summary>
      /// <typeparam name="TSource">The type of the elements of source.</typeparam>
      /// <param name="source">A sequence of values to determine the maximum value of.</param>
      /// <param name="selector">A transform function to apply to each element.</param>
      /// <param name="defaultValue">The default value.</param>
      /// <returns>The maximum value in the sequence or default value if sequence is empty.</returns>
      public static double MaxOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, double> selector, double defaultValue)
      {
          if (source.Any<TSource>())
              return source.Max<TSource>(selector);  

          return defaultValue;
      }
    }
}