一、标准的查询操作符
| 标准查询操作符 | 说明 |
|
Where OfType<TResult> |
筛选操作符定义了返回元素的条件。 |
|
Select SelectMany |
投射操作符用于把对象转换为另一个类型的新对象。 |
|
OrderBy ThenBy OrderByDescending ThenByDescending Reverse |
排序操作符改变所返回的元素的顺序。OrderBy升序，OrderByDescending降序。如果第一次排序的结果很类型，可以使用ThenBy和ThenBy Descending操作符进行第二次排序，Reverse翻转集合中元素的顺序。 |
|
Join GroupJoin |
连接操作符用于合并不直接相关的集合。Join操作符根据键选择器关联两个集合。GroupJoin操作符连接两个集合，组合其结果。 |
|
GroupBy ToLookup |
组合操作符把数据分组。GroupBy操作符组合有公共键的元素。ToLookup通过创建一个一对多字典，来组合元素。 |
|
Any All Contains |
如果元素序列满足指定的条件，限定符操作符就返回布尔值。Any确定集合中是否有满足谓词函数的元素；All确定集合中的所有元素是否都满足谓词函数；Contains检查某个元素是否存在集合中。 |
|
Take Skip TakeWhile SkipWhile |
分区操作符返回集合的一个子集。使用Take必须指定要从集合中提取的元素个数；Skip跳过指定的元素个数；TakeWhile提取条件为真的元素；SkipWhile跳过条件为真的元素。 |
|
Distinct Union Intersect Except Zip |
Set操作符返回一个集合。Distinct从集合中删除重复的元素；Union返回出现在其中一个集合中的唯一元素；Intersect返回两个集合中都有的元素；Except返回只出现在一个集合中的元素；Zip把两个集合合并成一个。 |
|
First FirstOrDefault Last LastOrDefault ElementAt ElementAtOrDefault Single SingleOrDefault |
元素操作符仅返回一个元素。First返回第一个满足条件的元素；FirstOrDefault类似于First，但如果没找到满足条件的元素就返回默认值；Last返回最后一个满足条件的元素；ElementAt制定了要返回的元素的位置。Single只返回一个满足条件的元素，如果有多个元素满足条件，则抛出一个异常。 |
|
Count Sum Min Max Average Aggregate |
聚合操作符计算集合的一个值。 |
|
ToArray AsEnumerable ToList ToDictionary Cast<TResult> |
这些转换操作符将集合转换为数组。 |
|
Empty Range Repeat |
生成操作符返回一个新集合。使用Empty时集合是空的；Range返回一系列数字；Repeat返回一个始终重复一个值的集合。 |
- 筛选操作符Where
, , , , , , , , };
var query = from r in arr
select r;
foreach (var item in query)
{
Console.WriteLine(item);
}
Console.ReadLine();
- 使用索引筛选
//找出数组中大于5且索引不为偶数的元素
, , , , , , , , };
&& index % != );
foreach (var item in query)
{
Console.WriteLine(item);
}
Console.ReadLine();
- 类型筛选
//筛选集合中为string类型的元素
, , };
var query = obj.OfType<string>();
foreach (var item in query)
{
Console.WriteLine(item);
}
Console.ReadLine();
- 复合的from子句
如果需要根据对象的一个成员进行筛选，而该成员本身又是一个集合，就可以使用符合的from子句。
var ferrariDrivers = from r in Formula1.GetChampions()
from c in r.Cars
where c == "Ferrari"
orderby r.LastName
select r.FirstName + " " + r.LastName;
foreach (var item in ferrariDrivers)
{
Console.WriteLine(item);
}
Console.ReadLine();
- 排序
, , , , , , , , };
var query = (from r in myInt
orderby r descending
);
foreach (var item in query)
{
Console.WriteLine(item);
}
Console.ReadLine();
- 分组
var countries = from r in Formula1.GetChampions()
group r by r.Country into g
orderby g.Count() descending, g.Key
select new
{
COUNTRY = g.Key,
COUNT = g.Count()
};
foreach (var item in countries)
{
Console.WriteLine("{0} {1}", item.COUNTRY, item.COUNT);
}
Console.ReadLine();
- 对嵌套对象分组
var countries = from r in Formula1.GetChampions()
group r by r.Country into g
orderby g.Count() descending, g.Key
select new
{
Country = g.Key,
Count = g.Count(),
Racers = from r1 in g
orderby r1.LastName
select r1.FirstName + " " + r1.LastName
};
foreach (var item in countries)
{
Console.WriteLine("{0} {1}", item.Country, item.Count);
foreach (var name in item.Racers)
{
Console.Write("{0};", name);
}
Console.WriteLine();
}
Console.ReadLine();
- 内连接
var racers = from r in Formula1.GetChampions()
from y in r.Years
select new
{
Year = y,
Name = r.FirstName + " " + r.LastName
};
var teams = from t in Formula1.GetContructorChampions()
from y in t.Years
select new
{
Year = y,
Name = t.Name
};
var racersAndTeams = (from r in racers
join t in teams on r.Year equals t.Year
select new
{
r.Year,
Champion = r.Name,
Constructor = t.Name
}).Take();
Console.WriteLine("Year World Champio\t Constructor Title");
foreach (var item in racersAndTeams)
{
Console.WriteLine("{0}:{1,-20} {2}", item.Year, item.Champion, item.Constructor);
}
Console.ReadLine();
- 左外连接
var racers = from r in Formula1.GetChampions()
from y in r.Years
select new
{
Year = y,
Name = r.FirstName + " " + r.LastName
};
var teams = from t in Formula1.GetContructorChampions()
from y in t.Years
select new
{
Year = y,
Name = t.Name
};
var racersAndTeams = (from r in racers
join t in teams on r.Year equals t.Year into rt
from t in rt.DefaultIfEmpty()
orderby r.Year
select new
{
r.Year,
Champion = r.Name,
Constructor = t == null ? "no constructor championship" : t.Name
}).Take();
Console.WriteLine("Year World Champio\t Constructor Title");
foreach (var item in racersAndTeams)
{
Console.WriteLine("{0}:{1,-20} {2}", item.Year, item.Champion, item.Constructor);
}
Console.ReadLine();
- 集合操作
Func<string, IEnumerable<Racer>> racersByCar = car => from r in Formula1.GetChampions()
from c in r.Cars
where c == car
orderby r.LastName
select r;
Console.WriteLine("World champion with Ferrari and McLaren");
foreach (var racer in racersByCar("Ferrari").Intersect(racersByCar("McLaren")))
{
Console.WriteLine(racer);
}
Console.ReadLine();
- 合并
var racerNames = from r in Formula1.GetChampions()
where r.Country == "Italy"
orderby r.Wins descending
select new
{
Name = r.FirstName + " " + r.LastName
};
var racerNamesAndStarts = from r in Formula1.GetChampions()
where r.Country == "Italy"
orderby r.Wins descending
select new
{
LastName = r.LastName,
Starts = r.Starts
};
var racers = racerNames.Zip(racerNamesAndStarts, (first, second) => first.Name + ",starts: " + second.Starts);
foreach (var r in racers)
{
Console.WriteLine(r);
}
Console.ReadLine();
- 分区
;
int numberPages = (int)Math.Ceiling(Formula1.GetChampions().Count() / (double)pageSize);
; page < numberPages; page++)
{
Console.WriteLine("Page {0}", page);
var racers = (from r in Formula1.GetChampions()
orderby r.LastName, r.FirstName
select r.FirstName + " " + r.LastName).Skip(page * pageSize).Take(pageSize);
foreach (var name in racers)
{
Console.WriteLine(name);
}
Console.WriteLine();
}
Console.ReadLine();
- 聚合操作符
var query = from r in Formula1.GetChampions()
let numberYears = r.Years.Count()
orderby numberYears descending, r.LastName
select new
{
Name = r.FirstName + " " + r.LastName,
TimesChampion = numberYears
};
foreach (var r in query)
{
Console.WriteLine("{0} {1}", r.Name, r.TimesChampion);
}
var countries = (from c in
from r in Formula1.GetChampions()
group r by r.Country into c
select new
{
Country = c.Key,
Wins = (from r1 in c select r1.Wins).Sum()
}
orderby c.Wins descending, c.Country
select c
).Take();
foreach (var country in countries)
{
Console.WriteLine("{0} {1}", country.Country, country.Wins);
}
Console.ReadLine();
- 转换操作符
在迭代中使用查询时，查询才会执行，使用转换操作符会立即执行查询，把查询结果放在数组、列表或字典中。
List<Racer> racers = (from r in Formula1.GetChampions()
orderby r.Starts descending
select r).ToList();
foreach (var racer in racers)
{
Console.WriteLine("{0} {0:S}", racer);
}
Console.ReadLine();
*如果在非类型化集合上使用LINQ查询，为了定义强类型化的查询，就可以使用Cast()方法。
- 生成操作符
, ).Select(n => n * );
foreach (var item in values)
{
Console.WriteLine("{0} ", item);
}
Console.ReadLine();