Join操作是将两个集合联合

Join操作两个集合，inner collection 和 outer collection

它返回一个集合（包含两个集合根据特定条件结合的所有元素），和SQL中的inner join一样

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);

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,
                                                        IEqualityComparer<TKey> comparer);

IList<string> strList1 = new List<string>() {
    "One",
    "Two",
    "Three",
    "Four"
};

IList<string> strList2 = new List<string>() {
    "One",
    "Two",
    "Five",
    "Six"
};

var innerJoin = strList1.Join(strList2,
                      str1 => str1,
                      str2 => str2,
                      (str1, str2) => str1);

public class Student{
    public int StudentID { get; set; }
    public string StudentName { get; set; }
    public int StandardID { get; set; }
}

public class Standard{
    public int StandardID { get; set; }
    public string StandardName { get; set; }
}

IList<Student> studentList = new List<Student>() {
    new Student() { StudentID = 1, StudentName = "John", StandardID =1 },
    new Student() { StudentID = 2, StudentName = "Moin", StandardID =1 },
    new Student() { StudentID = 3, StudentName = "Bill", StandardID =2 },
    new Student() { StudentID = 4, StudentName = "Ram" , StandardID =2 },
    new Student() { StudentID = 5, StudentName = "Ron"  }
};

IList<Standard> standardList = new List<Standard>() {
    new Standard(){ StandardID = 1, StandardName="Standard 1"},
    new Standard(){ StandardID = 2, StandardName="Standard 2"},
    new Standard(){ StandardID = 3, StandardName="Standard 3"}
};

var innerJoin = studentList.Join(// outer sequence
                      standardList,  // inner sequence
                      student => student.StandardID,    // outerKeySelector
                      standard => standard.StandardID,  // innerKeySelector
                      (student, standard) => new  // result selector
                                    {
                                        StudentName = student.StudentName,
                                        StandardName = standard.StandardName
                                    });

在上面的例子中，studentList是外部序列，因为先从它开始查询，Join的第一个参数是指定的内部序列，第二个和第三个参数指定对应的字段

外部序列的key选择器student => student.StandardID表明studentList中每个元素的StandardId字段必须与内部序列standard => standard.StandardID相对应

如果所有的key值相等则被包含在结果集中

最后一个参数是构建结果数据

查询语法

查询语法中的join和方法语法有些不同，它需要外部序列、内部序列、key选择器和结果选择器

from ... in outerSequence

join ... in innerSequence  

on outerKey equals innerKey

select ...

GroupJoin

GroupJoin和Join是一样的除了GroupJoin返回一个Group（根据特定的group key）

GroupJoin根据key联合两个序列并根据key分组

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);

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, IEqualityComparer<TKey> comparer);

IList<Student> studentList = new List<Student>() {
    new Student() { StudentID = 1, StudentName = "John", StandardID =1 },
    new Student() { StudentID = 2, StudentName = "Moin", StandardID =1 },
    new Student() { StudentID = 3, StudentName = "Bill", StandardID =2 },
    new Student() { StudentID = 4, StudentName = "Ram",  StandardID =2 },
    new Student() { StudentID = 5, StudentName = "Ron" }
};

IList<Standard> standardList = new List<Standard>() {
    new Standard(){ StandardID = 1, StandardName="Standard 1"},
    new Standard(){ StandardID = 2, StandardName="Standard 2"},
    new Standard(){ StandardID = 3, StandardName="Standard 3"}
};

var groupJoin = standardList.GroupJoin(studentList,  //inner sequence
                                std => std.StandardID, //outerKeySelector
                                s => s.StandardID,     //innerKeySelector
                                (std, studentsGroup) => new // resultSelector
                                {
                                    Students = studentsGroup,
                                    StandarFulldName = std.StandardName
                                });

foreach (var item in groupJoin)
{
    Console.WriteLine(item.StandarFulldName );

    foreach(var stud in item.Students)
        Console.WriteLine(stud.StudentName);
}

查询语法

from ... in outerSequence

join ... in innerSequence  

on outerKey equals innerKey

into groupedCollection    

select ...