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How to: Group Results in Various Ways (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 
    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. It is also possible to use a substring for the key. The grouping operation uses the default equality comparer for the type.

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

    // queryLastNames is an IEnumerable<IGrouping<string, DataClass.Student>> 
    // var is easier to type. 
    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: Feng
          Feng, Hanying
  Key: Garcia
          Garcia, Hugo
          Garcia, Cesar
          Garcia, Debra
  Key: Mortensen
          Mortensen, Sven
  Key: O'Donnell
          O'Donnell, Claire
  Key: Omelchenko
          Omelchenko, Svetlana
  Key: Tucker
          Tucker, Michael
          Tucker, Lance
 */  

The following example shows how to group source elements by using something other than a property of the object for the group key.

private static 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 first character:
        Key: O
                Omelchenko, Svetlana
                O'Donnell, Claire
        Key: G
                Garcia, Hugo
                Garcia, Cesar
                Garcia, Debra
        Key: M
                Mortensen, Sven
        Key: T
                Tucker, Michael
                Tucker, Lance
        Key: F
                Feng, Hanying
     */

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:

static int GetPercentile(Student s)
{
   double avg = s.Scores.Average();
   return avg > 0 ? (int)avg / 10 : 0;
}
private static 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
             Omelchenko, Svetlana
             O'Donnell, Claire
     Key: 80
             Garcia, Hugo
             Mortensen, Sven
             Garcia, Cesar
             Feng, Hanying
             Tucker, Lance
     Key: 90
             Tucker, Michael
     */

The following example shows how to group source elements by using a Boolean comparison expression. 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.

private static 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
                 Svetlana Omelchenko
                 Hugo Garcia
                 Sven Mortensen
                 Michael Tucker
                 Cesar Garcia
                 Hanying Feng
                 Lance Tucker
         Key: False
                 Claire O'Donnell
                 Debra Garcia
*/

The following example shows how to use an anonymous type to encapsulate a key that contains multiple values. In this case, 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.

private static 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 F who scored more than 85
                  Hanying Feng
          Name starts with G who scored more than 85
                  Hugo Garcia
                  Cesar 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 more than 85
                  Svetlana Omelchenko
          Name starts with O who scored less than 85
                  Claire O'Donnell
          Name starts with T who scored more than 85
                  Michael Tucker
          Name starts with T who scored less than 85
                  Lance Tucker
       */

This example contains references to objects that are defined in the sample application in How to: Query a Collection of Objects (C# Programming Guide). To compile and run this method, paste it into the StudentClass class in that application and add a call to it from the Main method.

When you adapt this method to your own application, remember that LINQ requires version 3.5 of the .NET Framework, and 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 usings and references. For more information, see How to: Create a LINQ Project.

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