Linq常见操作示例
2018-07-20 来源：open-open
static void DeferredQuery()
{
var names = new List<string> { "Nino", "Alberto", "Juan", "Mike", "Phil" };
var namesWithJ = from n in names
where n.StartsWith("J")
orderby n
select n;
Console.WriteLine("First iteration");
foreach (string name in namesWithJ)
{
Console.WriteLine(name);
}
Console.WriteLine();
names.Add("John");
names.Add("Jim");
names.Add("Jack");
names.Add("Denny");
Console.WriteLine("Second iteration");
foreach (string name in namesWithJ)
{
Console.WriteLine(name);
}
/*
First iteration
Juan
second iteration
Jack
Jim
John
Juan
*/
}
//如查询出来自巴西的所有世界冠军,并按照夺冠次数排序
static void LINQQuery()
{
var query = from r in Formula1.GetChampions()
where r.Country == "Brazil"
orderby r.Wins descending
select r;
foreach (var r in query)
{
Console.WriteLine("{0:A}", r);
}
}
//Zip合并功能
static void ZipOperation()
{
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);
//}
}
//根据赛手获得一级方程式冠军的次数,列出最成功的国家
static void Aggregate2()
{
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(5);
foreach (var country in countries)
{
Console.WriteLine("{0} {1}", country.Country, country.Wins);
}
}
//筛选赛手,只返回获得冠军次数超过3次的赛手
static void Aggregate()
{
var query = from r in Formula1.GetChampions()
where r.Years.Count() > 3
orderby r.Years.Count() descending
select new
{
Name = r.FirstName + " " + r.LastName,
TimesChampion = r.Years.Count()
};
foreach (var r in query)
{
Console.WriteLine("{0} {1}", r.Name, r.TimesChampion);
}
}
//Linq分页显示
static void Partitioning()
{
int pageSize = 5;
int numberPages = (int)Math.Ceiling(Formula1.GetChampions().Count() /
(double)pageSize);
for (int page = 0; page < numberPages; page++)
{
Console.WriteLine("Page {0}", page);
var racers =
(from r in Formula1.GetChampions()
orderby r.LastName
select r.FirstName + " " + r.LastName).
Skip(page * pageSize).Take(pageSize);
foreach (var name in racers)
{
Console.WriteLine(name);
}
Console.WriteLine();
}
}
//有驾驶法拉利和迈凯轮的冠军，集合交集
static void SetOperations()
{
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);
}
}
//列出每年的赛手冠军和车队冠军
static void Join()
{
var racers = from r in Formula1.GetChampions()
from y in r.Years
where y > 2003
select new
{
Year = y,
Name = r.FirstName + " " + r.LastName
};
var teams = from t in
Formula1.GetContructorChampions()
from y in t.Years
where y > 2003
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
{
Year = r.Year,
Racer = r.Name,
Team = t.Name
};
Console.WriteLine("Year Champion " + "Constructor Title");
foreach (var item in racersAndTeams)
{
Console.WriteLine("{0}: {1,-20} {2}",
item.Year, item.Racer, item.Team);
}
}
//现在一级方程式冠军应按照国家分组,并列出一个国家的冠军数以及包含赛手名序列
static void GroupingWithNestedObjects()
{
var countries = from r in Formula1.GetChampions()
group r by r.Country into g
orderby g.Count() descending, g.Key
where g.Count() >= 2
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, -10} {1}", item.Country, item.Count);
foreach (var name in item.Racers)
{
Console.Write("{0}; ", name);
}
Console.WriteLine();
}
}
static void Grouping()
{
var countries = from r in Formula1.GetChampions()
group r by r.Country into g
orderby g.Count() descending, g.Key
where g.Count() >= 2
select new
{
Country = g.Key,
Count = g.Count()
};
foreach (var item in countries)
{
Console.WriteLine("{0, -10} {1}", item.Country, item.Count);
}
}
//显示驾驶法拉利的所有一级方程式冠军名字
static void CompoundFrom()
{
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 racer in ferrariDrivers)
{
Console.WriteLine(racer);
}
}
static void TypeFiltering()
{
object[] data = { "one", 2, 3, "four", "five", 6 };
var query = data.OfType<string>();
foreach (var s in query)
{
Console.WriteLine(s);
}
/*
one
four
five
*/
}
//使用索引返回姓氏以A开头、索引为偶数的赛手
static void IndexFiltering()
{
var racers = Formula1.GetChampions().
Where((r, index) => r.LastName.StartsWith("A") && index % 2 != 0);
foreach (var r in racers)
{
Console.WriteLine("{0:A}", r);
}
}
//找出赢得至少 15场比赛的巴西和奥地利赛手
static void Filtering()
{
var racers = from r in Formula1.GetChampions()
where r.Wins > 15 && (r.Country == "Brazil" || r.Country == "Austria")
select r;
foreach (var r in racers)
{
Console.WriteLine("{0:A}", r);
}
}
//取消Linq并行操作
static void Cancellation()
{
const int arraySize = 100000000;
var data = new int[arraySize];
var r = new Random();
for (int i = 0; i < arraySize; i++)
{
data[i] = r.Next(40);
}
var cts = new CancellationTokenSource();
new Thread(() =>
{
try
{
var sum = (from x in data.AsParallel().WithCancellation(cts.Token)
where x < 80
select x).Sum();
Console.WriteLine("query finished, sum: {0}", sum);
}
catch (OperationCanceledException ex)
{
Console.WriteLine(ex.Message);
}
}).Start();
Console.WriteLine("query started");
Console.Write("cancel? ");
int input = Console.Read();
if (input == 'Y' || input == 'y')
{
// cancel!
cts.Cancel();
}
}
//Linq 并行操作
static void IntroParallel()
{
const int arraySize = 100000000;
var data = new int[arraySize];
var r = new Random();
for (int i = 0; i < arraySize; i++)
data[i] = r.Next(40);
Stopwatch watch = new Stopwatch();
watch.Start();
var q1 = (from x in Partitioner.Create(data).AsParallel()
where x < 80
select x).Sum();
watch.Stop();
Console.WriteLine(watch.ElapsedMilliseconds);
}
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