C# 关于Linq延迟查询

 demo:

 int Count = 0;string[] obj = { "item1", "item2", "item3", "item4", "item5", "item6" };var query = obj.Where(item => IsTrue(item));// 第一次遍历foreach (var item in query){Console.WriteLine(item);}Console.WriteLine();// 第二次遍历query.ToList(); //ToList() 会导致遍历整个查询// 第三次遍历if (query.Any()) // Any() 也会导致遍历，找到则返回{Console.WriteLine("At least one item is true.\r\n");}if (query.Count() > 0)//Count() 会全部遍历{Console.WriteLine("Count > 0");}bool IsTrue(string item){Count++;Console.WriteLine($"IsTrue is called:{Count},{Count % 2 == 0}---------{item}");return Count % 2 == 0; // 示例中总是返回true}

结果

IsTrue is called:1,False---------item1
IsTrue is called:2,True---------item2
item2
IsTrue is called:3,False---------item3
IsTrue is called:4,True---------item4
item4
IsTrue is called:5,False---------item5
IsTrue is called:6,True---------item6
item6IsTrue is called:7,False---------item1
IsTrue is called:8,True---------item2
IsTrue is called:9,False---------item3
IsTrue is called:10,True---------item4
IsTrue is called:11,False---------item5
IsTrue is called:12,True---------item6
IsTrue is called:13,False---------item1
IsTrue is called:14,True---------item2
At least one item is true.IsTrue is called:15,False---------item1
IsTrue is called:16,True---------item2
IsTrue is called:17,False---------item3
IsTrue is called:18,True---------item4
IsTrue is called:19,False---------item5
IsTrue is called:20,True---------item6
Count > 0