yield 与 IEnumerable<T> 结对出现, 可实现按需获取 , 迭代器模式

static void Main(string[] args)
         {
             try
             {
                 {
                     //Console.WriteLine("***************Collection**************");
                     CollectionDemo.Show();
                 }
                 {
                     Console.WriteLine("*******************Yield********************");
                     YieldDemo yieldDemo = new YieldDemo();
                     foreach (var item in yieldDemo.Power())
                     {
                         Console.WriteLine(item);//按需获取，要一个拿一个
                         if (item > 100)
                             break;
                     }
                     Console.WriteLine("*******************************************");
                     foreach (var item in yieldDemo.Common())
                     {
                         Console.WriteLine(yieldDemo.Common().Count());//先全部获取，然后一起返还
                         if (item > 100)
                             break;
                     }
                 }

{
     List<string> fruits =
     new List<string> { "apple", "passionfruit", "banana", "mango",
         "orange", "blueberry", "grape", "strawberry" };

IEnumerable<string> query = fruits.Where(fruit => fruit.Length < 6);
     foreach (var item in query)//遍历才会去查询比较   迭代器 yield
     {

}

IQueryable<string> queryable = fruits.AsQueryable<string>().Where(s => s.Length <6);
     foreach (var item in queryable)//表达式目录树的解析，延迟到遍历的时候才去执行  EF的延迟查询
     {

}
}
             }
             catch (Exception ex)
             {
                 Console.WriteLine(ex.Message);
             }
             Console.Read();
         }

public class YieldDemo
{
     public IEnumerable<int> Power()
     {
         for (int i = 0; i < 10; i++)
         {
             yield return this.Get(i);
             //yield return this.Get(i);

//Console.WriteLine("这里再来一次");

//yield return this.Get(i) + 1;
         }
     }

public IEnumerable<int> Common()
     {
         List<int> intList = new List<int>();
         for (int i = 0; i < 10; i++)
         {
             intList.Add(this.Get(i));
         }
         return intList;
     }

private int Get(int num)
     {
         Thread.Sleep(2000);
         return num * DateTime.Now.Second;
     }

}

public static class ExtendMethod
{
     public static IEnumerable<T> ElevenWhere<T>(this IEnumerable<T> source, Func<T, bool> func)
     {
         if (source == null)
         {
             throw new Exception("source is null");
         }
         if (func == null)
         {
             throw new Exception("func is null");
         }
         foreach (var item in source)
         {
             if (func.Invoke(item))
             {
                 yield return item;
             }
         }
     }
}