筛选信息

筛选器共两类：带有where子句的传统筛选器以及新的筛选器OfType。OfType用于筛选能够转换为特定类型的值。

WHERE子句

var numbers = new int[] { -1, -32, 3, 5, -8, 13, 7, -41 }; // 筛选绝对值大于 5 的数字 var magnitude = from n in numbers where Math.Abs(n) > 5 select n; foreach (var m in magnitude) Console.WriteLine(m);

OFTYPE

// 从数组中筛选能够转换成整型的元素 var randomData = new object[] { 1, "two", 3, "four", 5.5, "six", 7M }; var canBeInt = from r in randomData.OfType<int>() select r; foreach (var i in canBeInt) Console.WriteLine(i);

使用限定符 All, Any, Contains

All 判断是否每个成员都满足条件Any 指示是否有满足条件得成员Contains 用于查找符合条件的单个元素 class Program { static void Main(string[] args) { var household = new List<FamilyMember> { new FamilyMember{Name="Paul Kimmel", Species="Lunkhead in Chief", Gender="Male"}, new FamilyMember{Name="Dena Swanson", Species="Boss", Gender="Female"}, new FamilyMember{Name="Alex Kimmel", Species="Princess", Gender="Female"}, new FamilyMember{Name="Noah Kimmel", Species="Annoying Brother", Gender="Male"}, new FamilyMember{Name="Joe Swanson", Species="Homosapien", Gender="Male"}, new FamilyMember{Name="Ruby", Species="Canine", Gender="Female"}, new FamilyMember{Name="Leda", Species="Canine", Gender="Female"}, new FamilyMember{Name="Big Mama", Species="Feline", Gender="Female"} }; // 判断序列中是否有满足条件的子项 bool anyCats = household.Any(m => m.Species == "Feline"); Console.WriteLine("Any ? {0}", anyCats); // 判断序列中是否所有子项都满足指定条件 bool allCats = household.All(m => m.Species == "Feline"); Console.WriteLine("All ? {0}", allCats); bool containsCats = household.Contains(new FamilyMember{Name="Paul Kimmel", Species="Lunkhead in Chief", Gender="Male"}); Console.WriteLine("Contains ? {0}", containsCats); } } public class FamilyMember { public string Name { get; set; } public string Species { get; set; } public string Gender { get; set; } }

利用Skip和Take实现分区操作

Skip、SkipWhile、Take和TakeWhile用于将集合拆分成两个部分并返回其中一个。

var ints = new int[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 }; // 获取指定位置之后的元素(包含指定位置的元素) Console.WriteLine("+++++++++++++++++++++Skip+++++++++++++++++++++"); var resultSkip = ints.Skip(3); Array.ForEach(resultSkip.ToArray(), n => Console.Write(n + " ")); // 获取条件为false的元素 Console.WriteLine("+++++++++++++++++++++SkipWhile++++++++++++++++"); var resultSkipWhile = ints.SkipWhile(n => n <= 5); Array.ForEach(resultSkipWhile.ToArray(), n => Console.Write(n + " ")); // 获取指定位置之前的元素(不包含指定位置的元素) Console.WriteLine("+++++++++++++++++++++Take+++++++++++++++++++++"); var resultTake = ints.Take(3); Array.ForEach(resultTake.ToArray(), n => Console.Write(n + " ")); // 获取条件为true的元素 Console.WriteLine("+++++++++++++++++++++TakeWhile++++++++++++++++"); var resultTakeWhile = ints.TakeWhile(n => n <= 5); Array.ForEach(resultTakeWhile.ToArray(), n => Console.Write(n + " "));

结果 +++++++++++++++++++++Skip+++++++++++++++++++++ 3 4 5 6 7 8 9 10 +++++++++++++++++++++SkipWhile+++++++++++++++++ 6 7 8 9 10 +++++++++++++++++++++Take+++++++++++++++++++++ 0 1 2 +++++++++++++++++++++TakeWhile+++++++++++++++++ 0 1 2 3 4 5

使用生成运算

生成运算包括DefaultIfEmpty、Empty、Range以及Repeat。它们都是用于创建新的序列。

// 根据指定类型创建一个空的序列 var empties = Enumerable.Empty<int>(); Console.WriteLine("Type : {0}", empties.GetType()); Console.WriteLine("Count : {0}", empties.Count()); Console.WriteLine("========================================="); // 如果参数指定的序列为空，则使用指定类型的默认值创建一个数量为1的序列 var default_empties = Enumerable.DefaultIfEmpty<int>(Enumerable.Empty<int>()); Console.WriteLine("Type : {0}", default_empties.GetType()); Console.WriteLine("Count : {0}", default_empties.Count()); Array.ForEach(default_empties.ToArray(), n => Console.WriteLine(n)); Console.WriteLine("========================================="); var range = Enumerable.Range(10, 5); Array.ForEach(range.ToArray(), n => Console.WriteLine(n)); Console.WriteLine("Type : {0}", range.GetType()); Console.WriteLine("Count : {0}", range.Count()); Console.WriteLine("========================================="); var repeat = Enumerable.Repeat(10, 5); Array.ForEach(repeat.ToArray(), n => Console.WriteLine(n)); Console.WriteLine("Type : {0}", repeat.GetType()); Console.WriteLine("Count : {0}", repeat.Count());

结果 Type : System.Linq.EmptyPartition1[System.Int32] Count : 0 ========================================= Type : System.Linq.Enumerable+DefaultIfEmptyIterator1[System.Int32] Count : 1 0 ========================================= 10 11 12 13 14 Type : System.Linq.Enumerable+RangeIterator Count : 5 ========================================= 10 10 10 10 10 Type : System.Linq.Enumerable+RepeatIterator`1[System.Int32] Count : 5

相等性测试

var folder1 = Directory.GetDirectories("C:\\"); var folder2 = Directory.GetDirectories("C:\\Users\\Administrator"); // 比较两个序列中的所有子项是否相等 Console.WriteLine("Equal: " + folder1.SequenceEqual(folder2)); // 获取两个序列的差异，folder1中有而folder2中没有 var missingFolders = folder1.Except(folder2); Console.WriteLine("Missing Folders: {0}", missingFolders.Count()); Array.ForEach(missingFolders.ToArray(), folder => Console.WriteLine(folder));

从序列中获取特定元素

var numbers = new int[] { 1, 2, 3, 4, 5, 6, 7, 8, 9}; // 获取序列中的第一个元素 var first = (from n in numbers select n).First(); Console.WriteLine("First: {0}", first); // 获取序列中满足指定条件第一个的元素，如果都不满足条件则返回该类型的默认值 var firstOrDefault = (from n in numbers select n).FirstOrDefault(n => n > 10); Console.WriteLine("First or default: {0}", firstOrDefault); // 获取序列中的最后一个元素 var last = (from n in numbers select n).Last(); Console.WriteLine("Last: {0}", last); // 获取序列中满足指定条件最后一个的元素，如果都不满足条件则返回该类型的默认值 var lastOrDefault = (from n in numbers select n).LastOrDefault(n => n < 5); Console.WriteLine("Last or default: {0}", lastOrDefault); // 序列中有且包含一个元素 var single = (from n in numbers where n == 5 select n).Single(); Console.WriteLine("Single: {0}", single); var singleOrDefault = (from n in numbers select n).SingleOrDefault(n => n == 3); Console.WriteLine("Single or default: {0}", singleOrDefault); // 获取指定位置的元素 var element = (from n in numbers where n < 8 select n).ElementAt(6); Console.WriteLine("Element at 6: {0}", element); var elementOrDefault = (from n in numbers select n).ElementAtOrDefault(11); Console.WriteLine("Element at 11 or default: {0}", elementOrDefault);

通过Concat串联序列

var numbers = new int[] { 1, 2, 3, 4, 5, 6, 7, 8, 9 }; var moreNumbers = new int[] { 10, 11, 12, 13, 14, 15 }; foreach (var n in numbers.Concat(moreNumbers)) Console.WriteLine(n);