C#使用LINQ查詢操作符實例代碼(一)
更新時間：2022年06月14日 17:09:07 作者：springsnow
這篇文章介紹了C#使用LINQ查詢操作符的方法，文中通過示例代碼介紹的非常詳細。對大家的學習或工作具有一定的參考借鑒價值，需要的朋友可以參考下
相關閱讀
示例業(yè)務背景介紹
示例參考《C#高級編程（第六版）》LINQ章節(jié)（P267 - P296），進行改編。
打開示例代碼我們看到：
1、Racer.cs 車手冠軍信息類
定義一級方程式世界車手冠軍信息。
2、Team.cs 車隊冠軍信息類
定義一級方程式世界車隊冠軍信息。
3、Formula1.cs 一級方程式類
包含兩個重要靜態(tài)方法：（F1是"Formula One"的縮寫）
- GetChampions()：返回一組車手列表。這個列表包含了1950到2007年之間的所有一級方程式世界車手冠軍。
- GetContructorChampions()：返回一組車隊列表。這個列表包含了1985到2007年之間的所有一級方程式世界車隊冠軍。
[Serializable]
public class Racer : IComparable<Racer>, IFormattable, IEquatable<Racer>
{
public string FirstName { get; set; }
public string LastName { get; set; }
public int Wins { get; set; }
public string Country { get; set; }
public int Starts { get; set; }
public string[] Cars { get; set; }
public int[] Years { get; set; }
public string ToString(string format, IFormatProvider formatProvider)
{
switch (format)
{
case null:
case "N":
return ToString();
case "F":
return FirstName;
case "L":
return LastName;
case "C":
return Country;
case "S":
return Starts.ToString();
case "W":
return Wins.ToString();
case "A":
return String.Format("{0} {1}, {2}; starts: {3}, wins: {4}",
FirstName, LastName, Country, Starts, Wins);
default:
throw new FormatException(String.Format(
"Format {0} not supported", format));
}
}
public bool Equals(Racer other)
{
return this.FirstName == other.FirstName && this.LastName == other.LastName;
}
public int CompareTo(Racer other)
{
return this.LastName.CompareTo(other.LastName);
}
public string ToString(string format)
{
return ToString(format, null);
}
public override string ToString()
{
return String.Format("{0} {1}", FirstName, LastName);
}
}
public class Team
{
public string Name { get; set; }
public int[] Years { get; set; }
}
public class Formula1
{
//返回車手冠軍列表
public static List<Racer> GetChampions()
{
List<Racer> racers = new List<Racer>(40);
racers.Add(new Racer() { FirstName = "Nino", LastName = "Farina", Country = "Italy", Starts = 33, Wins = 5, Years = new int[] { 1950 }, Cars = new string[] { "Alfa Romeo" } });
racers.Add(new Racer() { FirstName = "Alberto", LastName = "Ascari", Country = "Italy", Starts = 32, Wins = 10, Years = new int[] { 1952, 1953 }, Cars = new string[] { "Ferrari" } });
racers.Add(new Racer() { FirstName = "Juan Manuel", LastName = "Fangio", Country = "Argentina", Starts = 51, Wins = 24, Years = new int[] { 1951, 1954, 1955, 1956, 1957 }, Cars = new string[] { "Alfa Romeo", "Maserati", "Mercedes", "Ferrari" } });
racers.Add(new Racer() { FirstName = "Mike", LastName = "Hawthorn", Country = "UK", Starts = 45, Wins = 3, Years = new int[] { 1958 }, Cars = new string[] { "Ferrari" } });
racers.Add(new Racer() { FirstName = "Phil", LastName = "Hill", Country = "USA", Starts = 48, Wins = 3, Years = new int[] { 1961 }, Cars = new string[] { "Ferrari" } });
racers.Add(new Racer() { FirstName = "John", LastName = "Surtees", Country = "UK", Starts = 111, Wins = 6, Years = new int[] { 1964 }, Cars = new string[] { "Ferrari" } });
racers.Add(new Racer() { FirstName = "Jim", LastName = "Clark", Country = "UK", Starts = 72, Wins = 25, Years = new int[] { 1963, 1965 }, Cars = new string[] { "Lotus" } });
racers.Add(new Racer() { FirstName = "Jack", LastName = "Brabham", Country = "Australia", Starts = 125, Wins = 14, Years = new int[] { 1959, 1960, 1966 }, Cars = new string[] { "Cooper", "Brabham" } });
racers.Add(new Racer() { FirstName = "Denny", LastName = "Hulme", Country = "New Zealand", Starts = 112, Wins = 8, Years = new int[] { 1967 }, Cars = new string[] { "Brabham" } });
racers.Add(new Racer() { FirstName = "Graham", LastName = "Hill", Country = "UK", Starts = 176, Wins = 14, Years = new int[] { 1962, 1968 }, Cars = new string[] { "BRM", "Lotus" } });
racers.Add(new Racer() { FirstName = "Jochen", LastName = "Rindt", Country = "Austria", Starts = 60, Wins = 6, Years = new int[] { 1970 }, Cars = new string[] { "Lotus" } });
racers.Add(new Racer() { FirstName = "Jackie", LastName = "Stewart", Country = "UK", Starts = 99, Wins = 27, Years = new int[] { 1969, 1971, 1973 }, Cars = new string[] { "Matra", "Tyrrell" } });
racers.Add(new Racer() { FirstName = "Emerson", LastName = "Fittipaldi", Country = "Brazil", Starts = 143, Wins = 14, Years = new int[] { 1972, 1974 }, Cars = new string[] { "Lotus", "McLaren" } });
racers.Add(new Racer() { FirstName = "James", LastName = "Hunt", Country = "UK", Starts = 91, Wins = 10, Years = new int[] { 1976 }, Cars = new string[] { "McLaren" } });
racers.Add(new Racer() { FirstName = "Mario", LastName = "Andretti", Country = "USA", Starts = 128, Wins = 12, Years = new int[] { 1978 }, Cars = new string[] { "Lotus" } });
racers.Add(new Racer() { FirstName = "Jody", LastName = "Scheckter", Country = "South Africa", Starts = 112, Wins = 10, Years = new int[] { 1979 }, Cars = new string[] { "Ferrari" } });
racers.Add(new Racer() { FirstName = "Alan", LastName = "Jones", Country = "Australia", Starts = 115, Wins = 12, Years = new int[] { 1980 }, Cars = new string[] { "Williams" } });
racers.Add(new Racer() { FirstName = "Keke", LastName = "Rosberg", Country = "Finland", Starts = 114, Wins = 5, Years = new int[] { 1982 }, Cars = new string[] { "Williams" } });
racers.Add(new Racer() { FirstName = "Niki", LastName = "Lauda", Country = "Austria", Starts = 173, Wins = 25, Years = new int[] { 1975, 1977, 1984 }, Cars = new string[] { "Ferrari", "McLaren" } });
racers.Add(new Racer() { FirstName = "Nelson", LastName = "Piquet", Country = "Brazil", Starts = 204, Wins = 23, Years = new int[] { 1981, 1983, 1987 }, Cars = new string[] { "Brabham", "Williams" } });
racers.Add(new Racer() { FirstName = "Ayrton", LastName = "Senna", Country = "Brazil", Starts = 161, Wins = 41, Years = new int[] { 1988, 1990, 1991 }, Cars = new string[] { "McLaren" } });
racers.Add(new Racer() { FirstName = "Nigel", LastName = "Mansell", Country = "UK", Starts = 187, Wins = 31, Years = new int[] { 1992 }, Cars = new string[] { "Williams" } });
racers.Add(new Racer() { FirstName = "Alain", LastName = "Prost", Country = "France", Starts = 197, Wins = 51, Years = new int[] { 1985, 1986, 1989, 1993 }, Cars = new string[] { "McLaren", "Williams" } });
racers.Add(new Racer() { FirstName = "Damon", LastName = "Hill", Country = "UK", Starts = 114, Wins = 22, Years = new int[] { 1996 }, Cars = new string[] { "Williams" } });
racers.Add(new Racer() { FirstName = "Jacques", LastName = "Villeneuve", Country = "Canada", Starts = 165, Wins = 11, Years = new int[] { 1997 }, Cars = new string[] { "Williams" } });
racers.Add(new Racer() { FirstName = "Mika", LastName = "Hakkinen", Country = "Finland", Starts = 160, Wins = 20, Years = new int[] { 1998, 1999 }, Cars = new string[] { "McLaren" } });
racers.Add(new Racer() { FirstName = "Michael", LastName = "Schumacher", Country = "Germany", Starts = 250, Wins = 91, Years = new int[] { 1994, 1995, 2000, 2001, 2002, 2003, 2004 }, Cars = new string[] { "Benetton", "Ferrari" } });
racers.Add(new Racer() { FirstName = "Fernando", LastName = "Alonso", Country = "Spain", Starts = 105, Wins = 19, Years = new int[] { 2005, 2006 }, Cars = new string[] { "Renault" } });
racers.Add(new Racer() { FirstName = "Kimi", LastName = "R?ikk?nen", Country = "Finland", Starts = 122, Wins = 15, Years = new int[] { 2007 }, Cars = new string[] { "Ferrari" } });
return racers;
}
//返回車隊冠軍列表
public static List<Team> GetContructorChampions()
{
List<Team> teams = new List<Team>(20);
teams.Add(new Team() { Name = "Vanwall", Years = new int[] { 1958 } });
teams.Add(new Team() { Name = "Cooper", Years = new int[] { 1959, 1960 } });
teams.Add(new Team() { Name = "Ferrari", Years = new int[] { 1961, 1964, 1975, 1976, 1977, 1979, 1982, 1983, 1999, 2000, 2001, 2002, 2003, 2004, 2007 } });
teams.Add(new Team() { Name = "BRM", Years = new int[] { 1962 } });
teams.Add(new Team() { Name = "Lotus", Years = new int[] { 1963, 1965, 1968, 1970, 1972, 1973, 1978 } });
teams.Add(new Team() { Name = "Brabham", Years = new int[] { 1966, 1967 } });
teams.Add(new Team() { Name = "Matra", Years = new int[] { 1969 } });
teams.Add(new Team() { Name = "Tyrrell", Years = new int[] { 1971 } });
teams.Add(new Team() { Name = "McLaren", Years = new int[] { 1974, 1984, 1985, 1988, 1989, 1990, 1991, 1998 } });
teams.Add(new Team() { Name = "Williams", Years = new int[] { 1980, 1981, 1986, 1987, 1992, 1993, 1994, 1996, 1997 } });
teams.Add(new Team() { Name = "Benetton", Years = new int[] { 1995 } });
teams.Add(new Team() { Name = "Renault", Years = new int[] { 2005, 2006 } });
return teams;
}
}一、篩選操作符
where子句合并多個表達式。 根據條件返回匹配元素的集合IEnumerable。
業(yè)務說明：找出贏得至少15場比賽的英國和奧地利賽車手。
var racers = from r in Formula1.GetChampions()
where r.Wins > 15 && (r.Country == "Brazil" || r.Country == "Austria")
select r;
foreach (var racer in racers)
{
Console.WriteLine("{0:A}", racer);
}下面代碼有Where擴展方法Where和Select調用。
var racers = Formula1.GetChampions().
Where(r => r.Wins > 15 && (r.Country == "Brazil" || r.Country == "Austria")).
Select(r => r);結果：
//Niki Lauda, Austria; starts: 173, wins: 25
//Nelson Piquet, Brazil; starts: 204, wins: 23
//Ayrton Senna, Brazil; starts: 161, wins: 41
1、索引器篩選
索引是篩選器返回的每個結果的計數器。下面由Where擴展方法調用， 使用索引返回。
業(yè)務說明：找出上述的、索引為偶數的賽車手
var racers1 = Formula1.GetChampions()
.Where((r, index)=> r.Wins > 15 && (r.Country == "Brazil" || r.Country == "Austria") && index % 2 != 0)
.Select(r => r);結果：
Nelson Piquet
2、類型篩選OfType
基于類型篩選，使用 OfType 擴展方法。
業(yè)務說明：取出下面所有int類型的參數
object[] data = { "one", 1, 2, "li" };
var query = data.OfType<int>();
foreach (var intValue in query)
{
Console.WriteLine("{0}", intValue);
}
//1
//2例2：取出下面racer類型的值
IList mixedList = new ArrayList();
mixedList.Add( Formula1.GetChampions()[0]);
mixedList.Add(Formula1.GetContructorChampions()[0]);
mixedList.Add("Two");
mixedList.Add(3);
var query = mixedList.OfType<Racer>();
foreach (var item in query)
{
Console.WriteLine("{0}", item);
}二、投影操作符
1、Select 子句
將序列的每個元素經過lambda表達式處理后投影到一個新類型元素上。
（與SelectMany不同在于，若單個元素投影到IEnumerable，Select不會對多個IEnumerable進行合并）
用Select方法創(chuàng)建匿名類型
var query2 = Formula1.GetChampions()
.GroupBy(r => r.Country)
.Select(g => new { Group = g, Count = g.Count() })
.OrderByDescending(g => g.Count)
.ThenBy(g => g.Group.Key)
.Where(g => g.Count > 1)
.Select(g => new
{
Country = g.Group.Key,
Count = g.Count
});
foreach (var racer in query2)
{
Console.WriteLine("{0:A}", racer);
}結果：
{ Country = UK, Count = 8 }
{ Country = Brazil, Count = 3 }
{ Country = Finland, Count = 3 }
{ Country = Australia, Count = 2 }
{ Country = Austria, Count = 2 }
{ Country = Italy, Count = 2 }
{ Country = USA, Count = 2 }
相應的lambda表達式：
var query3 = from r in Formula1.GetChampions()
group r by r.Country into g
let count=g.Count()
orderby count descending, g.Key
where count > 1
select new
{
Country = g.Key,
Count = count
};
foreach (var racer in query3)
{
Console.WriteLine("{0:A}", racer);
}2、復合的From子句
業(yè)務說明：篩選駕駛法拉利的所有冠軍
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("{0:A}", racer);
}結果：
Alberto Ascari
Juan Manuel Fangio
Mike Hawthorn
Phil Hill
Niki Lauda
Kimi Räikkönen
Jody Scheckter
Michael Schumacher
John Surtees
C#編譯器會把復合的from語句轉換為SelectMany方法。
var ferrariDrivers = Formula1.GetChampions()
.SelectMany(r => r.Cars, (r, c) => new { Racer = r, Car = c })
.Where(r => r.Car == "Ferrari")
.OrderBy(r => r.Racer.LastName)
.Select(r => r.Racer.FirstName + " " + r.Racer.LastName);符合from子句，將子集合與復集合在同一個表平行展示。
var ferrariDrivers = Formula1.GetChampions()
.SelectMany(r => r.Cars, (r, c) => new { Racer = r, Car = c })
.Where(r => r.Racer.Country == "UK")
.OrderBy(r => r.Racer.LastName)
.ThenBy(r => r.Car)
.Select(r => r);
foreach (var racer in ferrariDrivers)
{
Console.WriteLine("{0:A}", racer + " :" + racer.Racer.Cars);
}結果：
{ Racer = Jim Clark, Car = Lotus } :Lotus
{ Racer = Mike Hawthorn, Car = Ferrari } :Ferrari
{ Racer = Graham Hill, Car = BRM } :BRM
{ Racer = Graham Hill, Car = Lotus } :BRM
{ Racer = Damon Hill, Car = Williams } :Williams
{ Racer = James Hunt, Car = McLaren } :McLaren
{ Racer = Nigel Mansell, Car = Williams } :Williams
{ Racer = Jackie Stewart, Car = Matra } :Matra
{ Racer = Jackie Stewart, Car = Tyrrell } :Matra
{ Racer = John Surtees, Car = Ferrari } :Ferrari
三、let子句
上面的例子：
var query8 = from r in Formula1.GetChampions()
group r by r.Country into g
orderby g.Count() descending, g.Key
where g.Count() > 1
select new
{
Country = g.Key,
Count = g.Count()
};上面的分組查詢Count方法調用了多次。使用let子句可以改變這種形式
var query10 = from r in Formula1.GetChampions()
group r by r.Country into g
let count = g.Count()
orderby count descending, g.Key
where count > 1
select new
{
Country = g.Key,
Count = count
};四、排序操作符
業(yè)務說明：來自英國的賽車手按照贏得比賽的次數進行降序排序
var query6 = from r in Formula1.GetChampions()
where r.Country == "UK"
orderby r.Wins descending
select r;
foreach (var racer in query6)
{
Console.WriteLine("{0:A}", racer);
}結果：
Nigel Mansell, UK; starts: 187, wins: 31
Jackie Stewart, UK; starts: 99, wins: 27
Jim Clark, UK; starts: 72, wins: 25
Damon Hill, UK; starts: 114, wins: 22
Graham Hill, UK; starts: 176, wins: 14
James Hunt, UK; starts: 91, wins: 10
John Surtees, UK; starts: 111, wins: 6
Mike Hawthorn, UK; starts: 45, wins: 3
使用OrderBy()，OrderByDescending方式替換掉上面的寫法：
var racers2 = Formula1.GetChampions()
.Where(r => r.Country == "UK")
.OrderByDescending(r => r.Wins)
.Select(r => r);使用ThenBy() 和 ThenByDescending() 方法繼續(xù)排序進行二次排序
業(yè)務說明：獲取車手冠軍列表，并依次按照Country升序、LastName降序、FirstName升序進行排序。擴展方法 Take 提取前面 10 個元素。
// 查詢表達式
var racers = (from r in Formula1.GetChampions()
orderby r.Country, r.LastName descending, r.FirstName
select r).Take(10);
// 方法語法
var racers1 = Formula1.GetChampions()
.OrderBy(r => r.Country)
.ThenByDescending(r => r.LastName)
.ThenBy(r => r.FirstName).Take(10);
foreach (var racer in racers)
{
Console.WriteLine("{0:A}", racer);
}
//Juan Manuel Fangio, Argentina; starts: 51, wins: 24
//Alan Jones, Australia; starts: 115, wins: 12
//Jack Brabham, Australia; starts: 125, wins: 14
//Jochen Rindt, Austria; starts: 60, wins: 6
//Niki Lauda, Austria; starts: 173, wins: 25
//Ayrton Senna, Brazil; starts: 161, wins: 41
//Nelson Piquet, Brazil; starts: 204, wins: 23
//Emerson Fittipaldi, Brazil; starts: 143, wins: 14
//Jacques Villeneuve, Canada; starts: 165, wins: 11
//Keke Rosberg, Finland; starts: 114, wins: 5五、分組操作符
根據一個關鍵字值對查詢結果進行分組，使用 group 子句。
子句 group r by r.Country into g 根據 Country 屬性組合。并定義一個新的標識符g，它以后用于訪問分組的結果信息。
業(yè)務說明：列出每個國家的冠軍數
var countries = from r in Formula1.GetChampions()
group r by r.Country into g
orderby g.Count() descending, g.Key
where g.Count() > 1
select new
{
Country = g.Key,
Count = g.Count()
};
foreach (var country in countries)
{
Console.WriteLine(format: "{0,-10} {1}", arg0: country.Country, arg1: country.Count);
}結果：
UK 8
Brazil 3
Finland 3
Australia 2
Austria 2
Italy 2
USA 2
使用擴展方法，子句 group r by r.Country into g 解析為 GroupBy(r => r.Country) 返回分組序列。
var countries = Formual.GetChampions()
.GroupBy(r => r.Country)
.OrderByDescending(g => g.Count())
.ThenBy(g => g.Key)
.Where(g => g.Count() > 1)
.Select(g => new { Country = g.Key, Count = g.Count() });返回值為 IEnumerable。
1、對嵌套的對象分組
分組的對象包含嵌套的序列，可以改變 select 子句創(chuàng)建的匿名類型。
業(yè)務說明：分組查詢每個國家獲得冠軍的賽車手人數，人數必須大于3，返回國家名稱、國家賽車手冠軍數量、賽車手名稱。
var query = from r in Formula1.GetChampions()
group r by r.Country into g
orderby g.Count() descending, g.Key
where g.Count() >= 3
select new
{
Country = g.Key,
count = g.Count(),
RacerNickName = from r1 in g orderby r1.FirstName select r1.FirstName + " " + r1.LastName
};
foreach (var g in query)
{
Console.WriteLine("{0:A}",g.Country+" "+g.count);
foreach (var element in g.RacerNickName)
{
Console.WriteLine("---{0:A}",element);
}
}對應的擴展方法：
var query1 = Formula1.GetChampions()
.GroupBy(r => r.Country)
.OrderByDescending(g => g.Count())
.ThenBy(g => g.Key)
.Where(g => g.Count() >= 3)
.Select(g => new
{
Country = g.Key,
count = g.Count(),
RacerNickName = g.OrderBy(r1 => r1.FirstName).Select(r1 => r1.FirstName + " " + r1.LastName)
});結果：
UK 8
---Damon Hill
---Graham Hill
---Jackie Stewart
---James Hunt
---Jim Clark
---John Surtees
---Mike Hawthorn
---Nigel Mansell
Brazil 3
---Ayrton Senna
---Emerson Fittipaldi
---Nelson Piquet
Finland 3
---Keke Rosberg
---Kimi Räikkönen
---Mika Hakkinen
2、多字段分組
var query = from r in Formula1.GetChampions()
group r by new { r.Country, r.LastName } into g
orderby g.Count() descending, g.Key.Country, g.Key.LastName
where g.Count() >= 1
select new
{
MyGroup = g.Key,
count = g.Count(),
Racers = g
};
foreach (var g in query)
{
Console.WriteLine("{0:A}", g.MyGroup + " : " + g.count);
foreach (var element in g.Racers)
{
Console.WriteLine("---{0:A}", element);
}
}結果：
{ Country = UK, LastName = Hill } : 2
---Graham Hill, UK; starts: 176, wins: 14
---Damon Hill, UK; starts: 114, wins: 22
{ Country = Argentina, LastName = Fangio } : 1
---Juan Manuel Fangio, Argentina; starts: 51, wins: 24。。。
3、分組后再每組里面僅取滿足條件的行
var query = from r in Formula1.GetChampions()
group r by r.Country into g
let maxId = g.Max(a => a.Wins)
orderby g.Key
where g.Count() >= 1
from row in g
where row.Wins == maxId
select row;
foreach (var g in query)
{
Console.WriteLine("{0:A}", g);
}到此這篇關于C#使用LINQ查詢操作符的文章就介紹到這了。希望對大家的學習有所幫助，也希望大家多多支持腳本之家。
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