Standard Query Operators(2)
[索引页]
Standard Query Operators(1)
Standard Query Operators(2)
好现在我们来一个一个的看
Select
就是一般的查询返回数据。
先定义一个数组
name[] disk ={
new name{parent ="Micorsoft",child =new List<string>{".net","SQL Server"}},
new name{parent ="Sun",child =new List<string>{"netBean","MySQL"}},
new name{parent ="IBM",child =new List<string>{"eclipse","DB2"}}
};
SelectMany
提供多个from的查询,也就说如果你要使用Query Syntax的方式实现一样的功能,就要使用多个from的情况,就是一对多的,对多个子集合的查询.
他们两的代码在SelectMany button中
// QuerySyntax SelectMany
listBox1.Items.Add("_____________QuerySyntax SelectMany____");
![](/Images/OutliningIndicators/None.gif)
var query2 = from d in disk
select d;
![](/Images/OutliningIndicators/None.gif)
foreach (var q2 in query2.SelectMany(d2 => d2.child))
{ listBox1.Items.Add(q2); }
这个就是简单的查询child,下面两个是对应:一个是使用多个Select来实现对子集合的查询和用一个SelectMany实现
//MethodSyntax usge selectMany two
listBox2.Items.Add("____________MethodSyntax usge selectMany two");
![](/Images/OutliningIndicators/None.gif)
var methodquery2 = disk.Select(d => d);
![](/Images/OutliningIndicators/None.gif)
foreach (var q in methodquery2)
{
var subQuery = q.child.Select(qs => qs);
foreach (var sq in subQuery)
{
listBox2.Items.Add(sq);
}
}
//usga SelectMany one
![](/Images/OutliningIndicators/None.gif)
IEnumerable<string> query1 = disk.AsQueryable().SelectMany(d => d.child);
![](/Images/OutliningIndicators/None.gif)
foreach (var q in query1)
{
listBox3.Items.Add(q);
}
Sorting Data
其实排序在之前的一篇文章中我们有使用过.OrderBy,OrderByDesending是第一级的排序,ThenByDescending,ThenBy是子级的排序.Reverse是倒置.
//MethodSyntax Reverse
listBox2.Items.Add("____________MethodSyntax usge Reverse________");
![](/Images/OutliningIndicators/None.gif)
var methodQuery3 = disk.AsQueryable().SelectMany(d => d.child).Reverse();
foreach (var q in methodQuery3)
{
listBox2.Items.Add(q);
}
Joining Operators
结合或关联两个数据源对象的一些行为,他们的结合是通过一个共同的值或属性.而在LINQ中主要是通过key匹配.在面向对象编程时,就是对象之间的相关性,Join方法提供的Join和groupJoin.Join实现的是inner join,join比配的是另一个数据集或返回值.GroupJoin方法是扩展inner join和left outer join.一个left outer join是一个返回第一个数据源的所有元素.
![](/images/cnblogs_com/worksguo/97232/r_Bb397908.JoinCircles.png)
图(1)
这一部分以后会更深入讨论.
Join
就是在两个数据源的值比配的部分.就是会输出给A数据指定key与B数据的Key如果他们的key相比配的相应的数据.(就是inner join )
public static IEnumerable<TResult> Join<TOuter, TInner, TKey, TResult>(
this IEnumerable<TOuter> outer,
IEnumerable<TInner> inner,
Func<TOuter, TKey> outerKeySelector,
Func<TInner, TKey> innerKeySelector,
Func<TOuter, TInner, TResult> resultSelector
)
下面是多表的内连查询
Table<Contact> contact = context.GetTable<Contact>();
Table<Employee> emp = context.GetTable<Employee>();
Table<Individual> ind = context.GetTable<Individual>();
Table<Customer> cus = context.GetTable<Customer>();
//use table Contact,Individual,Customer;key is ID
var joinQuery2 = from c in contact
join em in emp on c.ContactID equals em.ContactID
join i in ind on c.ContactID equals (i.ContactID-791)
join cu in cus on i.CustomerID equals cu.CustomerID
where c.FirstName.StartsWith("S")
orderby c.LastName
select new { i.CustomerID,cu.AccountNumber,c.ContactID,c.LastName};//em.EmployeeID,c.FirstName,c.EmailAddress};
foreach(var j2 in joinQuery2)
{
listBox3.Items.Add(j2.LastName+"_______" + j2.CustomerID +"___"+ j2.AccountNumber);
}
SQL是:
SELECT [t0].[ContactID]
FROM [Person].[Contact] AS [t0]
INNER JOIN [HumanResources].[Employee] AS [t1] ON [t0].[ContactID] = [t1].[ContactID]
INNER JOIN [Sales].[Individual] AS [t2] ON [t0].[ContactID] = [t2].[ContactID]-791
INNER JOIN [Sales].[Customer] AS [t3] ON [t2].[CustomerID] = [t3].[CustomerID]
WHERE [t0].[FirstName] LIKE 'S%'
ORDER BY [t0].[LastName]
下面他们是一对 ,基本的内连查询,使用两中方式QuerySyntax和MethodSyntax
//Usage QuerySyntax join
var joinQuery = from c in contact
join es in emp
on c.ContactID equals es.ContactID
where c.FirstName.StartsWith("S")
orderby c.LastName
select new { c.FirstName, c.LastName, c.MiddleName, es.Title };
![](/Images/OutliningIndicators/None.gif)
foreach (var j in joinQuery)
{
listBox1.Items.Add(j.FirstName);
}
![](/Images/OutliningIndicators/None.gif)
//MethodSyntax join
listBox2.Items.Add("______________MethodSyntax join_______");
var joinMethodQuery = contact.Join(emp, con => con.ContactID, em => em.ContactID, (con, em) => new { Contact = con.FirstName, myEmployee = em.ContactID });
foreach (var q4 in joinMethodQuery)
{
listBox2.Items.Add(q4.Contact);
}
GroupJoin:
就是第一个数据源中加入相应的第二个数据的值并将他们一起返回,主要是使用在有层次结构的数据,可以输出有层次的结果,保留他们次序.就是outer里面的元素与inner里面的匹配的元素的集合一起配对.(就是group join. )
看GroupJoin方法
public static IEnumerable<TResult> GroupJoin<TOuter, TInner, TKey, TResult>(
this IEnumerable<TOuter> outer,
IEnumerable<TInner> inner,
Func<TOuter, TKey> outerKeySelector,
Func<TInner, TKey> innerKeySelector,
Func<TOuter, IEnumerable<TInner>, TResult> resultSelector
)
我们主要来研究参数:
它是要被延迟执行的!
outer 类型是IEnumerable(TOuter)是的第一数据源,其实这一个是不需要指定的,因为是this IEnumerable<TOuter> outer
inner 类型是IEnumerable(Tinner)第二个数据源Join到第一个数据源,也就是第一个数据与第二个数据的inner
outerKeySelector 类型为Func(TOuter,Tkey),从第一个数据中提取key.
innerKeySelector 类型为Func(TInner,TKey),从第二个数据中提取key.
resultSelector类型为System.Func(TOuter, IEnumerable(TInner), TResult) 创建一个由第个数据的一个元素和在第二个数据中匹配key的元素集合.是两个参数值组成的匿名方法.
返回类型是IEnumerable<TResult>
看代码
class Person
{
public string Name { get; set; }
}
![](/Images/OutliningIndicators/None.gif)
class Pet
{
public string Name { get; set; }
public Person Owner { get; set; }
}
class Son
{
public string Name { get; set; }
public Pet Owner { get; set; }
}
//Groupjoin example
public void GroupJoinEx1()
{
Person magnus = new Person { Name = "Hedlund, Magnus" };
Person terry = new Person { Name = "Adams, Terry" };
Person charlotte = new Person { Name = "Weiss, Charlotte" };
![](/Images/OutliningIndicators/InBlock.gif)
Pet barley = new Pet { Name = "Barley", Owner = terry};
Pet boots = new Pet { Name = "Boots", Owner = terry};
Pet whiskers = new Pet { Name = "Whiskers", Owner = charlotte};
Pet daisy = new Pet { Name = "Daisy", Owner = magnus};
Son Massif = new Son { Name = "Massif", Owner = barley };
Son Mass = new Son { Name = "Mass", Owner = boots };
Son Mono = new Son { Name = "Mono", Owner = whiskers };
Son Marie = new Son { Name = "Marie", Owner = daisy };
List<Person> people = new List<Person> { magnus, terry, charlotte };
List<Pet> pets = new List<Pet> { barley, boots, whiskers, daisy };
List<Son> Sons = new List<Son> { Massif, Mass, Mono, Marie };
var query =
people.GroupJoin(pets,
person => person,
pet => pet.Owner,
(person, petCollection) =>
new
{
OwnerName = person.Name,
//Pets = petCollection
Pets = petCollection.Select(pet => pet.Name)
//Pets = petCollection.Select(pet => pet.Owner)
});
![](/Images/OutliningIndicators/InBlock.gif)
foreach (var obj in query)
{
![](/Images/OutliningIndicators/InBlock.gif)
listBox2.Items.Add(obj.OwnerName);
foreach (var pet in obj.Pets)
listBox2.Items.Add("_______________"+pet);
}
var query2 =
pets.GroupJoin(Sons,
p=> p,
s =>s.Owner,
(myPet, sonCollection) =>
new
{ //他们是同一级别的
OwnerName = myPet.Owner.Name,
//Pets = petCollection
Pets = sonCollection.Select(so => so.Owner.Name),
//Pets = petCollection.Select(pet => pet.Owner)
});
foreach (var obj in query2)
{
listBox2.Items.Add(obj.OwnerName);
foreach (var pet in obj.Pets)
{
listBox2.Items.Add("_______________" + pet);
}
}
}
基本的内连查询,使用两中方式QuerySyntax和MethodSyntax.
定义三个List,而他们的List的关系是通过Owner来传递的,Son里面持有Pet的实例,而Pet的实现中又持有Peson的实例,在第一个中查询person名和pet名,在第二个中也是查询person名和pet名,但他们都是通过同一层的对象的子一级对象来来查找的.
inner = {System.Collections.Generic.List<CSharpLanguage_C_app.LinqAPP.Pet>}
innerKeySelector = {System.Func<CSharpLanguage_C_app.LinqAPP.Pet,CSharpLanguage_C_app.LinqAPP.Person>}
outer = {System.Collections.Generic.List<CSharpLanguage_C_app.LinqAPP.Person>}
outerKeySelector = {System.Func<CSharpLanguage_C_app.LinqAPP.Person,CSharpLanguage_C_app.LinqAPP.Person>}
resultSelector = {System.Func<CSharpLanguage_C_app.LinqAPP.Person,System.Collections.Generic.IEnumerable<CSharpLanguage_C_app.LinqAPP.Pet>,<string,System.Collections.Generic.IEnumerable<string>> >}
首先你要看上面的图(1)整个过程就是给出outer,outer是Person,在给出inner是之前的Person与Pet才能找到inner,然后给出outer的key,inner的key,Func(TInner,TKey)是提取TInner中指定类型值,之后指定一个outer和一个层次结构数据的次级的数据.
再回到我们对表的操作:
Table<Contact> contact = context.GetTable<Contact>();
Table<Employee> emp = context.GetTable<Employee>();
![](/Images/OutliningIndicators/None.gif)
var groupJoinQuery = contact.GroupJoin(emp, con => con.ContactID, em => em.ContactID,
(myCon, myEm) => new { newCon = myCon.ContactID,
newEm = myEm.Select(lastEm => lastEm.ContactID) });
![](/Images/OutliningIndicators/None.gif)
foreach (var j3 in groupJoinQuery)
{
listBox3.Items.Add(j3.newCon);
foreach (var order in j3.newEm)
{
listBox3.Items.Add(order);
}
}
GroupBy
就是按指定的值分组,以前我们谈过.
Table<Contact> contact = context.GetTable<Contact>();
// MethodSyntax Groupby
var queryGroup = contact.Where(c => c.ContactID > 1500&&c.FirstName.StartsWith("S")).GroupBy(c => c.ContactID);
listBox2.Items.Add("MethodSyntax Groupby");
foreach(var q in queryGroup)
{
listBox2.Items.Add(q.Key);
foreach (var cust in q)
{
listBox2.Items.Add(""+cust.LastName);
}
}
//QuerySyntax
![](/Images/OutliningIndicators/None.gif)
var queryGroup2 = (from c in contact where c.ContactID > 1500 && c.FirstName.StartsWith("S") select c).GroupBy(c => c.ContactID);
listBox2.Items.Add("MethodSyntax Groupby");
foreach (var q2 in queryGroup2)
{
listBox1.Items.Add(q2.Key);
foreach (var cust2 in q2)
{
listBox1.Items.Add("" + cust2.LastName);
}
}
Concat
串联两个数据.
Concat
![](/Images/OutliningIndicators/None.gif)
串联两个数据.
public static IEnumerable<TSource> Concat<TSource>(
this IEnumerable<TSource> first,
IEnumerable<TSource> second
)
Table<Contact> contact = context.GetTable<Contact>();
Table<Employee> emp = context.GetTable<Employee>();
// MethodSyntax
var contactQuery = contact.Select(con => con.ContactID).Concat(emp.Select(em => em.EmployeeID));
foreach(var q in contactQuery)
{
listBox2.Items.Add("+"+q);
}
//QuerySyntax
var contactQuery2 = (from con in contact select con.ContactID).Concat(from em in emp select em.EmployeeID);
foreach (var q2 in contactQuery)
{
listBox1.Items.Add("+" + q2);
}
Aggregating
聚集两个数据.遍历集合中的元素,并通过一个指定函数与一个seed value(可迁移的值)对照,保留符合条件的.
public static TSource Aggregate<TSource>(
this IEnumerable<TSource> source,
Func<TSource, TSource, TSource> func
)
int[] randomNumbers = { 1,2,3,4,5,6,7,8,9,10,11,12,13,14,15};
var queryAgg = randomNumbers.Select(r => r).Aggregate(0, (total, next) =>
next + next >= 25 ? total + 1 : total);
listBox1.Items.Add(queryAgg);
其实我们可以在方法里面处理一些逻辑后再返回一些相应的值。
还有Average,Count,Max,Min,Sum基本就和以前一样.
Set Operators
Distinct
是移出相同的值;
Union
结合数据里面唯一的元素
Intersect
这个是交集,
Except
相同数据之外的部分.
DefaultIfEmpty方法和Empty方法
Empty方法
返回一个空的集合
Empty(TResult)()方法缓存着一个TResult类型的数据.当对象返回时,它就已经没有元素.
public static IEnumerable<TResult> Empty<TResult>()
DefaultIfEmpty方法
如果你的集合是Empty就用指定的默认集合去代替它,为一个空集合提供一个默认元素.
public static IEnumerable<TSource> DefaultIfEmpty<TSource>(
this IEnumerable<TSource> source
)
Range
创建一个集合来产生numbers数据.
public static IEnumerable<int> Range(
int start,
int count
)
Repeat
创建一个单独的值去替换替换自己的某部分.
public static IEnumerable<TResult> Repeat<TResult>(
TResult element,
int count
)
Conversion Operators
AsEnumerable
这里返回查询输出类型为IEnumerable(T),就说你能改变数据源类,全部该成IEnumerable(T)类型输出,(相应的泛型).
解决方法是指定 where 的客户端泛型 IEnumerable<T> 实现以替换泛型 IQueryable<T>。
public static IEnumerable<TSource> AsEnumerable<TSource>(
this IEnumerable<TSource> source
)
其实这个是很常用的!
Table<Contact> contact = context.GetTable<Contact>();
IEnumerable<Contact> query =
contact.AsEnumerable().Where(con => con.FirstName.Contains("K"));
![](/Images/OutliningIndicators/None.gif)
IEnumerable<Contact> query1 = contact.Where(con=> con.FirstName.Contains("K"));
![](/Images/OutliningIndicators/None.gif)
foreach (Contact item in query)
{ listBox1.Items.Add(item.FirstName); }
上面一句
query = {System.Linq.Enumerable.WhereIterator<LINQSQO_WFormsApp.Form1.Contact>}
下面一句
Method = {System.Linq.IQueryable`1[LINQSQO_WFormsApp.Form1+Contact]
Where[Contact](System.Linq.IQueryable`1[LINQSQO_WFormsApp.Form1+Contact], System.Linq.Expressions.Expression`1[System.Func`2[LINQSQO_WFormsApp.Form1+Contact,System.Boolean]])}
这里我不能很好的总结成一,两句话就说明白,所以给一个长一点的.
我们在没有实现IEnumerable<TSource>前我们调用在System.Query.Sequence 类中的方法如Where, Select, and SelectMany,但实现后它后我们就能调用公有Where方法.
看下面这个实现IEnumerable<TSource>的查询
IEnumerable<Contact> query =
contact.AsEnumerable().Where(con => con.FirstName.Contains("K"));
和这个对应
[System.Linq.Expressions.MethodCallExpression] = {Table(Contact).Where(con => con.FirstName.Contains("K"))}
没有实现它的查询:
IEnumerable<Contact> query1 = contact.Where(con=> con.FirstName.Contains("K"));
query = {System.Linq.Enumerable.WhereIterator<LINQSQO_WFormsApp.Form1.Contact>}
Method = {System.Linq.IQueryable`1[LINQSQO_WFormsApp.Form1+Contact] Where[Contact](System.Linq.IQueryable`1[LINQSQO_WFormsApp.Form1+Contact], System.Linq.Expressions.Expression`1[System.Func`2[LINQSQO_WFormsApp.Form1+Contact,System.Boolean]])}
我想你应该清楚他们的区别了.
还有LINQ一个个并行执行的结构,有本地对象集合是local queries ,而远程数据源查询是interpreted queries,所以你需要分清是那种查询,操作集合你要实现IEnumerable<T>.而且查询操作符号是Enumerable类里面.而remote execution的,操作符号则要实现IQueryable<T>. expression tree转换成为SQL使用的树时候是被remote execution,所以我们会调用带有"predicate argument"(谓词参数);但如果remote execution不能执行,我们就要调用本地执行的方法!其实我们SQL是要到服务器端去执行的,可以隐藏自己定义的方法,使用standard query operators.(在以后LINQ to SQL中我们还会谈到它!).好拉这一下"同拉""爽拉"!
Cast和OfType在上篇文章中有很好的描述也有很好的事例.ToArray,ToList之前我们已经用熟悉的!
这里我们来使用ToDictionary
可以将序列转化为字典,从IEnumerable(T)中创建一个Dictionary(TKey,TValue)按照指定的selector function.
public static Dictionary<TKey, TSource> ToDictionary<TSource, TKey>(
this IEnumerable<TSource> source,
Func<TSource, TKey> keySelector
)
我们来实践一下:
Table<Contact> contact = context.GetTable<Contact>();
![](/Images/OutliningIndicators/None.gif)
Dictionary<int, string> dict = contact.ToDictionary(con => con.ContactID, con => con.FirstName);
![](/Images/OutliningIndicators/None.gif)
foreach (KeyValuePair<int, string> item in dict)
{
listBox1.Items.Add(item.Key + " "+item.Value);
}
ToLookup(TSource,Tkey)
一个一对多的字典,是一个key对Collections的值(明白来吧!),按照一个指定的Key从IEnumerable(T)中创建一个Lookup(Tkey,TElment).
public static ILookup<TKey, TSource> ToLookup<TSource, TKey>(
this IEnumerable<TSource> source,
Func<TSource, TKey> keySelector
)
ElementAt
返回一个指定index所对应的值.但在查询表达方式中是不支持的.
ElementAtOrDefault
返回一个指定index所对应的值,如果这个值不存在,就返回默认值.查询表达方式中是不支持.
你可以在本地集合中使用它,这里就不说哦!
Last和First
分别是返回最后一个元素和第一个元素.
FirstOrDefault
返回的是第一个元素,如果没有被找到,那就返回默认值.
LastOrDefault
返回最后一个元素,如果没有,就返回默认值.查询表达方式中是不支持.
Table<Contact> contact = context.GetTable<Contact>();
var query = from c in contact
where c.FirstName.StartsWith("S")
select c.FirstName;
listBox1.Items.Add(query.First());
![](/Images/OutliningIndicators/None.gif)
var query2 = from c in contact
where c.FirstName.StartsWith("ZZ")
select c.FirstName;
listBox1.Items.Add("+ "+query2.FirstOrDefault()+" +");
Single
返回一个数据的一个单独的元素,就是这个集合中必须是一个元素.如果在数据中不是个元素那就会抛出异常!(感觉这个方法是用来判断集合里面是不只有元素样!)
Table<Contact> contact = context.GetTable<Contact>();
var query3 = from c in contact
where c.LastName.StartsWith("Kobylinski")
select c.FirstName;
try
{
listBox1.Items.Add(query3.Single());
}
catch (System.InvalidOperationException)
{
listBox1.Items.Add("it's contain more than one in the sequence");
![](/Images/OutliningIndicators/InBlock.gif)
}
![](/Images/OutliningIndicators/None.gif)
var query = from c in contact
where c.LastName.StartsWith("Kobylinski")
select c.FirstName;
listBox1.Items.Add(query.SingleOrDefault());
Quantifier Operators
All:集合中的值是否满足一个指定条件.
Any:
返回值是一个boolean值,集合中任何值符合指定条件,就返回true;
Table<Contact> contact = context.GetTable<Contact>();
![](/Images/OutliningIndicators/None.gif)
bool query2 = contact.All(c => c.FirstName.StartsWith("K"));
listBox2.Items.Add(query2.ToString());
var query = from c in contact
where c.LastName.StartsWith("Z")
select c.FirstName;
listBox1.Items.Add(query.Any());
Contains
在集合中中是否包括指定的元素
Table<Contact> contact = context.GetTable<Contact>();
var query = from c in contact
select c.LastName;
listBox1.Items.Add(query.Contains("Kleinerman"));
Skip 跳过给定的数目的元素返回其余的的元素.其实就是可以输出剩余元素.
SkipWhile:有指定条件的去跳过一些数据.上面的是没有逻辑控制的,而这一个我们给一些比较复杂的控制Take.
Table<Contact> contact = context.GetTable<Contact>();
listBox2.Items.Add("____________Skip__________");
var query1 = contact.Select(c => c.ContactID).Skip(15000);
foreach (int q in query1)
listBox2.Items.Add(q);
![](/Images/OutliningIndicators/None.gif)
![](/Images/OutliningIndicators/None.gif)
listBox1.Items.Add("____________Skip__________");
int[] randomNumbers = { 86, 2, 77, 94, 100, 65, 5, 22, 70, 55, 81, 66, 45 };
IEnumerable<int> skipLessThan50 =
randomNumbers.OrderBy(num => num).SkipWhile(num =>
num < 50);
foreach (int number in skipLessThan50)
listBox1.Items.Add("__"+number);
Take:是取回指定个数的元素.
TakeWhile:是用其判断条件去依次判断数据中的元素取回符合条件的元素.
Table<Contact> contact = context.GetTable<Contact>();
listBox2.Items.Add("____________Take__________");
var query1 = contact.Select(c => c.ContactID).Take(6);
foreach (int q in query1)
{ listBox2.Items.Add(q); }
int[] randomNumberss = { 86, 2, 77, 94, 100, 65, 5, 22, 70, 55, 81, 66, 45 };
listBox1.Items.Add("____________Take__________");
IEnumerable<int> query2 =randomNumberss.OrderBy(num => num).TakeWhile(num => num >2);
foreach (int number in query2)
listBox1.Items.Add("__" + number.ToString());
worksguo
请多指教!
程序下载
Standard Query Operators(1)
Standard Query Operators(1)
Standard Query Operators(2)
好现在我们来一个一个的看
Select
就是一般的查询返回数据。
先定义一个数组
![](/Images/OutliningIndicators/ExpandedBlockStart.gif)
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SelectMany
提供多个from的查询,也就说如果你要使用Query Syntax的方式实现一样的功能,就要使用多个from的情况,就是一对多的,对多个子集合的查询.
他们两的代码在SelectMany button中
![](/Images/OutliningIndicators/None.gif)
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Sorting Data
其实排序在之前的一篇文章中我们有使用过.OrderBy,OrderByDesending是第一级的排序,ThenByDescending,ThenBy是子级的排序.Reverse是倒置.
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Joining Operators
结合或关联两个数据源对象的一些行为,他们的结合是通过一个共同的值或属性.而在LINQ中主要是通过key匹配.在面向对象编程时,就是对象之间的相关性,Join方法提供的Join和groupJoin.Join实现的是inner join,join比配的是另一个数据集或返回值.GroupJoin方法是扩展inner join和left outer join.一个left outer join是一个返回第一个数据源的所有元素.
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图(1)
这一部分以后会更深入讨论.
Join
就是在两个数据源的值比配的部分.就是会输出给A数据指定key与B数据的Key如果他们的key相比配的相应的数据.(就是inner join )
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SQL是:
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下面他们是一对 ,基本的内连查询,使用两中方式QuerySyntax和MethodSyntax
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GroupJoin:
就是第一个数据源中加入相应的第二个数据的值并将他们一起返回,主要是使用在有层次结构的数据,可以输出有层次的结果,保留他们次序.就是outer里面的元素与inner里面的匹配的元素的集合一起配对.(就是group join. )
看GroupJoin方法
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我们主要来研究参数:
它是要被延迟执行的!
outer 类型是IEnumerable(TOuter)是的第一数据源,其实这一个是不需要指定的,因为是this IEnumerable<TOuter> outer
inner 类型是IEnumerable(Tinner)第二个数据源Join到第一个数据源,也就是第一个数据与第二个数据的inner
outerKeySelector 类型为Func(TOuter,Tkey),从第一个数据中提取key.
innerKeySelector 类型为Func(TInner,TKey),从第二个数据中提取key.
resultSelector类型为System.Func(TOuter, IEnumerable(TInner), TResult) 创建一个由第个数据的一个元素和在第二个数据中匹配key的元素集合.是两个参数值组成的匿名方法.
返回类型是IEnumerable<TResult>
看代码
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定义三个List,而他们的List的关系是通过Owner来传递的,Son里面持有Pet的实例,而Pet的实现中又持有Peson的实例,在第一个中查询person名和pet名,在第二个中也是查询person名和pet名,但他们都是通过同一层的对象的子一级对象来来查找的.
inner = {System.Collections.Generic.List<CSharpLanguage_C_app.LinqAPP.Pet>}
innerKeySelector = {System.Func<CSharpLanguage_C_app.LinqAPP.Pet,CSharpLanguage_C_app.LinqAPP.Person>}
outer = {System.Collections.Generic.List<CSharpLanguage_C_app.LinqAPP.Person>}
outerKeySelector = {System.Func<CSharpLanguage_C_app.LinqAPP.Person,CSharpLanguage_C_app.LinqAPP.Person>}
resultSelector = {System.Func<CSharpLanguage_C_app.LinqAPP.Person,System.Collections.Generic.IEnumerable<CSharpLanguage_C_app.LinqAPP.Pet>,<string,System.Collections.Generic.IEnumerable<string>> >}
首先你要看上面的图(1)整个过程就是给出outer,outer是Person,在给出inner是之前的Person与Pet才能找到inner,然后给出outer的key,inner的key,Func(TInner,TKey)是提取TInner中指定类型值,之后指定一个outer和一个层次结构数据的次级的数据.
再回到我们对表的操作:
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GroupBy
就是按指定的值分组,以前我们谈过.
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Concat
串联两个数据.
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Aggregating
聚集两个数据.遍历集合中的元素,并通过一个指定函数与一个seed value(可迁移的值)对照,保留符合条件的.
public static TSource Aggregate<TSource>(
this IEnumerable<TSource> source,
Func<TSource, TSource, TSource> func
)
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还有Average,Count,Max,Min,Sum基本就和以前一样.
Set Operators
Distinct
是移出相同的值;
Union
结合数据里面唯一的元素
Intersect
这个是交集,
Except
相同数据之外的部分.
DefaultIfEmpty方法和Empty方法
Empty方法
返回一个空的集合
Empty(TResult)()方法缓存着一个TResult类型的数据.当对象返回时,它就已经没有元素.
public static IEnumerable<TResult> Empty<TResult>()
DefaultIfEmpty方法
如果你的集合是Empty就用指定的默认集合去代替它,为一个空集合提供一个默认元素.
public static IEnumerable<TSource> DefaultIfEmpty<TSource>(
this IEnumerable<TSource> source
)
Range
创建一个集合来产生numbers数据.
public static IEnumerable<int> Range(
int start,
int count
)
Repeat
创建一个单独的值去替换替换自己的某部分.
public static IEnumerable<TResult> Repeat<TResult>(
TResult element,
int count
)
Conversion Operators
AsEnumerable
这里返回查询输出类型为IEnumerable(T),就说你能改变数据源类,全部该成IEnumerable(T)类型输出,(相应的泛型).
解决方法是指定 where 的客户端泛型 IEnumerable<T> 实现以替换泛型 IQueryable<T>。
public static IEnumerable<TSource> AsEnumerable<TSource>(
this IEnumerable<TSource> source
)
其实这个是很常用的!
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上面一句
query = {System.Linq.Enumerable.WhereIterator<LINQSQO_WFormsApp.Form1.Contact>}
下面一句
Method = {System.Linq.IQueryable`1[LINQSQO_WFormsApp.Form1+Contact]
Where[Contact](System.Linq.IQueryable`1[LINQSQO_WFormsApp.Form1+Contact], System.Linq.Expressions.Expression`1[System.Func`2[LINQSQO_WFormsApp.Form1+Contact,System.Boolean]])}
这里我不能很好的总结成一,两句话就说明白,所以给一个长一点的.
我们在没有实现IEnumerable<TSource>前我们调用在System.Query.Sequence 类中的方法如Where, Select, and SelectMany,但实现后它后我们就能调用公有Where方法.
看下面这个实现IEnumerable<TSource>的查询
IEnumerable<Contact> query =
contact.AsEnumerable().Where(con => con.FirstName.Contains("K"));
和这个对应
[System.Linq.Expressions.MethodCallExpression] = {Table(Contact).Where(con => con.FirstName.Contains("K"))}
没有实现它的查询:
IEnumerable<Contact> query1 = contact.Where(con=> con.FirstName.Contains("K"));
query = {System.Linq.Enumerable.WhereIterator<LINQSQO_WFormsApp.Form1.Contact>}
Method = {System.Linq.IQueryable`1[LINQSQO_WFormsApp.Form1+Contact] Where[Contact](System.Linq.IQueryable`1[LINQSQO_WFormsApp.Form1+Contact], System.Linq.Expressions.Expression`1[System.Func`2[LINQSQO_WFormsApp.Form1+Contact,System.Boolean]])}
我想你应该清楚他们的区别了.
还有LINQ一个个并行执行的结构,有本地对象集合是local queries ,而远程数据源查询是interpreted queries,所以你需要分清是那种查询,操作集合你要实现IEnumerable<T>.而且查询操作符号是Enumerable类里面.而remote execution的,操作符号则要实现IQueryable<T>. expression tree转换成为SQL使用的树时候是被remote execution,所以我们会调用带有"predicate argument"(谓词参数);但如果remote execution不能执行,我们就要调用本地执行的方法!其实我们SQL是要到服务器端去执行的,可以隐藏自己定义的方法,使用standard query operators.(在以后LINQ to SQL中我们还会谈到它!).好拉这一下"同拉""爽拉"!
Cast和OfType在上篇文章中有很好的描述也有很好的事例.ToArray,ToList之前我们已经用熟悉的!
这里我们来使用ToDictionary
可以将序列转化为字典,从IEnumerable(T)中创建一个Dictionary(TKey,TValue)按照指定的selector function.
public static Dictionary<TKey, TSource> ToDictionary<TSource, TKey>(
this IEnumerable<TSource> source,
Func<TSource, TKey> keySelector
)
我们来实践一下:
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ToLookup(TSource,Tkey)
一个一对多的字典,是一个key对Collections的值(明白来吧!),按照一个指定的Key从IEnumerable(T)中创建一个Lookup(Tkey,TElment).
public static ILookup<TKey, TSource> ToLookup<TSource, TKey>(
this IEnumerable<TSource> source,
Func<TSource, TKey> keySelector
)
ElementAt
返回一个指定index所对应的值.但在查询表达方式中是不支持的.
ElementAtOrDefault
返回一个指定index所对应的值,如果这个值不存在,就返回默认值.查询表达方式中是不支持.
你可以在本地集合中使用它,这里就不说哦!
Last和First
分别是返回最后一个元素和第一个元素.
FirstOrDefault
返回的是第一个元素,如果没有被找到,那就返回默认值.
LastOrDefault
返回最后一个元素,如果没有,就返回默认值.查询表达方式中是不支持.
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Single
返回一个数据的一个单独的元素,就是这个集合中必须是一个元素.如果在数据中不是个元素那就会抛出异常!(感觉这个方法是用来判断集合里面是不只有元素样!)
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Quantifier Operators
All:集合中的值是否满足一个指定条件.
Any:
返回值是一个boolean值,集合中任何值符合指定条件,就返回true;
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Contains
在集合中中是否包括指定的元素
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Skip 跳过给定的数目的元素返回其余的的元素.其实就是可以输出剩余元素.
SkipWhile:有指定条件的去跳过一些数据.上面的是没有逻辑控制的,而这一个我们给一些比较复杂的控制Take.
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Take:是取回指定个数的元素.
TakeWhile:是用其判断条件去依次判断数据中的元素取回符合条件的元素.
![](/Images/OutliningIndicators/None.gif)
![](/Images/OutliningIndicators/None.gif)
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![](/Images/OutliningIndicators/ExpandedBlockStart.gif)
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![](/Images/OutliningIndicators/None.gif)
![](/Images/OutliningIndicators/None.gif)
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worksguo
请多指教!
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Standard Query Operators(1)