使用方法：使用 LINQ 查询或修改集合 - C#

如何查找两个列表之间的差集

此示例演示如何使用 LINQ 对两个字符串列表进行比较，并输出那些位于第一个集合（而不是第二个集合）中的行。名称的第一个集合存储在文件 names1.txt 中：
Bankov, Peter
Holm, Michael
Garcia, Hugo
Potra, Cristina
Noriega, Fabricio
Aw, Kam Foo
Beebe, Ann
Toyoshima, Tim
Guy, Wey Yuan
Garcia, Debra
名称的第二个集合存储在文件 names2.txt 中。 一些名称同时出现在两个序列中。
Liu, Jinghao
Bankov, Peter
Holm, Michael
Garcia, Hugo
Beebe, Ann
Gilchrist, Beth
Myrcha, Jacek
Giakoumakis, Leo
McLin, Nkenge
El Yassir, Mehdi
以下代码演示如何使用 Enumerable.Except 方法查找在第一个列表中，但不在第二个列表中的元素：
// Create the IEnumerable data sources.
string[] names1 = File.ReadAllLines("names1.txt");
string[] names2 = File.ReadAllLines("names2.txt");
// Create the query. Note that method syntax must be used here.
var differenceQuery = names1.Except(names2);
// Execute the query.
Console.WriteLine("The following lines are in names1.txt but not names2.txt");
foreach (string s in differenceQuery)
    Console.WriteLine(s);
/* Output:
 The following lines are in names1.txt but not names2.txt
 Potra, Cristina
 Noriega, Fabricio
 Aw, Kam Foo
 Toyoshima, Tim
 Guy, Wey Yuan
 Garcia, Debra
某些类型的查询操作（例如 Except、Distinct、Union 和 Concat）只能用基于方法的语法表示。
如何合并和比较字符串集合
此示例演示如何合并包含文本行的文件，并对结果排序。 具体而言，此示例演示如何对两组文本行执行串联、联合和交集。 它使用相同的两个文本文件，如前面的示例所示。 该代码显示 Enumerable.Concat、Enumerable.Union 和 Enumerable.Except 的示例。
//Put text files in your solution folder
string[] fileA = File.ReadAllLines("names1.txt");
string[] fileB = File.ReadAllLines("names2.txt");
//Simple concatenation and sort. Duplicates are preserved.
var concatQuery = fileA.Concat(fileB).OrderBy(s => s);
// Pass the query variable to another function for execution.
OutputQueryResults(concatQuery, "Simple concatenate and sort. Duplicates are preserved:");
// Concatenate and remove duplicate names based on
// default string comparer.
var uniqueNamesQuery = fileA.Union(fileB).OrderBy(s => s);
OutputQueryResults(uniqueNamesQuery, "Union removes duplicate names:");
// Find the names that occur in both files (based on
// default string comparer).
var commonNamesQuery = fileA.Intersect(fileB);
OutputQueryResults(commonNamesQuery, "Merge based on intersect:");
// Find the matching fields in each list. Merge the two
// results by using Concat, and then
// sort using the default string comparer.
string nameMatch = "Garcia";
var tempQuery1 = from name in fileA
                 let n = name.Split(',')
                 where n[0] == nameMatch
                 select name;
var tempQuery2 = from name2 in fileB
                 let n2 = name2.Split(',')
                 where n2[0] == nameMatch
                 select name2;
var nameMatchQuery = tempQuery1.Concat(tempQuery2).OrderBy(s => s);
OutputQueryResults(nameMatchQuery, $"""Concat based on partial name match "{nameMatch}":""");
static void OutputQueryResults(IEnumerable<string> query, string message)
    Console.WriteLine(Environment.NewLine + message);
    foreach (string item in query)
        Console.WriteLine(item);
    Console.WriteLine($"{query.Count()} total names in list");
/* Output:
    Simple concatenate and sort. Duplicates are preserved:
    Aw, Kam Foo
    Bankov, Peter
    Bankov, Peter
    Beebe, Ann
    Beebe, Ann
    El Yassir, Mehdi
    Garcia, Debra
    Garcia, Hugo
    Garcia, Hugo
    Giakoumakis, Leo
    Gilchrist, Beth
    Guy, Wey Yuan
    Holm, Michael
    Holm, Michael
    Liu, Jinghao
    McLin, Nkenge
    Myrcha, Jacek
    Noriega, Fabricio 
    Potra, Cristina
    Toyoshima, Tim
    20 total names in list
    Union removes duplicate names:
    Aw, Kam Foo
    Bankov, Peter
    Beebe, Ann
    El Yassir, Mehdi
    Garcia, Debra
    Garcia, Hugo
    Giakoumakis, Leo
    Gilchrist, Beth
    Guy, Wey Yuan
    Holm, Michael
    Liu, Jinghao
    McLin, Nkenge
    Myrcha, Jacek
    Noriega, Fabricio
    Potra, Cristina
    Toyoshima, Tim
    16 total names in list
    Merge based on intersect:
    Bankov, Peter
    Holm, Michael
    Garcia, Hugo
    Beebe, Ann
    4 total names in list
    Concat based on partial name match "Garcia":
    Garcia, Debra
    Garcia, Hugo
    Garcia, Hugo
    3 total names in list
如何从多个源填充对象集合
本示例演示如何将来自不同源的数据合并到一系列新的类型。
请勿尝试将内存中数据或文件系统中的数据与仍在数据库中的数据进行联接。 这种跨域联接可能产生未定义的结果，因为可能为数据库查询和其他类型的源定义了联接操作的不同方式。 此外，如果数据库中的数据量足够大，这样的操作还存在可能导致内存不足的异常的风险。 若要将数据库中的数据联接到内存数据，首先对数据库查询调用 ToList 或 ToArray，然后对返回的集合执行联接。
此示例使用两个文件。 第一个 names.csv 文件包含学生名称和学生 ID。
Omelchenko,Svetlana,111
O'Donnell,Claire,112
Mortensen,Sven,113
Garcia,Cesar,114
Garcia,Debra,115
Fakhouri,Fadi,116
Feng,Hanying,117
Garcia,Hugo,118
Tucker,Lance,119
Adams,Terry,120
Zabokritski,Eugene,121
Tucker,Michael,122
第二个 scores.csv 文件包含第一列中的学生 ID，后跟考试分数。
111, 97, 92, 81, 60
112, 75, 84, 91, 39
113, 88, 94, 65, 91
114, 97, 89, 85, 82
115, 35, 72, 91, 70
116, 99, 86, 90, 94
117, 93, 92, 80, 87
118, 92, 90, 83, 78
119, 68, 79, 88, 92
120, 99, 82, 81, 79
121, 96, 85, 91, 60
122, 94, 92, 91, 91
下面的示例演示如何使用命名记录 Student 存储来自两个内存字符串集合（模拟 .csv 格式的电子表格数据）的合并数据。 该 ID 用作将学生映射到其分数的键。
// Each line of names.csv consists of a last name, a first name, and an
// ID number, separated by commas. For example, Omelchenko,Svetlana,111
string[] names = File.ReadAllLines("names.csv");
// Each line of scores.csv consists of an ID number and four test
// scores, separated by commas. For example, 111, 97, 92, 81, 60
string[] scores = File.ReadAllLines("scores.csv");
// Merge the data sources using a named type.
// var could be used instead of an explicit type. Note the dynamic
// creation of a list of ints for the ExamScores member. The first item
// is skipped in the split string because it is the student ID,
// not an exam score.
IEnumerable<Student> queryNamesScores = from nameLine in names
                                        let splitName = nameLine.Split(',')
                                        from scoreLine in scores
                                        let splitScoreLine = scoreLine.Split(',')
                                        where Convert.ToInt32(splitName[2]) == Convert.ToInt32(splitScoreLine[0])
                                        select new Student
                                            FirstName: splitName[0],
                                            LastName: splitName[1],
                                            ID: Convert.ToInt32(splitName[2]),
                                            ExamScores: (from scoreAsText in splitScoreLine.Skip(1)
                                                         select Convert.ToInt32(scoreAsText)
                                                        ).ToArray()
// Optional. Store the newly created student objects in memory
// for faster access in future queries. This could be useful with
// very large data files.
List<Student> students = queryNamesScores.ToList();
// Display each student's name and exam score average.
foreach (var student in students)
    Console.WriteLine($"The average score of {student.FirstName} {student.LastName} is {student.ExamScores.Average()}.");
/* Output:
The average score of Omelchenko Svetlana is 82.5.
The average score of O'Donnell Claire is 72.25.
The average score of Mortensen Sven is 84.5.
The average score of Garcia Cesar is 88.25.
The average score of Garcia Debra is 67.
The average score of Fakhouri Fadi is 92.25.
The average score of Feng Hanying is 88.
The average score of Garcia Hugo is 85.75.
The average score of Tucker Lance is 81.75.
The average score of Adams Terry is 85.25.
The average score of Zabokritski Eugene is 83.
The average score of Tucker Michael is 92.
在“选择”子句中，将从两个源中的数据初始化每个新的 Student 对象。
如果不需要存储查询的结果，那么与命名的类型相比，元组或匿名类型使用起来更方便。 下面的示例执行与前面示例相同的任务，但使用的是元组，而不是命名的类型：
// Merge the data sources by using an anonymous type.
// Note the dynamic creation of a list of ints for the
// ExamScores member. We skip 1 because the first string
// in the array is the student ID, not an exam score.
var queryNamesScores2 = from nameLine in names
                        let splitName = nameLine.Split(',')
                        from scoreLine in scores
                        let splitScoreLine = scoreLine.Split(',')
                        where Convert.ToInt32(splitName[2]) == Convert.ToInt32(splitScoreLine[0])
                        select (FirstName: splitName[0], 
                                LastName: splitName[1], 
                                ExamScores: (from scoreAsText in splitScoreLine.Skip(1)
                                             select Convert.ToInt32(scoreAsText))
                                             .ToList()
// Display each student's name and exam score average.
foreach (var student in queryNamesScores2)
    Console.WriteLine($"The average score of {student.FirstName} {student.LastName} is {student.ExamScores.Average()}.");
如何使用 LINQ 查询 ArrayList
如果使用 LINQ 来查询非泛型 IEnumerable 集合（例如 ArrayList），必须显式声明范围变量的类型，以反映集合中对象的特定类型。 如果有 Student 对象的 ArrayList，那么 from 子句应如下所示：
var query = from Student s in arrList
//...
通过指定范围变量的类型，可将 ArrayList 中的每项强制转换为 Student。
在查询表达式中使用显式类型范围变量等效于调用 Cast 方法。 如果无法执行指定的强制转换，Cast 将引发异常。 Cast 和 OfType 是两个标准查询运算符方法，可对非泛型 IEnumerable 类型执行操作。 有关详细信息，请参阅 LINQ 查询操作中的类型关系。 下面的示例演示对 ArrayList 进行查询。
ArrayList arrList = new ArrayList();
arrList.Add(
    new Student
        FirstName: "Svetlana",
        LastName: "Omelchenko",
        ExamScores: new int[] { 98, 92, 81, 60 }
arrList.Add(
    new Student
        FirstName: "Claire",
        LastName: "O’Donnell",
        ExamScores: new int[] { 75, 84, 91, 39 }
arrList.Add(
    new Student
        FirstName: "Sven",
        LastName: "Mortensen",
        ExamScores: new int[] { 88, 94, 65, 91 }
arrList.Add(
    new Student
        FirstName: "Cesar",
        LastName: "Garcia",
        ExamScores: new int[] { 97, 89, 85, 82 }
var query = from Student student in arrList
            where student.ExamScores[0] > 95
            select student;
foreach (Student s in query)
    Console.WriteLine(s.LastName + ": " + s.ExamScores[0]);