11.5 【Linq 】连接

11.5.1 使用 join 子句的内连接

如果你打算把一个巨大的序列连接到一个极小的序列上，应尽可能把小序列作为右边序列

    class Program
    {
        static void Main(string[] args)
        {
            var query = from defect in SampleData.AllDefects
                        join subscription in SampleData.AllSubscriptions
                        on defect.Project equals subscription.Project
                        select new { defect.Summary, subscription.EmailAddress };

            foreach (var item in query)
            {
                Console.WriteLine("{0} {1}", item.Summary, item.EmailAddress);
            }

            Console.ReadKey();
        }
    }

我们通常需要对序列进行过滤，而在连接前进行过滤比在连接后过滤效率要高得多。
在这个阶段，假如只有左边的序列需要进行过滤，查询表达式相对简单一些。例如，如果我们只想显示已经关闭的缺陷，我们可以使用下面的查询表达式

    class Program
    {
        static void Main(string[] args)
        {
            var query = from defect in SampleData.AllDefects
                        where defect.Status == Status.Closed
                        join subscription in SampleData.AllSubscriptions
                        on defect.Project equals subscription.Project
                        select new { defect.Summary, subscription.EmailAddress };

            var query2 = from subscription in SampleData.AllSubscriptions
                         join defect in (from defect in SampleData.AllDefects
                                         where defect.Status == Status.Closed
                                         select defect)
                         on subscription.Project equals defect.Project
                         select new { defect.Summary, subscription.EmailAddress };

            foreach (var item in query)
            {
                Console.WriteLine("{0} {1}", item.Summary, item.EmailAddress);
            }

            Console.ReadKey();
        }
    }

11.5.2 使用 join...into 子句进行分组连接

    class Program
    {
        static void Main(string[] args)
        {
            var query = from defect in SampleData.AllDefects
                        join subscription in SampleData.AllSubscriptions
                        on defect.Project equals subscription.Project
                        into groupSubscriptions
                        select new { Defect = defect, Subscription = groupSubscriptions };

            foreach (var entry in query)
            {
                Console.WriteLine(entry.Defect.Summary);
                foreach (var subscription in entry.Subscription)
                {
                    Console.WriteLine("         {0} ", subscription.EmailAddress);
                }
            }

            Console.ReadKey();
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var dates = new DateTimeRange(SampleData.Start, SampleData.End);

            var query = from date in dates
                        join defect in SampleData.AllDefects
                        on date equals defect.Created.Date
                        into joined
                        select new { Date = date, Count = joined.Count() };

            var query2 = dates.GroupJoin(SampleData.AllDefects,
                                        date => date,
                                        defect => defect.Created.Date,
                                        (date, joined) => new { Date = date, Count = joined.Count() });

            foreach (var item in query)
            {
                Console.WriteLine("{0:d} {1}", item.Date, item.Count);
            }

            Console.ReadKey();
        }
    }

11.5.3 使用多个 from 子句进行交叉连接和合并序列

    class Program
    {
        static void Main(string[] args)
        {
            var query = from left in Enumerable.Range(1, 4)
                        from right in Enumerable.Range(11, left)
                        select new { Left = left, Right = right };

            var query2 = Enumerable.Range(1, 4)
                .SelectMany(left => Enumerable.Range(11, left),
                            (left, right) => new { Left = left, Right = right });

            foreach (var item in query)
            {
                Console.WriteLine("Left= {0} Right= {1} ", item.Left, item.Right);
            }

            Console.ReadKey();
        }
    }

SampleData代码，如下

    public class SampleData
    {
        static List<Defect> defects;
        static List<User> users;
        static List<Project> projects;
        static List<NotificationSubscription> subscriptions;

        public static readonly DateTime Start = May(1);
        public static readonly DateTime End = May(31);

        public static IEnumerable<Defect> AllDefects
        {
            get { return defects; }
        }

        public static IEnumerable<User> AllUsers
        {
            get { return users; }
        }

        public static IEnumerable<Project> AllProjects
        {
            get { return projects; }
        }

        public static IEnumerable<NotificationSubscription> AllSubscriptions
        {
            get { return subscriptions; }
        }

        public static class Projects
        {
            public static readonly Project SkeetyMediaPlayer = new Project { Name = "Skeety Media Player" };
            public static readonly Project SkeetyTalk = new Project { Name = "Skeety Talk" };
            public static readonly Project SkeetyOffice = new Project { Name = "Skeety Office" };
        }

        public static class Users
        {
            public static readonly User TesterTim = new User("Tim Trotter", UserType.Tester);
            public static readonly User TesterTara = new User("Tara Tutu", UserType.Tester);
            public static readonly User DeveloperDeborah = new User("Deborah Denton", UserType.Developer);
            public static readonly User DeveloperDarren = new User("Darren Dahlia", UserType.Developer);
            public static readonly User ManagerMary = new User("Mary Malcop", UserType.Manager);
            public static readonly User CustomerColin = new User("Colin Carton", UserType.Customer);
        }

        static SampleData()
        {
            projects = new List<Project>
            {
                Projects.SkeetyMediaPlayer,
                Projects.SkeetyTalk,
                Projects.SkeetyOffice
            };

            users = new List<User>
            {
                Users.TesterTim,
                Users.TesterTara,
                Users.DeveloperDeborah,
                Users.DeveloperDarren,
                Users.ManagerMary,
                Users.CustomerColin
            };

            subscriptions = new List<NotificationSubscription>
            {
                new NotificationSubscription { Project=Projects.SkeetyMediaPlayer, EmailAddress="media-bugs@skeetysoft.com" },
                new NotificationSubscription { Project=Projects.SkeetyTalk, EmailAddress="talk-bugs@skeetysoft.com" },
                new NotificationSubscription { Project=Projects.SkeetyOffice, EmailAddress="office-bugs@skeetysoft.com" },
                new NotificationSubscription { Project=Projects.SkeetyMediaPlayer, EmailAddress="theboss@skeetysoft.com"}
            };

            defects = new List<Defect>
            {
                new Defect
                {
                    Project = Projects.SkeetyMediaPlayer,
                    Created = May(1),
                    CreatedBy = Users.TesterTim,
                    Summary = "MP3 files crash system",
                    Severity = Severity.Showstopper,
                    AssignedTo = Users.DeveloperDarren,
                    Status = Status.Accepted,
                    LastModified = May(23)
                },

                new Defect
                {
                    Project = Projects.SkeetyMediaPlayer,
                    Created = May(3),
                    CreatedBy = Users.DeveloperDeborah,
                    Summary = "Text is too big",
                    Severity = Severity.Trivial,
                    AssignedTo = null,
                    Status = Status.Closed,
                    LastModified = May(9)
                },

                new Defect
                {
                    Project = Projects.SkeetyTalk,
                    Created = May(3),
                    CreatedBy = Users.CustomerColin,
                    Summary = "Sky is wrong shade of blue",
                    Severity = Severity.Minor,
                    AssignedTo = Users.TesterTara,
                    Status = Status.Fixed,
                    LastModified = May(19)
                },

                new Defect
                {
                    Project = Projects.SkeetyMediaPlayer,
                    Created = May(4),
                    CreatedBy = Users.DeveloperDarren,
                    Summary = "Can't play files more than 200 bytes long",
                    Severity = Severity.Major,
                    AssignedTo = Users.DeveloperDarren,
                    Status = Status.Reopened,
                    LastModified = May(23)
                },

                new Defect
                {
                    Project = Projects.SkeetyMediaPlayer,
                    Created = May(6),
                    CreatedBy = Users.TesterTim,
                    Summary = "Installation is slow",
                    Severity = Severity.Trivial,
                    AssignedTo = Users.TesterTim,
                    Status = Status.Fixed,
                    LastModified = May(15)
                },

                new Defect
                {
                    Project = Projects.SkeetyMediaPlayer,
                    Created = May(7),
                    CreatedBy = Users.ManagerMary,
                    Summary = "DivX is choppy on Pentium 100",
                    Severity = Severity.Major,
                    AssignedTo = Users.DeveloperDarren,
                    Status = Status.Accepted,
                    LastModified = May(29)
                },

                new Defect
                {
                    Project = Projects.SkeetyTalk,
                    Created = May(8),
                    CreatedBy = Users.DeveloperDeborah,
                    Summary = "Client acts as virus",
                    Severity = Severity.Showstopper,
                    AssignedTo = null,
                    Status = Status.Closed,
                    LastModified = May(10)
                },

                new Defect
                {
                    Project = Projects.SkeetyMediaPlayer,
                    Created = May(8),
                    CreatedBy = Users.DeveloperDarren,
                    Summary = "Subtitles only work in Welsh",
                    Severity = Severity.Major,
                    AssignedTo = Users.TesterTim,
                    Status = Status.Fixed,
                    LastModified = May(23)
                },

                new Defect
                {
                    Project = Projects.SkeetyTalk,
                    Created = May(9),
                    CreatedBy = Users.CustomerColin,
                    Summary = "Voice recognition is confused by background noise",
                    Severity = Severity.Minor,
                    AssignedTo = null,
                    Status = Status.Closed,
                    LastModified = May(15)
                },

                new Defect
                {
                    Project = Projects.SkeetyTalk,
                    Created = May(9),
                    CreatedBy = Users.TesterTim,
                    Summary = "User interface should be more caramelly",
                    Severity = Severity.Trivial,
                    AssignedTo = Users.DeveloperDarren,
                    Status = Status.Created,
                    LastModified = May(9)
                },

                new Defect
                {
                    Project = Projects.SkeetyMediaPlayer,
                    Created = May(10),
                    CreatedBy = Users.ManagerMary,
                    Summary = "Burning a CD makes the printer catch fire",
                    Severity = Severity.Showstopper,
                    AssignedTo = null,
                    Status = Status.Closed,
                    LastModified = May(29)
                },

                new Defect
                {
                    Project = Projects.SkeetyTalk,
                    Created = May(10),
                    CreatedBy = Users.TesterTara,
                    Summary = "Peer to peer pairing passes parameters poorly",
                    Severity = Severity.Minor,
                    AssignedTo = Users.DeveloperDarren,
                    Status = Status.Accepted,
                    LastModified = May(12)
                },

                new Defect
                {
                    Project = Projects.SkeetyTalk,
                    Created = May(11),
                    CreatedBy = Users.DeveloperDarren,
                    Summary = "Delay when sending message",
                    Severity = Severity.Minor,
                    AssignedTo = Users.TesterTara,
                    Status = Status.Fixed,
                    LastModified = May(20)
                },

                new Defect
                {
                    Project = Projects.SkeetyMediaPlayer,
                    Created = May(11),
                    CreatedBy = Users.ManagerMary,
                    Summary = "Volume control needs to go to 11",
                    Severity = Severity.Minor,
                    AssignedTo = Users.DeveloperDarren,
                    Status = Status.Created,
                    LastModified = May(11)
                },

                new Defect
                {
                    Project = Projects.SkeetyMediaPlayer,
                    Created = May(11),
                    CreatedBy = Users.CustomerColin,
                    Summary = "Splash screen fades too quickly",
                    Severity = Severity.Minor,
                    AssignedTo = Users.TesterTara,
                    Status = Status.Fixed,
                    LastModified = May(15)
                },

                new Defect
                {
                    Project = Projects.SkeetyTalk,
                    Created = May(12),
                    CreatedBy = Users.DeveloperDeborah,
                    Summary = "Text box doesn't keep up with fast typing",
                    Severity = Severity.Major,
                    AssignedTo = Users.DeveloperDeborah,
                    Status = Status.Accepted,
                    LastModified = May(12)
                },

                new Defect
                {
                    Project = Projects.SkeetyTalk,
                    Created = May(12),
                    CreatedBy = Users.DeveloperDarren,
                    Summary = "Password displayed in plain text",
                    Severity = Severity.Showstopper,
                    AssignedTo = null,
                    Status = Status.Closed,
                    LastModified = May(13)
                },

                new Defect
                {
                    Project = Projects.SkeetyMediaPlayer,
                    Created = May(12),
                    CreatedBy = Users.TesterTim,
                    Summary = "Play button points the wrong way",
                    Severity = Severity.Major,
                    AssignedTo = Users.TesterTim,
                    Status = Status.Fixed,
                    LastModified = May(17)
                },

                new Defect
                {
                    Project = Projects.SkeetyMediaPlayer,
                    Created = May(13),
                    CreatedBy = Users.CustomerColin,
                    Summary = "Wizard needed for CD burning",
                    Severity = Severity.Minor,
                    AssignedTo = Users.CustomerColin,
                    Status = Status.Fixed,
                    LastModified = May(20)
                },

                new Defect
                {
                    Project = Projects.SkeetyMediaPlayer,
                    Created = May(13),
                    CreatedBy = Users.ManagerMary,
                    Summary = "Subtitles don't display during fast forward",
                    Severity = Severity.Trivial,
                    AssignedTo = Users.DeveloperDarren,
                    Status = Status.Accepted,
                    LastModified = May(14)
                },

                new Defect
                {
                    Project = Projects.SkeetyMediaPlayer,
                    Created = May(13),
                    CreatedBy = Users.DeveloperDarren,
                    Summary = "Memory leak when watching Memento",
                    Severity = Severity.Trivial,
                    AssignedTo = Users.DeveloperDeborah,
                    Status = Status.Created,
                    LastModified = May(13)
                },

                new Defect
                {
                    Project = Projects.SkeetyTalk,
                    Created = May(13),
                    CreatedBy = Users.DeveloperDeborah,
                    Summary = "Profile screen shows login count of -1",
                    Severity = Severity.Major,
                    AssignedTo = Users.DeveloperDeborah,
                    Status = Status.Accepted,
                    LastModified = May(20)
                },

                new Defect
                {
                    Project = Projects.SkeetyTalk,
                    Created = May(13),
                    CreatedBy = Users.TesterTim,
                    Summary = "Server crashes under heavy load (3 users)",
                    Severity = Severity.Major,
                    AssignedTo = Users.DeveloperDeborah,
                    Status = Status.Accepted,
                    LastModified = May(17)
                },

                new Defect
                {
                    Project = Projects.SkeetyMediaPlayer,
                    Created = May(15),
                    CreatedBy = Users.TesterTara,
                    Summary = "Unable to connect to any media server",
                    Severity = Severity.Showstopper,
                    AssignedTo = Users.DeveloperDarren,
                    Status = Status.Reopened,
                    LastModified = May(18)
                },

                new Defect
                {
                    Project = Projects.SkeetyMediaPlayer,
                    Created = May(15),
                    CreatedBy = Users.DeveloperDeborah,
                    Summary = "UI turns black and white when playing old films",
                    Severity = Severity.Minor,
                    AssignedTo = Users.TesterTara,
                    Status = Status.Fixed,
                    LastModified = May(25)
                },

                new Defect
                {
                    Project = Projects.SkeetyTalk,
                    Created = May(16),
                    CreatedBy = Users.ManagerMary,
                    Summary = "Password reset changes passwords for all users",
                    Severity = Severity.Showstopper,
                    AssignedTo = null,
                    Status = Status.Closed,
                    LastModified = May(18)
                },

                new Defect
                {
                    Project = Projects.SkeetyMediaPlayer,
                    Created = May(17),
                    CreatedBy = Users.TesterTim,
                    Summary = "Modern music sounds rubbish",
                    Severity = Severity.Trivial,
                    AssignedTo = Users.DeveloperDarren,
                    Status = Status.Created,
                    LastModified = May(17)
                },

                new Defect
                {
                    Project = Projects.SkeetyTalk,
                    Created = May(18),
                    CreatedBy = Users.TesterTim,
                    Summary = "Webcam makes me look bald",
                    Severity = Severity.Showstopper,
                    AssignedTo = Users.TesterTim,
                    Status = Status.Fixed,
                    LastModified = May(27)
                },

                new Defect
                {
                    Project = Projects.SkeetyTalk,
                    Created = May(18),
                    CreatedBy = Users.CustomerColin,
                    Summary = "Sound is distorted when speakers are underwater",
                    Severity = Severity.Major,
                    AssignedTo = Users.DeveloperDarren,
                    Status = Status.Created,
                    LastModified = May(18)
                },

                new Defect
                {
                    Project = Projects.SkeetyTalk,
                    Created = May(19),
                    CreatedBy = Users.DeveloperDarren,
                    Summary = "Japanese characters don't display properly",
                    Severity = Severity.Major,
                    AssignedTo = Users.DeveloperDeborah,
                    Status = Status.Accepted,
                    LastModified = May(23)
                },

                new Defect
                {
                    Project = Projects.SkeetyMediaPlayer,
                    Created = May(20),
                    CreatedBy = Users.TesterTara,
                    Summary = "Video takes 100% of CPU",
                    Severity = Severity.Major,
                    AssignedTo = Users.DeveloperDeborah,
                    Status = Status.Accepted,
                    LastModified = May(22)
                },

                new Defect
                {
                    Project = Projects.SkeetyMediaPlayer,
                    Created = May(22),
                    CreatedBy = Users.TesterTim,
                    Summary = "DVD Easter eggs unavailable",
                    Severity = Severity.Trivial,
                    AssignedTo = Users.DeveloperDarren,
                    Status = Status.Created,
                    LastModified = May(22)
                },

                new Defect
                {
                    Project = Projects.SkeetyTalk,
                    Created = May(23),
                    CreatedBy = Users.ManagerMary,
                    Summary = "Transparency is high for menus to be readable",
                    Severity = Severity.Minor,
                    AssignedTo = Users.DeveloperDeborah,
                    Status = Status.Accepted,
                    LastModified = May(25)
                },

                new Defect
                {
                    Project = Projects.SkeetyMediaPlayer,
                    Created = May(24),
                    CreatedBy = Users.CustomerColin,
                    Summary = "About box is missing version number",
                    Severity = Severity.Minor,
                    AssignedTo = Users.CustomerColin,
                    Status = Status.Fixed,
                    LastModified = May(29)
                },

                new Defect
                {
                    Project = Projects.SkeetyTalk,
                    Created = May(25),
                    CreatedBy = Users.TesterTim,
                    Summary = "Logs record confidential conversations",
                    Severity = Severity.Major,
                    AssignedTo = Users.DeveloperDarren,
                    Status = Status.Reopened,
                    LastModified = May(30)
                },

                new Defect
                {
                    Project = Projects.SkeetyTalk,
                    Created = May(27),
                    CreatedBy = Users.DeveloperDeborah,
                    Summary = "Profanity filter is too aggressive",
                    Severity = Severity.Minor,
                    AssignedTo = Users.TesterTara,
                    Status = Status.Fixed,
                    LastModified = May(29)
                },

                new Defect
                {
                    Project = Projects.SkeetyMediaPlayer,
                    Created = May(27),
                    CreatedBy = Users.TesterTara,
                    Summary = "Full screen mode fails on dual monitors",
                    Severity = Severity.Minor,
                    AssignedTo = Users.DeveloperDeborah,
                    Status = Status.Created,
                    LastModified = May(27)
                },

                new Defect
                {
                    Project = Projects.SkeetyMediaPlayer,
                    Created = May(28),
                    CreatedBy = Users.CustomerColin,
                    Summary = "Visualization hypnotises pets",
                    Severity = Severity.Minor,
                    AssignedTo = Users.DeveloperDeborah,
                    Status = Status.Accepted,
                    LastModified = May(29)
                },

                new Defect
                {
                    Project = Projects.SkeetyTalk,
                    Created = May(29),
                    CreatedBy = Users.ManagerMary,
                    Summary = "Resizing while typing loses input",
                    Severity = Severity.Trivial,
                    AssignedTo = Users.DeveloperDarren,
                    Status = Status.Created,
                    LastModified = May(29)
                },

                new Defect
                {
                    Project = Projects.SkeetyMediaPlayer,
                    Created = May(30),
                    CreatedBy = Users.TesterTim,
                    Summary = "Network is saturated when playing WAV file",
                    Severity = Severity.Minor,
                    AssignedTo = Users.TesterTim,
                    Status = Status.Fixed,
                    LastModified = May(31)
                },

                new Defect
                {
                    Project = Projects.SkeetyMediaPlayer,
                    Created = May(31),
                    CreatedBy = Users.TesterTara,
                    Summary = "Media library tells user to keep the noise down",
                    Severity = Severity.Major,
                    AssignedTo = Users.DeveloperDarren,
                    Status = Status.Created,
                    LastModified = May(31)
                }
            };
        }

        public static DateTime May(int day)
        {
            return new DateTime(2013, 5, day);
        }

    }
    public enum UserType : byte
    {
        Customer,
        Developer,
        Tester,
        Manager,
    }
    public class Defect
    {
        public Project Project { get; set; }
        /// <summary>
        /// Which user is this defect currently assigned to? Should not be null until the status is Closed.
        /// </summary>
        public User AssignedTo { get; set; }
        public string Summary { get; set; }
        public Severity Severity { get; set; }
        public Status Status { get; set; }
        public DateTime Created { get; set; }
        public DateTime LastModified { get; set; }
        public User CreatedBy { get; set; }
        public int ID { get; private set; }

        public Defect()
        {
            ID = StaticCounter.Next();
        }

        public override string ToString()
        {
            return string.Format("{0,2}: {1}
    ({2:d}-{3:d}, {4}/{5}, {6} -> {7})",
                ID, Summary, Created, LastModified, Severity, Status, CreatedBy.Name,
                AssignedTo == null ? "n/a" : AssignedTo.Name);
        }
    }
    public class NotificationSubscription
    {
        /// <summary>
        /// Project for which this subscriber is notified
        /// </summary>
        public Project Project { get; set; }

        /// <summary>
        /// The address to send the notification to
        /// </summary>
        public string EmailAddress { get; set; }
    }
    public class Project
    {
        public string Name { get; set; }

        public override string ToString()
        {
            return string.Format("Project: {0}", Name);
        }
    }
    public enum Severity : byte
    {
        Trivial,
        Minor,
        Major,
        Showstopper,
    }
    public static class StaticCounter
    {
        static int next = 1;
        public static int Next()
        {
            return next++;
        }
    }
    public enum Status : byte
    {
        /// <summary>
        /// Defect has been opened, but not verified as reproducible or an issue.
        /// </summary>
        Created,
        /// <summary>
        /// Defect has been verified as an issue requiring work.
        /// </summary>
        Accepted,
        /// <summary>
        /// Defect has been fixed in code, but not verified other than through developer testing.
        /// </summary>
        Fixed,
        /// <summary>
        /// Defect was fixed, but has now been reopened due to failing verification.
        /// </summary>
        Reopened,
        /// <summary>
        /// Defect has been fixed and tested; the fix is satisfactory.
        /// </summary>
        Closed,
    }
    public class User
    {
        public string Name { get; set; }
        public UserType UserType { get; set; }

        public User(string name, UserType userType)
        {
            Name = name;
            UserType = userType;
        }

        public override string ToString()
        {
            return string.Format("User: {0} ({1})", Name, UserType);
        }
    }
    static class Extensions
    {
        public static Dummy<T> Where<T>(this Dummy<T> dummy,
                                        Func<T, bool> predicate)
        {
            Console.WriteLine("Where called");
            return dummy;
        }
    }
    class Dummy<T>
    {
        public Dummy<U> Select<U>(Func<T, U> selector)
        {
            Console.WriteLine("Select called");
            return new Dummy<U>();
        }
    }
    public class DateTimeRange : IEnumerable<DateTime>
    {
        private readonly DateTime start;
        private readonly DateTime end;

        public DateTimeRange(DateTime start, DateTime end)
        {
            this.start = start;
            this.end = end;
        }

        public IEnumerator<DateTime> GetEnumerator()
        {
            for (DateTime current = start; current <= end; current = current.AddDays(1))
            {
                yield return current;
            }
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            return GetEnumerator();
        }
    }