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("	{0}	{1}	{2}	{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}	{1}	{2}	{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}	{1}	{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}	{1}	{2}	{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}	{1}	{2}	{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}	{1}	{2}	{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}	{1}	{2}	{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("	{0}	{1}	{2}	{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("	{0}	{1}	{2}	{3}", item.ID, item.Name, item.Region, item.Price));