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(, )
from right in Enumerable.Range(, left)
select new { Left = left, Right = right }; var query2 = Enumerable.Range(, )
.SelectMany(left => Enumerable.Range(, 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();
public static readonly DateTime End = May(); 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(),
CreatedBy = Users.TesterTim,
Summary = "MP3 files crash system",
Severity = Severity.Showstopper,
AssignedTo = Users.DeveloperDarren,
Status = Status.Accepted,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyMediaPlayer,
Created = May(),
CreatedBy = Users.DeveloperDeborah,
Summary = "Text is too big",
Severity = Severity.Trivial,
AssignedTo = null,
Status = Status.Closed,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyTalk,
Created = May(),
CreatedBy = Users.CustomerColin,
Summary = "Sky is wrong shade of blue",
Severity = Severity.Minor,
AssignedTo = Users.TesterTara,
Status = Status.Fixed,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyMediaPlayer,
Created = May(),
CreatedBy = Users.DeveloperDarren,
Summary = "Can't play files more than 200 bytes long",
Severity = Severity.Major,
AssignedTo = Users.DeveloperDarren,
Status = Status.Reopened,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyMediaPlayer,
Created = May(),
CreatedBy = Users.TesterTim,
Summary = "Installation is slow",
Severity = Severity.Trivial,
AssignedTo = Users.TesterTim,
Status = Status.Fixed,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyMediaPlayer,
Created = May(),
CreatedBy = Users.ManagerMary,
Summary = "DivX is choppy on Pentium 100",
Severity = Severity.Major,
AssignedTo = Users.DeveloperDarren,
Status = Status.Accepted,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyTalk,
Created = May(),
CreatedBy = Users.DeveloperDeborah,
Summary = "Client acts as virus",
Severity = Severity.Showstopper,
AssignedTo = null,
Status = Status.Closed,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyMediaPlayer,
Created = May(),
CreatedBy = Users.DeveloperDarren,
Summary = "Subtitles only work in Welsh",
Severity = Severity.Major,
AssignedTo = Users.TesterTim,
Status = Status.Fixed,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyTalk,
Created = May(),
CreatedBy = Users.CustomerColin,
Summary = "Voice recognition is confused by background noise",
Severity = Severity.Minor,
AssignedTo = null,
Status = Status.Closed,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyTalk,
Created = May(),
CreatedBy = Users.TesterTim,
Summary = "User interface should be more caramelly",
Severity = Severity.Trivial,
AssignedTo = Users.DeveloperDarren,
Status = Status.Created,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyMediaPlayer,
Created = May(),
CreatedBy = Users.ManagerMary,
Summary = "Burning a CD makes the printer catch fire",
Severity = Severity.Showstopper,
AssignedTo = null,
Status = Status.Closed,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyTalk,
Created = May(),
CreatedBy = Users.TesterTara,
Summary = "Peer to peer pairing passes parameters poorly",
Severity = Severity.Minor,
AssignedTo = Users.DeveloperDarren,
Status = Status.Accepted,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyTalk,
Created = May(),
CreatedBy = Users.DeveloperDarren,
Summary = "Delay when sending message",
Severity = Severity.Minor,
AssignedTo = Users.TesterTara,
Status = Status.Fixed,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyMediaPlayer,
Created = May(),
CreatedBy = Users.ManagerMary,
Summary = "Volume control needs to go to 11",
Severity = Severity.Minor,
AssignedTo = Users.DeveloperDarren,
Status = Status.Created,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyMediaPlayer,
Created = May(),
CreatedBy = Users.CustomerColin,
Summary = "Splash screen fades too quickly",
Severity = Severity.Minor,
AssignedTo = Users.TesterTara,
Status = Status.Fixed,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyTalk,
Created = May(),
CreatedBy = Users.DeveloperDeborah,
Summary = "Text box doesn't keep up with fast typing",
Severity = Severity.Major,
AssignedTo = Users.DeveloperDeborah,
Status = Status.Accepted,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyTalk,
Created = May(),
CreatedBy = Users.DeveloperDarren,
Summary = "Password displayed in plain text",
Severity = Severity.Showstopper,
AssignedTo = null,
Status = Status.Closed,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyMediaPlayer,
Created = May(),
CreatedBy = Users.TesterTim,
Summary = "Play button points the wrong way",
Severity = Severity.Major,
AssignedTo = Users.TesterTim,
Status = Status.Fixed,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyMediaPlayer,
Created = May(),
CreatedBy = Users.CustomerColin,
Summary = "Wizard needed for CD burning",
Severity = Severity.Minor,
AssignedTo = Users.CustomerColin,
Status = Status.Fixed,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyMediaPlayer,
Created = May(),
CreatedBy = Users.ManagerMary,
Summary = "Subtitles don't display during fast forward",
Severity = Severity.Trivial,
AssignedTo = Users.DeveloperDarren,
Status = Status.Accepted,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyMediaPlayer,
Created = May(),
CreatedBy = Users.DeveloperDarren,
Summary = "Memory leak when watching Memento",
Severity = Severity.Trivial,
AssignedTo = Users.DeveloperDeborah,
Status = Status.Created,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyTalk,
Created = May(),
CreatedBy = Users.DeveloperDeborah,
Summary = "Profile screen shows login count of -1",
Severity = Severity.Major,
AssignedTo = Users.DeveloperDeborah,
Status = Status.Accepted,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyTalk,
Created = May(),
CreatedBy = Users.TesterTim,
Summary = "Server crashes under heavy load (3 users)",
Severity = Severity.Major,
AssignedTo = Users.DeveloperDeborah,
Status = Status.Accepted,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyMediaPlayer,
Created = May(),
CreatedBy = Users.TesterTara,
Summary = "Unable to connect to any media server",
Severity = Severity.Showstopper,
AssignedTo = Users.DeveloperDarren,
Status = Status.Reopened,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyMediaPlayer,
Created = May(),
CreatedBy = Users.DeveloperDeborah,
Summary = "UI turns black and white when playing old films",
Severity = Severity.Minor,
AssignedTo = Users.TesterTara,
Status = Status.Fixed,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyTalk,
Created = May(),
CreatedBy = Users.ManagerMary,
Summary = "Password reset changes passwords for all users",
Severity = Severity.Showstopper,
AssignedTo = null,
Status = Status.Closed,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyMediaPlayer,
Created = May(),
CreatedBy = Users.TesterTim,
Summary = "Modern music sounds rubbish",
Severity = Severity.Trivial,
AssignedTo = Users.DeveloperDarren,
Status = Status.Created,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyTalk,
Created = May(),
CreatedBy = Users.TesterTim,
Summary = "Webcam makes me look bald",
Severity = Severity.Showstopper,
AssignedTo = Users.TesterTim,
Status = Status.Fixed,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyTalk,
Created = May(),
CreatedBy = Users.CustomerColin,
Summary = "Sound is distorted when speakers are underwater",
Severity = Severity.Major,
AssignedTo = Users.DeveloperDarren,
Status = Status.Created,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyTalk,
Created = May(),
CreatedBy = Users.DeveloperDarren,
Summary = "Japanese characters don't display properly",
Severity = Severity.Major,
AssignedTo = Users.DeveloperDeborah,
Status = Status.Accepted,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyMediaPlayer,
Created = May(),
CreatedBy = Users.TesterTara,
Summary = "Video takes 100% of CPU",
Severity = Severity.Major,
AssignedTo = Users.DeveloperDeborah,
Status = Status.Accepted,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyMediaPlayer,
Created = May(),
CreatedBy = Users.TesterTim,
Summary = "DVD Easter eggs unavailable",
Severity = Severity.Trivial,
AssignedTo = Users.DeveloperDarren,
Status = Status.Created,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyTalk,
Created = May(),
CreatedBy = Users.ManagerMary,
Summary = "Transparency is high for menus to be readable",
Severity = Severity.Minor,
AssignedTo = Users.DeveloperDeborah,
Status = Status.Accepted,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyMediaPlayer,
Created = May(),
CreatedBy = Users.CustomerColin,
Summary = "About box is missing version number",
Severity = Severity.Minor,
AssignedTo = Users.CustomerColin,
Status = Status.Fixed,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyTalk,
Created = May(),
CreatedBy = Users.TesterTim,
Summary = "Logs record confidential conversations",
Severity = Severity.Major,
AssignedTo = Users.DeveloperDarren,
Status = Status.Reopened,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyTalk,
Created = May(),
CreatedBy = Users.DeveloperDeborah,
Summary = "Profanity filter is too aggressive",
Severity = Severity.Minor,
AssignedTo = Users.TesterTara,
Status = Status.Fixed,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyMediaPlayer,
Created = May(),
CreatedBy = Users.TesterTara,
Summary = "Full screen mode fails on dual monitors",
Severity = Severity.Minor,
AssignedTo = Users.DeveloperDeborah,
Status = Status.Created,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyMediaPlayer,
Created = May(),
CreatedBy = Users.CustomerColin,
Summary = "Visualization hypnotises pets",
Severity = Severity.Minor,
AssignedTo = Users.DeveloperDeborah,
Status = Status.Accepted,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyTalk,
Created = May(),
CreatedBy = Users.ManagerMary,
Summary = "Resizing while typing loses input",
Severity = Severity.Trivial,
AssignedTo = Users.DeveloperDarren,
Status = Status.Created,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyMediaPlayer,
Created = May(),
CreatedBy = Users.TesterTim,
Summary = "Network is saturated when playing WAV file",
Severity = Severity.Minor,
AssignedTo = Users.TesterTim,
Status = Status.Fixed,
LastModified = May()
}, new Defect
{
Project = Projects.SkeetyMediaPlayer,
Created = May(),
CreatedBy = Users.TesterTara,
Summary = "Media library tells user to keep the noise down",
Severity = Severity.Major,
AssignedTo = Users.DeveloperDarren,
Status = Status.Created,
LastModified = May()
}
};
} public static DateTime May(int day)
{
return new DateTime(, , 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}\r\n ({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 = ;
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())
{
yield return current;
}
} IEnumerator IEnumerable.GetEnumerator()
{
return GetEnumerator();
}
}
