IEnumerable的一些基本方法
在说明用法之后，先要弄点数据。
class Product { public int ID { get; set; } public string Name { get; set; } public string Region { get; set; } public decimal Price { get; set; } public bool IsFavorite { get; set; } } List<Product> products = new List<Product> { new Product { ID=1, Name="路易十八比萨饼", Region="意大利", Price=79961, IsFavorite = false }, new Product { ID=2, Name="澳洲胡桃", Region="澳洲", Price=195, IsFavorite = false }, new Product { ID=3, Name="Almas鱼子酱", Region="伊朗", Price=129950, IsFavorite = false }, new Product { ID=4, Name="和牛肉", Region="日本", Price=3250, IsFavorite = true }, new Product { ID=5, Name="麝香猫咖啡豆", Region="印尼", Price=2000, IsFavorite = true }, new Product { ID=6, Name="大红袍茶叶", Region="中国", Price=208000, IsFavorite = true }, new Product { ID=7, Name="Kona Nigari矿泉水", Region="美国", Price=13000, IsFavorite = true }, new Product { ID=8, Name="Diva伏特加", Region="北欧", Price=6500, IsFavorite = false }, new Product { ID=9, Name="番红花的雄蕊", Region="地中海", Price=38986, IsFavorite = false }, };
一、ALL和ANY
bool allChina = products.All(p => p.Region == "中国");//所有项Region都要是中国，结果：False bool anyChina = products.Any(p => p.Region == "中国");//某一项Region是中国，结果：True
二、聚集
int countIdGreater5 = products.Count(p => p.ID > 5);//ID大于5的记录数，结果：4 decimal maxPrice = products.Max(p => p.Price);//金额最高，结果：208000 int minId = products.Min(p => p.ID);//编号最小，结果：1 decimal avgPrice = products.Average(p => p.Price);//金额平均值，结果：53538 decimal sumPrice = products.Sum(p => p.Price);//金额总值 结果：481842
三、累加器
Product aggregate1 = products.Aggregate((total, next) =>//累加器，对products中每一个元素执行一次Func { total.Price += next.Price; return total; });
上面的代码可以做一下优化
decimal aggregate2 = products.Aggregate(2000M, (total, next) =>//累加器可以给初始值，这里给的值是2000 { total += next.Price; return total;//这里返回的类型和初始值一致 });
累加器操作的时候尽量用值类型，上面2段代码如果一起执行，aggregate2的值就会出现异常。
四、SELECT
string[] select1 = products.Select(p => p.Name).ToArray();//选择单列，可以转换成数组 var select2 = products.Select(p => new { p.ID, p.Name }).ToDictionary(d => d.ID);//选择两列，可以转换成键值对 var selectMore = products.Select(p => new { p.ID, p.Name, p.Price }).ToList();//选择多列，可以转换成对象 //键值对必须要保证键值是唯一的，在键值不唯一的情况可以使用ToLookup方法 var lookup = products.ToLookup(l => l.IsFavorite, p => new { p.ID, p.Name, p.Region, p.Price }).ToList(); lookup.ForEach(l => { Console.WriteLine(l.Key ? "已收藏" : "未收藏"); l.ToList().ForEach(item => Console.WriteLine("\t{0}\t{1}\t{2}\t{3}", item.ID, item.Name, item.Region, item.Price)); });
五、ORDER BY
var rightOrder = products.OrderBy(p => p.IsFavorite).ThenByDescending(p => p.ID).ToList();//主IsFavorite，次ID var errorOrder = products.OrderBy(p => p.IsFavorite).OrderByDescending(p => p.ID).ToList();//主ID，次IsFavorite
六、GROUP BY
var group = products.GroupBy(p => p.IsFavorite).Select(g => new { IsFavorite = g.Key, SumPrice = g.Sum(item => item.Price), CountItem = g.Count() }).ToList();
当然在写拉姆达表达式的时候，也顺便说一个LINQ的用法
var groupLinq = (from p in products group p by p.IsFavorite into g select new { IsFavorite = g.Key, SumPrice = g.Sum(item => item.Price), CountItem = g.Count() }).ToList();
七、WHERE
List<Product> distinct = products.Distinct().ToList();//去掉重复的记录 List<Product> take = products.Take(3).ToList();//顺序取3条记录 List<Product> takeWhile = products.TakeWhile(p => p.ID <= 4).ToList();//只要不满足条件了，返回所有当前记录 List<Product> skip = products.Skip(3).ToList();//顺序跳过3条记录 List<Product> skipWhile = products.SkipWhile(p => p.Price < 100000).ToList();//只要不满足条件了，返回所有剩余记录 List<Product> contains = products.Where(p => p.Name.Contains("红")).ToList();//包含“红”的集合 Product first = products.Where(p => p.Name.StartsWith("大")).First();//“大”开头的第一条记录 如果无记录，直接报异常 Product lastDefault = products.Where(p => p.Name.EndsWith("胡")).LastOrDefault();//“胡”结尾的最后一条记录 如果无记录，返回默认值（对象返回null）不会报异常 Product single = products.Where(p => p.ID == 1).SingleOrDefault();//取单条记录，有多条时会报异常 Product elementDefault = products.ElementAtOrDefault(10);//返回第10条记录 如果没有第10条记录，返回默认值（对象返回null）不会报异常
八、默认
products.DefaultIfEmpty(new Product { ID = 999, Name = "默认产品", Region = "默认地区", Price = 0 });//判断是否为空，是返回默认值，否返回products
单集合操作讲得差不多了，下面说一下多集合操作的，还是老套路，先弄点数据，这里我们数据用最普遍的DataTable格式
DataTable table1 = new DataTable(); table1.Columns.Add("ID"); table1.Columns.Add("Name"); table1.Columns.Add("Amount"); table1.Columns.Add("Description"); table1.Rows.Add("1", "张三", "200", "不知道和张三丰有什么关系？"); table1.Rows.Add("2", "李四", "4", "无"); table1.Rows.Add("3", "王五", "5", "是住你家隔壁的那位吗？"); DataTable table2 = new DataTable(); table2.Columns.Add("ID"); table2.Columns.Add("Name"); table2.Columns.Add("Amount"); table2.Columns.Add("Description"); table2.Rows.Add("1", "张三", "200", "不知道和张三丰有什么关系？"); table2.Rows.Add("3", "老王", "15000", "这才是隔壁那位吧"); table2.Rows.Add("5", "老刘", "20", "无");
九、JOIN
//两表内联，结果有2条记录 var joinTable = table1.AsEnumerable().Join(table2.AsEnumerable(), left => left["ID"].ToString(), right => right["ID"].ToString(), (left, right) => new { LeftID = left["ID"].ToString(), RightID = right["ID"].ToString(), LeftName = left["Name"].ToString(), RightName = right["Name"].ToString() }).ToList(); joinTable.ForEach(t => Console.WriteLine("{0}\t{1}\t{2}\t{3}", t.LeftID, t.RightID, t.LeftName, t.RightName));
十、GROUPJOIN
//以第一个表为基准，对第二个表进行分组 var groupJoinTable = table1.AsEnumerable().GroupJoin(table2.AsEnumerable(), left => left["Description"].ToString(), right => right["Description"].ToString(), (key, g) => new { Key = key["Description"].ToString(), Count = g.Count(), TotalAmount = g.Where(s => decimal.Parse(s["Amount"].ToString()) > 20).Sum(s => decimal.Parse(s["Amount"].ToString())) }).ToList(); groupJoinTable.ForEach(t => Console.WriteLine("{0}\t{1}\t{2}", t.Key, t.Count, t.TotalAmount));
这里的统计不会包括第一个表的记录，第一个表只何为一个索引使用
十一、比较两个表是否相等
bool isEqual = table1.AsEnumerable().Where(t => t["ID"].ToString() == "1") .SequenceEqual(table2.AsEnumerable().Where(t => t["ID"].ToString() == "1"), DataRowComparer.Default); Console.WriteLine(isEqual);
这里只是做了单条记录的比较，为的只是返回一个TRUE，整个集合比较也是可以的
十二、连接两个表，不去重复，列取公共部分
var concatTable = table1.AsEnumerable().Concat(table2.AsEnumerable()).ToList(); concatTable.ForEach(t => Console.WriteLine("{0}\t{1}\t{2}\t{3}", t["ID"], t["Name"], t["Amount"], t["Description"]));
十三、差集、交集、并集
//两表的差集 var exceptTable = table1.AsEnumerable().Except(table2.AsEnumerable(), DataRowComparer.Default).ToList(); exceptTable.ForEach(t => Console.WriteLine("{0}\t{1}\t{2}\t{3}", t["ID"], t["Name"], t["Amount"], t["Description"])); //两表的交集 var intersectTable = table1.AsEnumerable().Intersect(table2.AsEnumerable(), DataRowComparer.Default).ToList(); intersectTable.ForEach(t => Console.WriteLine("{0}\t{1}\t{2}\t{3}", t["ID"], t["Name"], t["Amount"], t["Description"])); //两表的并集 var unionTable = table1.AsEnumerable().Union(table2.AsEnumerable(), DataRowComparer.Default).ToList(); unionTable.ForEach(t => Console.WriteLine("{0}\t{1}\t{2}\t{3}", t["ID"], t["Name"], t["Amount"], t["Description"]));
最后说一个比较好玩的方法，还是老规矩，先弄数据
List<Store> stores = new List<Store> { new Store { ID = 1, Name = "城北", Products = new List<Product> { new Product { ID=1, Name="路易十八比萨饼", Region="意大利", Price=79961, IsFavorite = false }, new Product { ID=2, Name="澳洲胡桃", Region="澳洲", Price=195, IsFavorite = false }, new Product { ID=3, Name="Almas鱼子酱", Region="伊朗", Price=129950, IsFavorite = false } } }, new Store { ID = 1, Name = "城南", Products = new List<Product> { new Product { ID=4, Name="和牛肉", Region="日本", Price=3250, IsFavorite = true }, new Product { ID=5, Name="麝香猫咖啡豆", Region="印尼", Price=2000, IsFavorite = true }, new Product { ID=6, Name="大红袍茶叶", Region="中国", Price=208000, IsFavorite = true } } }, new Store { ID = 1, Name = "城东", Products = new List<Product> { new Product { ID=7, Name="Kona Nigari矿泉水", Region="美国", Price=13000, IsFavorite = true }, new Product { ID=8, Name="Diva伏特加", Region="北欧", Price=6500, IsFavorite = false }, new Product { ID=9, Name="番红花的雄蕊", Region="地中海", Price=38986, IsFavorite = false } } } };
我把上面的9个产品分到了3个仓库里面存在，当我要查找金额小于10000的所有产品时，按以前的做法就要写2个FOREACH循环，现在用到SELECTMANY就方便多了
var selectMany = stores.SelectMany(s => s.Products).Where(p => p.Price < 10000).ToList(); selectMany.ForEach(item => Console.WriteLine("\t{0}\t{1}\t{2}\t{3}", item.ID, item.Name, item.Region, item.Price));
当然，也可以用LINQ的方式
var linqSelectMany = from s in stores from p in s.Products where p.Price < 10000 select p; linqSelectMany.ToList().ForEach(item => Console.WriteLine("\t{0}\t{1}\t{2}\t{3}", item.ID, item.Name, item.Region, item.Price));
