在本节中，您将学习一些复杂的LINQ查询。我们将使用以下学生和标准集合进行查询。
IList<Student> studentList = new List<Student>() {
new Student() { StudentID = 1, StudentName = "John", Age = 18, StandardID = 1 } ,
new Student() { StudentID = 2, StudentName = "Steve", Age = 21, StandardID = 1 } ,
new Student() { StudentID = 3, StudentName = "Bill", Age = 18, StandardID = 2 } ,
new Student() { StudentID = 4, StudentName = "Ram" , Age = 20, StandardID = 2 } ,
new Student() { StudentID = 5, StudentName = "Ron" , Age = 21 }
};
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"}
};示例：多个Select和where运算符
var studentNames = studentList.Where(s => s.Age > 18) .Select(s => s) .Where(st => st.StandardID > 0) .Select(s => s.StudentName);
Steve Ram
以下查询返回仅具有StudentName属性的匿名对象的Enumerable：
var teenStudentsName = from s in studentList
where s.age > 12 && s.age < 20
select new { StudentName = s.StudentName };
teenStudentsName.ToList().ForEach(s => Console.WriteLine(s.StudentName));John Bill
以下查询返回按StandardID列出的学生组：
var studentsGroupByStandard = from s in studentList
group s by s.StandardID into sg
orderby sg.Key
select new { sg.Key, sg };
foreach (var group in studentsGroupByStandard)
{
Console.WriteLine("StandardID {0}:", group.Key);
group.sg.ToList().ForEach(st => Console.WriteLine(st.StudentName ));
}StandardID 0: Ron StandardID 1: John Steve StandardID 2: Bill Ram
输出包括没有任何 StandardID 的 Ron，因此 Ron 属于 StandardID 0。
要删除没有StandardID的学生，请在组运算符之前使用where运算符：
var studentsGroupByStandard = from s in studentList
where s.StandardID > 0
group s by s.StandardID into sg
orderby sg.Key
select new { sg.Key, sg };StandardID 1: John Steve StandardID 2: Bill Ram
使用左外部联接(Left outer join)显示每个标准下的学生。即使没有分配该标准的学生，也要显示标准名称。
var studentsGroup = from stad in standardList
join s in studentList
on stad.StandardID equals s.StandardID
into sg
select new {
StandardName = stad.StandardName,
Students = sg
};
foreach (var group in studentsGroup)
{
Console.WriteLine(group.StandardName);
group.Students.ToList().ForEach(st => Console.WriteLine(st.StudentName));
}Standard 1: John Steve Standard 2: Bill Ram Standard 3:
在下面的group by查询示例中，我们对组进行排序并只选择StudentName:
var studentsWithStandard = from stad in standardList
join s in studentList
on stad.StandardID equals s.StandardID
into sg
from std_grp in sg
orderby stad.StandardName, std_grp.StudentName
select new {
StudentName = std_grp.StudentName,
StandardName = stad.StandardName
};
foreach (var group in studentsWithStandard)
{
Console.WriteLine("{0} is in {1}", group.StudentName, group.StandardName);
}John is in Standard 1 Steve is in Standard 1 Bill is in Standard 2 Ram is in Standard 2
以下查询按StandardID和Age的升序返回学生列表。
var sortedStudents = from s in studentList
orderby s.StandardID, s.age
select new {
StudentName = s.StudentName,
Age = s.age,
StandardID = s.StandardID };
sortedStudents.ToList().ForEach(s => Console.WriteLine("Student Name: {0}, Age: {1}, StandardID: {2}", s.StudentName, s.Age , s.StandardID));Student Name: Ron, Age: 21, StandardID: 0 Student Name: John, Age: 18, StandardID: 1 Student Name: Steve, Age: 21, StandardID: 1 Student Name: Bill, Age: 18, StandardID: 2 Student Name: Ram, Age: 20, StandardID: 2
var studentWithStandard = from s in studentList
join stad in standardList
on s.StandardID equals stad.StandardID
select new {
StudentName = s.StudentName,
StandardName = stad.StandardName
};
studentWithStandard.ToList().ForEach(s => Console.WriteLine("{0} is in {1}", s.StudentName, s.StandardName ));John is in Standard 1 Steve is in Standard 1 Bill is in Standard 2 Ram is in Standard 2
var nestedQueries = from s in studentList where s.age > 18 && s.StandardID == (from std in standardList where std.StandardName == "Standard 1" select std.StandardID).FirstOrDefault() select s; nestedQueries.ToList().ForEach(s => Console.WriteLine(s.StudentName));
Steve