The ‘GroupBy’ feature in LINQ is amazing and very powerful. When you use a ‘GroupBy’ in LINQ, internally, it calls an extension method which returns a sequence of System.Collections.Generic.IEnumerable<(Of <(IGrouping<(Of <(TKey, TSource>)>)>)>).

The GroupBy<(Of <(TSource, TKey>)>)(IEnumerable<(Of <(TSource>)>), Func<(Of <(TSource, TKey>)>)) method returns a collection of IGrouping<(Of <(TKey, TElement>)>) objects, one for each distinct key that was encountered. The key represents the attribute that is common to each value in the IGrouping<(Of <(TKey, TElement>)>) and can be accessed using a ForEach loop.

In order to understand GroupBy in LINQ, let’s take an example. Linda is an HR in a small private firm. To facilitate the HR process, she wants a simple console application to obtain some quick results. She needs the following details of employees:

Raw list of employees
List of employees grouped by the first letter of their first name
List of employees grouped by the year in which they were born
List of employees grouped by the year and month in which they were born
Total count of employees having birthdays in the same year
Sex ratio

Let’s take these requirements one by one and see how they can be easily achieved using ‘GroupBy’ in LINQ. We will first create a simple list of employees (List<Employees>) and add some data to it.

class Program 
{ 
    static void Main(string[] args) 
    { 
        List<Employee> empList = new List<Employee>(); 
        empList.Add(new Employee() { ID = 1, FName = "John", MName = "", LName = "Shields", 
                                     DOB = DateTime.Parse("12/11/1971"), Sex = 'M' }); 
        empList.Add(new Employee() { ID = 2, FName = "Mary", 
                                     MName = "Matthew", LName = "Jacobs", 
                                     DOB = DateTime.Parse("01/17/1961"), Sex = 'F' }); 
        empList.Add(new Employee() { ID = 3, FName = "Amber", 
                                     MName = "Carl", LName = "Agar", 
                                     DOB = DateTime.Parse("12/23/1971"), Sex = 'M' }); 
        empList.Add(new Employee() { ID = 4, FName = "Kathy", MName = "", LName = "Berry", 
                                     DOB = DateTime.Parse("11/15/1976"), Sex = 'F' }); 
        empList.Add(new Employee() { ID = 5, FName = "Lena", 
                                     MName = "Ashco", LName = "Bilton", 
                                     DOB = DateTime.Parse("05/11/1978"), Sex = 'F' }); 
        empList.Add(new Employee() { ID = 6, FName = "Susanne", 
                                     MName = "", LName = "Buck", 
                                     DOB = DateTime.Parse("03/7/1965"), Sex = 'F' }); 
        empList.Add(new Employee() { ID = 7, FName = "Jim", MName = "", LName = "Brown", 
                                     DOB = DateTime.Parse("09/11/1972"), Sex = 'M' }); 
        empList.Add(new Employee() { ID = 8, FName = "Jane", MName = "G", LName = "Hooks", 
                                     DOB = DateTime.Parse("12/11/1972"), Sex = 'F' }); 
        empList.Add(new Employee() { ID = 9, FName = "Robert", MName = "", LName = "", 
                                     DOB = DateTime.Parse("06/28/1964"), Sex = 'M' }); 
        empList.Add(new Employee() { ID = 10, FName = "Cindy", MName = "Preston", 
                                     LName = "Fox", 
                                     DOB = DateTime.Parse("01/11/1978"), Sex = 'M' }); 

        // Printing the List 
        Console.WriteLine("\n{0,2} {1,7}    {2,8}      {3,8}      {4,23}      {5,3}", 
                          "ID", "FName", "MName", "LName", "DOB", "Sex"); 
        empList.ForEach(delegate(Employee e) 
        { 
            Console.WriteLine("{0,2} {1,7}    {2,8}      {3,8}      {4,23}    {5,3}", 
                              e.ID, e.FName, e.MName, e.LName, e.DOB, e.Sex); 
        }); 

        Console.ReadLine(); 

} 

class Employee 
{ 
    public int ID { get; set; } 
    public string FName { get; set; } 
    public string MName { get; set; } 
    public string LName { get; set; } 
    public DateTime DOB { get; set; } 
    public char Sex { get; set; } 
}

List of employees grouped by the first letter of their first name

To display a list of employees group by the first alphabet of their first name, use this query:

// Group People by the First Letter of their FirstName 
var grpOrderedFirstLetter = empList.GroupBy(employees => 
    new String(employees.FName[0], 1)).OrderBy(employees => 
    employees.Key.ToString());; 

foreach (var employee in grpOrderedFirstLetter) 
{ 
    Console.WriteLine("\n'Employees having First Letter {0}':", 
                      employee.Key.ToString()); 
    foreach (var empl in employee) 
    { 
        Console.WriteLine(empl.FName); 
    } 
} 

Console.ReadLine();

List of employees grouped by the year in which they were born

In order to group the employees based on the year in which they were born, use this query:

// Group People by the Year in which they were born            
var grpOrderedYr = empList.GroupBy(employees => 
    employees.DOB.Year).OrderBy(employees => employees.Key); 

foreach (var employee in grpOrderedYr) 
{ 
    Console.WriteLine("\nEmployees Born In the Year " + employee.Key); 
    foreach (var empl in employee) 
    { 
        Console.WriteLine("{0,2} {1,7}", empl.ID, empl.FName); 
               } 
} 
Console.ReadLine();

List of employees grouped by the year and month in which they were born

In order to group the employees based on the year and then the month in which they were born, use this query:

// Group people by the Year and Month in which they were born 
var grpOrderedYrMon = empList.GroupBy(employees => 
    new DateTime(employees.DOB.Year, 
      employees.DOB.Month, 1)).OrderBy(employees => employees.Key); ; 

foreach (var employee in grpOrderedYrMon) 
{ 
    Console.WriteLine("\nEmployees Born in Year {0} - Month {1} is/are :", 
                      employee.Key.Year, employee.Key.Month); 
           foreach (var empl in employee) 
    { 
        Console.WriteLine("{0}: {1}", empl.ID, empl.FName); 
    } 
} 
Console.ReadLine();

Total number of birthdays each year

To get the total of the employees born in the same year, use this query:

// Count people grouped by the Year in which they were born 
var grpCountYrMon = empList.GroupBy(employees => employees.DOB.Year) 
    .Select(lst => new {Year = lst.Key, Count = lst.Count()} ); 

foreach (var employee in grpCountYrMon) 
{ 
    Console.WriteLine("\n{0} were born in {1}", employee.Count, employee.Year);
} 
Console.ReadLine();

Sex ratio

To find the sex ratio in the company, use this query:

// Sex Ratio 
var ratioSex = empList.GroupBy(ra => ra.Sex) 
  .Select( emp => new 
  { 
      Sex = emp.Key, 
      Ratio = (emp.Count() * 100) / empList.Count 
  }); 

foreach (var ratio in ratioSex) 
      { 
    Console.WriteLine("\n{0} are {1}%", ratio.Sex, ratio.Ratio); 
} 
Console.ReadLine();

How to Use LINQ GroupBy