Linq知识大全

select的源码public static IEnumerable<TResult> Select<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, TResult> selector);

 class Program
{
static void Main(string[] args)
{
    var customers = new[] {  
        new { CustomerID = 1, FirstName = "Orlando", LastName = "Gee",  
                CompanyName = "A Bike Store" },
        new { CustomerID = 2, FirstName = "Keith", LastName = "Harris",  
                CompanyName = "Bike World" },
        new { CustomerID = 3, FirstName = "Donna", LastName = "Carreras",  
                CompanyName = "A Bike Store" },
        new { CustomerID = 4, FirstName = "Janet", LastName = "Gates",  
                    CompanyName = "Fitness Hotel" },
        new { CustomerID = 5, FirstName = "Lucy", LastName = "Harrington",  
                CompanyName = "Grand Industries" },
        new { CustomerID = 6, FirstName = "David", LastName = "Liu",  
                CompanyName = "Bike World" },
        new { CustomerID = 7, FirstName = "Donald", LastName = "Blanton",  
                CompanyName = "Grand Industries" },
        new { CustomerID = 8, FirstName = "Jackie", LastName = "Blackwell",  
                CompanyName = "Fitness Hotel" },
        new { CustomerID = 9, FirstName = "Elsa", LastName = "Leavitt",  
                CompanyName = "Grand Industries" },
        new { CustomerID = 10, FirstName = "Eric", LastName = "Lang",  
                    CompanyName = "Distant Inn" }  
    };
    var addresses = new[] {  
        new { CompanyName = "A Bike Store", City = "New York", Country = "United States"},
        new { CompanyName = "Bike World", City = "Chicago", Country = "United States"},
        new { CompanyName = "Fitness Hotel", City = "Ottawa", Country = "Canada"},
        new { CompanyName = "Grand Industries", City = "London",Country = "United Kingdom"},
        new { CompanyName = "Distant Inn", City = "Tetbury", Country = "United Kingdom"}
    };

    //查询数据
    IEnumerable<string> customerFirstNames = customers.Select(cust => cust.FirstName);
    //IEnumerable<string> customerFirstNames = from custs in customers select custs.FirstName;
    foreach (string name in customerFirstNames)
    {
        Console.WriteLine(name);
    }
    Console.WriteLine(" ");

    IEnumerable<string> customerFullNames = customers.Select(cust => cust.FirstName + "  " + cust.LastName);
    //IEnumerable<string> customerFullNames = from custs in customers select custs.FirstName + "  " + custs.LastName;
    foreach (string name in customerFullNames)
    {
        Console.WriteLine(name);
    }

    Console.WriteLine(" ");
    var customerNames = customers.Select(cust => new { FirstName = cust.FirstName, LastName = cust.LastName });
    //var customerNames = from custs in customers select new { FirstName = custs.FirstName, LastName = custs.LastName };
    foreach (var name in customerNames)
    {
        Console.WriteLine(name);
    }

    //数据筛选
    Console.WriteLine(" ");
    IEnumerable<string> usCompanies = addresses.Where(addr => String.Equals(addr.Country, "United States"))
                                                .Select(usComp => usComp.CompanyName);
    //IEnumerable<string> usCompanies = from adr in addresses
    //                                  where String.Equals(adr.Country, "United States")
    //                                  select adr.CompanyName;
    foreach (string name in usCompanies)
    {
        Console.WriteLine(name);
    }

    //数据排序
    Console.WriteLine(" ");
    IEnumerable<string> companyNames = addresses.OrderBy(addr => addr.CompanyName).Select(comp => comp.CompanyName);
    //IEnumerable<string> companyNames = from adr in addresses orderby adr.CompanyName select adr.CompanyName;
    foreach (string name in companyNames)
    {
        Console.WriteLine(name);
    }

    Console.WriteLine(" ");
    //数据分组---分组之后变成二维数组
    var companiesGroupedByCountry = addresses.GroupBy(addrs => addrs.Country);
    //var companiesGroupedByCountry = from adr in addresses group adr by adr.Country;
    foreach (var companiesPerCountry in companiesGroupedByCountry)
    {
        Console.WriteLine("Country: {0} {1} companies",
                companiesPerCountry.Key, companiesPerCountry.Count());
        foreach (var companies in companiesPerCountry)
        {
            Console.WriteLine(" {0}", companies.CompanyName);
        }
    }

    //数据统计
    Console.WriteLine(" ");
    int numberOfCountries = addresses.Select(addr => addr.Country).Distinct().Count();
    //int numberOfCountries = (from adr in addresses select adr.Country).Distinct().Count();
    Console.WriteLine("Number of countries: {0}", numberOfCountries);

    //联合查询--内联查询
    Console.WriteLine(" ");
    var citiesAndCustomers = customers.Select(c =>
        new { c.FirstName, c.LastName, c.CompanyName })
        .Join(addresses, custs => custs.CompanyName, addrs => addrs.CompanyName, (custs, addrs) => new { custs.FirstName, custs.LastName, addrs.Country });

    //var citiesAndCustomers = from cuts in customers
    //                         join adr in addresses
    //                             on cuts.CompanyName equals adr.CompanyName
    //                         select new { cuts.FirstName, cuts.LastName, adr.Country };
    foreach (var row in citiesAndCustomers)
    {
        Console.WriteLine(row);
    }

    Console.ReadKey();
    }
}
}

 private static void Orderby()
        {
            int[] intAry1 = { 4, 3, 556, 34, 6, 2, 67 };
            var query1 = from var1 in intAry1
                         orderby var1 ascending
                         select var1;
            query1.ToList().ForEach(p => Console.Write(p + Environment.NewLine));
            var query2 = from var1 in intAry1
                         orderby var1 descending
                         select var1;
            query2.ToList().ForEach(p => Console.Write(p + Environment.NewLine));
            Console.WriteLine();
        }

        private static void Join左外部连接()
        {
            int[] intAry1 = { 5, 15, 25, 30, 33, 40 };
            int[] intAry2 = { 5, 20, 30, 30, 50, 70, 80 };
            var query1 = from var1 in intAry1
                         join var2 in intAry2 on var1 equals var2 into var2Grp //左连接 右连接相反
                         from grp in var2Grp.DefaultIfEmpty()
                         select new { a = var1 != 0 ? var1 : 100, b = grp };
            query1.ToList().ForEach(p => Console.Write("{0}:{1}", p.a, p.b + Environment.NewLine));
            Console.WriteLine();
        }

        private static void Join分组连接()
        {
            int[] intAry1 = { 5, 15, 25, 30, 33, 40 };
            int[] intAry2 = { 5, 20, 30, 30, 50, 70, 80 };
            var query1 = from var1 in intAry1
                         join var2 in intAry2 on var1 equals var2 into var2Grp
                         select new { a = var1, b = var2Grp };
            foreach (var item in query1)
            {
                Console.Write("{0}:", item.a);
                foreach (var o in item.b)
                {
                    Console.Write("{0} ", o);
                }
                Console.WriteLine();
            }
            Console.WriteLine();
        }

        private static void Join内部连接()
        {
            int[] intAry1 = { 5, 15, 25, 30, 33, 40 };
            int[] intAry2 = { 5, 20, 30, 30, 50, 70, 80 };
            var query1 = from var1 in intAry1
                         join var2 in intAry2 on var1 equals var2
                         select new { a = var1, b = var2 };
            query1.ToList().ForEach(p => Console.Write(p.a + Environment.NewLine));
            Console.WriteLine();
        }