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查询表达式实际上是由编译器“预处理”为“普通”的C#代码,接着以完全普通的方式进行编译。这种巧妙的发式将查询集合到了语言中,而无须把语义改得乱七八糟
#region 11-1打印出所有用户的袖珍查询
var query = from user in SampleData.AllUsers
select user;
foreach (var user in query)
{
Console.WriteLine(user);
}
#endregion
#region 11-2将11-1的查询表达式被转译为一个方法调用
var query = SampleData.AllUsers.Select(user => user);//编译器不要求Select必须为一个方法,或AllUsers必须为属性,只要转换后的代码可以编译就可以了
foreach (var user in query)
{
Console.WriteLine(user);
}
#endregion
#region 11-3编译器转译调用伪LINQ实现中的方法
static class Extensions
{
public static Dummy<T> Where<T>(this Dummy<T> dummy, Func<T, bool> predicate)//声明Where扩展方法
{
Console.WriteLine("Where called");
return dummy;
}
}
class Dummy<T>
{
public Dummy<U> Select<U>(Func<T, U> selector)//声明Select实例方法
{
Console.WriteLine("Select called");
return new Dummy<U>();
}
}
#endregion
#region 11-3
var source = new Dummy<string>();//创建用于查询的数据源
var query = from dummy in source
where dummy.ToString() == "Ignored"
select "Anything";//通过查询表达式来调用方法var query=source.where(dummy=>dummy.ToString()=="Ignored").Select(dummy=>"Anything")
#endregion
#region 11-4仅选择user对象名称的查询
IEnumerable<string> query = from user in SampleData.AllUsers
select user.Name;
foreach (string name in query)
{
Console.WriteLine(name);
}
#endregion
#region 11-5使用Cast和OfType来处理弱类型集合
ArrayList list = new ArrayList { "First", "Second", "Third" };
IEnumerable<string> strings = list.Cast<string>();
foreach (string item in strings)
{
Console.WriteLine(item);
}
list = new ArrayList { 1, "First", ‘d‘, "dsds", 3 };
strings = list.OfType<string>();
foreach (string item in strings)
{
Console.WriteLine(item);
}
#endregion
#region 11-6使用显示类型的范围变量来自动调节Cast
ArrayList list = new ArrayList { "First", "Second", "Third" };
var strings = from string entry in list
select entry.Substring(0, 3);//IEnumerable<string> strings = from entry in source.Cast<string>() select entry.Substring(0, 3);
foreach (string start in strings)
{
Console.WriteLine(start);
}
#endregion
#region 11-7使用多个where字句的查询表达式
User tim = SampleData.Users.TesterTim;
var query = from defect in SampleData.AllDefects
where defect.Status != Status.Closed
where defect.AssignedTo == tim
select defect.Summary;
foreach (var summary in query)
{
Console.WriteLine(summary);
}
#endregion
#region 11-8按缺陷严重度的优先级从高到低的顺序排序
User tim = SampleData.Users.TesterTim;
var query = from defect in SampleData.AllDefects
where defect.Status != Status.Closed
where defect.AssignedTo == tim
orderby defect.Severity descending
select defect;
foreach (var defect in query)
{
Console.WriteLine("{0},{1}", defect.Severity, defect.Summary);
}
#endregion
#region 11-9先按严重度排序,而后按最后修改时间排序
User tim = SampleData.Users.TesterTim;
var query = from defect in SampleData.AllDefects
where defect.Status != Status.Closed
where defect.AssignedTo == tim
orderby defect.Severity descending, defect.LastModified
select defect;
foreach (var defect in query)
{
Console.WriteLine("{0},{1}({2:d})", defect.Severity, defect.Summary, defect.LastModified);
}
#endregion
#region 11-10在不使用let子句的情况下,按用户名称长度来排序
var query = from user in SampleData.AllUsers//两次使用了Length
orderby user.Name.Length
select user.Name;
foreach (var name in query)
{
Console.WriteLine("{0}:{1}", name.Length, name);
}
#endregion
#region 11-11使用let子句来消除冗余的计算
var query = from user in SampleData.AllUsers
let length = user.Name.Length//引入length范围变量
orderby length
select new { Name = user.Name, Length = length };
foreach (var name in query)
{
Console.WriteLine("{0}:{1}", name.Name, name.Length);
}
#endregion
#region 11-12根据项目把缺陷和通知订阅连接在一起
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 entry in query)
{
Console.WriteLine("{0}:{1}", entry.EmailAddress, entry.Summary);
}
#endregion
#region 11-13使用分组连接把缺陷的订阅连接到一起
var query = from defect in SampleData.AllDefects
join subscription in SampleData.AllSubscriptions
on defect.Project equals subscription.Project
into gtoupedSubscription
select new { Defece = defect, Subscription = gtoupedSubscription };
foreach (var enrty in query)
{
Console.WriteLine(enrty.Defece.Summary);
foreach (var subscription in enrty.Subscription)
{
Console.WriteLine("{0}", subscription.EmailAddress);
}
}
#endregion
#region 11-15用户和项目的交叉连接
var query = from user in SampleData.AllUsers
from project in SampleData.AllProjects
select new { User = user, Project = project };
foreach (var pair in query)
{
Console.WriteLine("{0}/{1}", pair.User.Name, pair.Project.Name);
}
#endregion
#region 11-16右边序列依赖于左边元素的交叉连接
var query = from left in Enumerable.Range(1, 4)
from right in Enumerable.Range(11, left)
select new { Left = left, Right = right };
foreach (var pair in query)
{
Console.WriteLine("Left={0};Rigth={1}", pair.Left, pair.Right);
}
#endregion
#region 11-17用分配来分组缺陷——无比简单的投影
var query = from defect in SampleData.AllDefects
where defect.AssignedTo != null//过滤未分配的缺陷
group defect by defect.AssignedTo;//用分配者来分组
foreach (var entry in query)
{
Console.WriteLine(entry.Key.Name);
foreach (var defect in entry)
{
Console.WriteLine("({0}) {1}", defect.Severity, defect.Summary);
}
Console.WriteLine();
}
#endregion
#region 11-18按分配者来分组缺陷——投影只保留概要信息
var query = from defect in SampleData.AllDefects
where defect.AssignedTo != null
group defect.Summary by defect.AssignedTo;
foreach (var entry in query)
{
Console.WriteLine(entry.Key.Name);
foreach (var summary in entry)
{
Console.WriteLine(" {0}", summary);
}
Console.WriteLine();
}
#endregion
#region 11-19使用另外一个投影来延续分组结果
var query = from defect in SampleData.AllDefects
where defect.AssignedTo != null
group defect by defect.AssignedTo into grouped
select new { Assignee = grouped.Key, Count = grouped.Count() };//在第二部分使用grouped范围变量,不过defect范围变量不在可用——它已经超出了它的作用域
foreach (var entry in query)
{
Console.WriteLine("{0}:{1}", entry.Assignee.Name, entry.Count);
}
#endregion
#region 11-20在group和select子句之后的查询表达式延续
var query = from defect in SampleData.AllDefects
where defect.AssignedTo != null
group defect by defect.AssignedTo into grouped
select new
{
Assignee = grouped.Key,
Count = grouped.Count()
} into result
orderby result.Count descending
select result;
foreach (var entry in query)
{
Console.WriteLine("{0}:{1}", entry.Assignee.Name, entry.Count);
}
#endregion
#region 只能用于点标记操作
var que = SampleData.AllUsers.Where(I1 => I1.Name.Length % 2 == 0).Select((I1, I2) => new { I1, I2 });
foreach (var entry in que)
{
Console.WriteLine(entry.I2);
}
Console.WriteLine();
foreach (var entry in que)
{
Console.WriteLine(entry.I1);
}
Console.ReadKey();
#endregion
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原文地址:http://www.cnblogs.com/Tan-sir/p/5169224.html