查询对象的集合

  • 项目

此示例演示如何对一系列 Student 对象执行简单查询的示例。 每个 Student 对象均包含一些关于学生的基本信息和表示学生在四门考试中的分数的列表。

备注

本节中的许多其他示例使用相同的 Student 类和 students 集合。

class Student
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public int ID { get; set; }
    public GradeLevel? Year { get; set; }
    public List<int> ExamScores { get; set; }

    public Student(string FirstName, string LastName, int ID, GradeLevel Year, List<int> ExamScores)
    {
        this.FirstName = FirstName;
        this.LastName = LastName;
        this.ID = ID;
        this.Year = Year;
        this.ExamScores = ExamScores;
    }

    public Student(string FirstName, string LastName, int StudentID, List<int>? ExamScores = null)
    {
        this.FirstName = FirstName;
        this.LastName = LastName;
        ID = StudentID;
        this.ExamScores = ExamScores ?? Enumerable.Empty<int>().ToList();
    }

    public static List<Student> students = new()
    {
        new(
            FirstName: "Terry", LastName: "Adams", ID: 120,
            Year: GradeLevel.SecondYear,
            ExamScores: new() { 99, 82, 81, 79 }
        ),
        new(
            "Fadi", "Fakhouri", 116,
            GradeLevel.ThirdYear,
            new() { 99, 86, 90, 94 }
        ),
        new(
            "Hanying", "Feng", 117,
            GradeLevel.FirstYear,
            new() { 93, 92, 80, 87 }
        ),
        new(
            "Cesar", "Garcia", 114,
            GradeLevel.FourthYear,
            new() { 97, 89, 85, 82 }
        ),
        new(
            "Debra", "Garcia", 115,
            GradeLevel.ThirdYear,
            new() { 35, 72, 91, 70 }
        ),
        new(
            "Hugo", "Garcia", 118,
            GradeLevel.SecondYear,
            new() { 92, 90, 83, 78 }
        ),
        new(
            "Sven", "Mortensen", 113,
            GradeLevel.FirstYear,
            new() { 88, 94, 65, 91 }
        ),
        new(
            "Claire", "O'Donnell", 112,
            GradeLevel.FourthYear,
            new() { 75, 84, 91, 39 }
        ),
        new(
            "Svetlana", "Omelchenko", 111,
            GradeLevel.SecondYear,
            new() { 97, 92, 81, 60 }
        ),
        new(
            "Lance", "Tucker", 119,
            GradeLevel.ThirdYear,
            new() { 68, 79, 88, 92 }
        ),
        new(
            "Michael", "Tucker", 122,
            GradeLevel.FirstYear,
            new() { 94, 92, 91, 91 }
        ),
        new(
            "Eugene", "Zabokritski", 121,
            GradeLevel.FourthYear,
            new() { 96, 85, 91, 60 }
        )
    };
}

enum GradeLevel
{
    FirstYear = 1,
    SecondYear,
    ThirdYear,
    FourthYear
};

示例

以下查询将返回在第一堂考试中得分为 90 或更高的学生。

void QueryHighScores(int exam, int score)
{
    var highScores =
        from student in students
        where student.ExamScores[exam] > score
        select new
        {
            Name = student.FirstName,
            Score = student.ExamScores[exam]
        };

    foreach (var item in highScores)
    {
        Console.WriteLine($"{item.Name,-15}{item.Score}");
    }
}

QueryHighScores(1, 90);

该查询有意简单，以使你可以进行实验。 例如，你可以在 where 子句中尝试其他条件或使用 orderby 子句对结果进行排序。

请参阅