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Query a collection of objects

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This topic shows an example of how to perform a simple query over a list of Student objects. Each Student object contains some basic information about the student, and a list that represents the student's scores on four examinations.

Note

Many other examples in this section use the same Student class and students collection.

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
};

Example

The following query returns the students who received a score of 90 or greater on their first exam.

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);

This query is intentionally simple to enable you to experiment. For example, you can try more conditions in the where clause, or use an orderby clause to sort the results.

See also