将两个LINQ查询转换为单个查询

public class Student
{
    public int StudentID { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

public class Enrollment
{
    public int StudentID { get; set; }
    public int CourseID { get; set; }
    public decimal Grade { get; set; }
}

// 假设我们有一个学生列表和一个选课记录列表
List<Student> students = ...;
List<Enrollment> enrollments = ...;

// 原始的两个LINQ查询
var studentsWithCourse1 = from s in students
                           where s.StudentID == 1
                           select s;

var gradesForCourse1 = from e in enrollments
                       where e.CourseID == 1
                       select e.Grade;

// 合并后的单个LINQ查询
var combinedQuery = from s in students
                    join e in enrollments on s.StudentID equals e.StudentID
                    where e.CourseID == 1
                    select new { Student = s, Grade = e.Grade };