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How to: Group Query Results (C# Programming Guide)

Grouping is one of the most powerful capabilities of LINQ. The following examples show how to group data in various ways:

  • By a single property.

  • By the first letter of a string property.

  • By a computed numeric range.

  • By Boolean predicate or other expression.

  • By a compound key.

In addition, the last two queries project their results into a new anonymous type that contains only the student's first and last name. For more information, see the group clause (C# Reference).

All the examples in this topic use the following helper classes and data sources.


public class StudentClass
{
    #region data
    protected enum GradeLevel { FirstYear = 1, SecondYear, ThirdYear, FourthYear };
    protected class Student
    {
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public int ID { get; set; }
        public GradeLevel Year;
        public List<int> ExamScores;
    }

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

    //Helper method, used in GroupByRange.
    protected static int GetPercentile(Student s)
    {
        double avg = s.ExamScores.Average();
        return avg > 0 ? (int)avg / 10 : 0;
    }



    public 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("{0,-15}{1}", item.Name, item.Score);
        }
    }
}

public class Program
{
    public static void Main()
    {
        StudentClass sc = new StudentClass();
        sc.QueryHighScores(1, 90);

        // Keep the console window open in debug mode.
        Console.WriteLine("Press any key to exit");
        Console.ReadKey();
    }
}


The following example shows how to group source elements by using a single property of the element as the group key. In this case the key is a string, the student's last name. It is also possible to use a substring for the key. The grouping operation uses the default equality comparer for the type.

Paste the following method into the StudentClass class. Change the calling statement in the Main method to sc.GroupBySingleProperty().


public void GroupBySingleProperty()
{
    Console.WriteLine("Group by a single property in an object:");

    // Variable queryLastNames is an IEnumerable<IGrouping<string, 
    // DataClass.Student>>. 
    var queryLastNames =
        from student in students
        group student by student.LastName into newGroup
        orderby newGroup.Key
        select newGroup;

    foreach (var nameGroup in queryLastNames)
    {
        Console.WriteLine("Key: {0}", nameGroup.Key);
        foreach (var student in nameGroup)
        {
            Console.WriteLine("\t{0}, {1}", student.LastName, student.FirstName);
        }
    }
}
/* Output:
    Group by a single property in an object:
    Key: Adams
            Adams, Terry
    Key: Fakhouri
            Fakhouri, Fadi
    Key: Feng
            Feng, Hanying
    Key: Garcia
            Garcia, Cesar
            Garcia, Debra
            Garcia, Hugo
    Key: Mortensen
            Mortensen, Sven
    Key: O'Donnell
            O'Donnell, Claire
    Key: Omelchenko
            Omelchenko, Svetlana
    Key: Tucker
            Tucker, Lance
            Tucker, Michael
    Key: Zabokritski
            Zabokritski, Eugene
*/


The following example shows how to group source elements by using something other than a property of the object for the group key. In this example, the key is the first letter of the student's last name.

Paste the following method into the StudentClass class. Change the calling statement in the Main method to sc.GroupBySubstring().


public void GroupBySubstring()
{            
    Console.WriteLine("\r\nGroup by something other than a property of the object:");

    var queryFirstLetters =
        from student in students
        group student by student.LastName[0];

    foreach (var studentGroup in queryFirstLetters)
    {
        Console.WriteLine("Key: {0}", studentGroup.Key);
        // Nested foreach is required to access group items.
        foreach (var student in studentGroup)
        {
            Console.WriteLine("\t{0}, {1}", student.LastName, student.FirstName);
        }
    }           
}
/* Output:
    Group by something other than a property of the object:
    Key: A
            Adams, Terry
    Key: F
            Fakhouri, Fadi
            Feng, Hanying
    Key: G
            Garcia, Cesar
            Garcia, Debra
            Garcia, Hugo
    Key: M
            Mortensen, Sven
    Key: O
            O'Donnell, Claire
            Omelchenko, Svetlana
    Key: T
            Tucker, Lance
            Tucker, Michael
    Key: Z
            Zabokritski, Eugene
*/


The following example shows how to group source elements by using a numeric range as a group key. The query then projects the results into an anonymous type that contains only the first and last name and the percentile range to which the student belongs. An anonymous type is used because it is not necessary to use the complete Student object to display the results. GetPercentile is a helper function that calculates a percentile based on the student's average score. The method returns an integer between 0 and 10.


//Helper method, used in GroupByRange.
protected static int GetPercentile(Student s)
{
    double avg = s.ExamScores.Average();
    return avg > 0 ? (int)avg / 10 : 0;
}


Paste the following method into the StudentClass class. Change the calling statement in the Main method to sc.GroupByRange().


public void GroupByRange()
{            
    Console.WriteLine("\r\nGroup by numeric range and project into a new anonymous type:");

    var queryNumericRange =
        from student in students
        let percentile = GetPercentile(student)
        group new { student.FirstName, student.LastName } by percentile into percentGroup
        orderby percentGroup.Key
        select percentGroup;

    // Nested foreach required to iterate over groups and group items.
    foreach (var studentGroup in queryNumericRange)
    {
        Console.WriteLine("Key: {0}", (studentGroup.Key * 10));
        foreach (var item in studentGroup)
        {
            Console.WriteLine("\t{0}, {1}", item.LastName, item.FirstName);
        }
    }            
}
/* Output:
    Group by numeric range and project into a new anonymous type:
    Key: 60
            Garcia, Debra
    Key: 70
            O'Donnell, Claire
    Key: 80
            Adams, Terry
            Feng, Hanying
            Garcia, Cesar
            Garcia, Hugo
            Mortensen, Sven
            Omelchenko, Svetlana
            Tucker, Lance
            Zabokritski, Eugene
    Key: 90
            Fakhouri, Fadi
            Tucker, Michael
*/


The following example shows how to group source elements by using a Boolean comparison expression. In this example, the Boolean expression tests whether a student's average exam score is greater than 75. As in previous examples, the results are projected into an anonymous type because the complete source element is not needed. Note that the properties in the anonymous type become properties on the Key member and can be accessed by name when the query is executed.

Paste the following method into the StudentClass class. Change the calling statement in the Main method to sc.GroupByBoolean().


public void GroupByBoolean()
{            
    Console.WriteLine("\r\nGroup by a Boolean into two groups with string keys");
    Console.WriteLine("\"True\" and \"False\" and project into a new anonymous type:");
    var queryGroupByAverages = from student in students
                               group new { student.FirstName, student.LastName }
                                    by student.ExamScores.Average() > 75 into studentGroup
                               select studentGroup;

    foreach (var studentGroup in queryGroupByAverages)
    {
        Console.WriteLine("Key: {0}", studentGroup.Key);
        foreach (var student in studentGroup)
            Console.WriteLine("\t{0} {1}", student.FirstName, student.LastName);
    }            
}
/* Output:
    Group by a Boolean into two groups with string keys
    "True" and "False" and project into a new anonymous type:
    Key: True
            Terry Adams
            Fadi Fakhouri
            Hanying Feng
            Cesar Garcia
            Hugo Garcia
            Sven Mortensen
            Svetlana Omelchenko
            Lance Tucker
            Michael Tucker
            Eugene Zabokritski
    Key: False
            Debra Garcia
            Claire O'Donnell
*/


The following example shows how to use an anonymous type to encapsulate a key that contains multiple values. In this example, the first key value is the first letter of the student's last name. The second key value is a Boolean that specifies whether the student scored over 85 on the first exam. You can order the groups by any property in the key.

Paste the following method into the StudentClass class. Change the calling statement in the Main method to sc.GroupByCompositeKey().


public void GroupByCompositeKey()
{

    var queryHighScoreGroups =
        from student in students
        group student by new { FirstLetter = student.LastName[0], 
            Score = student.ExamScores[0] > 85 } into studentGroup
        orderby studentGroup.Key.FirstLetter
        select studentGroup;

    Console.WriteLine("\r\nGroup and order by a compound key:");
    foreach (var scoreGroup in queryHighScoreGroups)
    {
        string s = scoreGroup.Key.Score == true ? "more than" : "less than";
        Console.WriteLine("Name starts with {0} who scored {1} 85", scoreGroup.Key.FirstLetter, s);
        foreach (var item in scoreGroup)
        {
            Console.WriteLine("\t{0} {1}", item.FirstName, item.LastName);
        }
    }
}
/* Output:
    Group and order by a compound key:
    Name starts with A who scored more than 85
            Terry Adams
    Name starts with F who scored more than 85
            Fadi Fakhouri
            Hanying Feng
    Name starts with G who scored more than 85
            Cesar Garcia
            Hugo Garcia
    Name starts with G who scored less than 85
            Debra Garcia
    Name starts with M who scored more than 85
            Sven Mortensen
    Name starts with O who scored less than 85
            Claire O'Donnell
    Name starts with O who scored more than 85
            Svetlana Omelchenko
    Name starts with T who scored less than 85
            Lance Tucker
    Name starts with T who scored more than 85
            Michael Tucker
    Name starts with Z who scored more than 85
            Eugene Zabokritski
*/


Copy and paste each method that you want to test into the StudentClass class. Add a calling statement for the method to the Main method and press F5.

When you adapt these methods to your own application, remember that LINQ requires version 3.5 or 4 of the .NET Framework, and that the project must contain a reference to System.Core.dll and a using directive for System.Linq. LINQ to SQL, LINQ to XML, and LINQ to DataSet types require additional using directives and references. For more information, see How to: Create a LINQ Project.

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