Grouping Operators: GroupBy & ToLookup

The grouping operators do the same thing as the GroupBy clause of SQL query. The grouping operators create a group of elements based on the given key. This group is contained in a special type of collection that implements an IGrouping<TKey,TSource> interface where TKey is a key value, on which the group has been formed and TSource is the collection of elements that matches with the grouping key value.

Grouping Operators	Description
GroupBy	The GroupBy operator returns groups of elements based on some key value. Each group is represented by IGrouping<TKey, TElement> object.
ToLookup	ToLookup is the same as GroupBy; the only difference is the execution of GroupBy is deferred whereas ToLookup execution is immediate.

GroupBy

The GroupBy operator returns a group of elements from the given collection based on some key value. Each group is represented by IGrouping<TKey, TElement> object. Also, the GroupBy method has eight overload methods, so you can use appropriate extension method based on your requirement in method syntax.

Note:

A LINQ query can end with a GroupBy or Select clause.

The result of GroupBy operators is a collection of groups. For example, GroupBy returns IEnumerable<IGrouping<TKey,Student>> from the Student collection:

GroupBy in Query Syntax

The following example creates a groups of students who have same age. Students of the same age will be in the same collection and each grouped collection will have a key and inner collection, where the key will be the age and the inner collection will include students whose age is matched with a key.

Example: GroupBy in Query syntax C#
IList<Student> studentList = new List<Student>() { 
        new Student() { StudentID = 1, StudentName = "John", Age = 18 } ,
        new Student() { StudentID = 2, StudentName = "Steve",  Age = 21 } ,
        new Student() { StudentID = 3, StudentName = "Bill",  Age = 18 } ,
        new Student() { StudentID = 4, StudentName = "Ram" , Age = 20 } ,
        new Student() { StudentID = 5, StudentName = "Abram" , Age = 21 } 
    };

var groupedResult = from s in studentList
                    group s by s.Age;

//iterate each group        
foreach (var ageGroup in groupedResult)
{
    Console.WriteLine("Age Group: {0}", ageGroup .Key); //Each group has a key 
             
    foreach(Student s in ageGroup) // Each group has inner collection
        Console.WriteLine("Student Name: {0}", s.StudentName);
}
Try it 

Output:

AgeGroup: 18 StudentName: John StudentName: Bill AgeGroup: 21 StudentName: Steve StudentName: Abram AgeGroup: 20 StudentName: Ram

As you can see in the above example, you can iterate the group using a 'foreach' loop, where each group contains a key and inner collection. The following figure shows the result in debug view.

Grouped collection with key and inner collection

Use "Into Group" with the 'Group By' clause in VB.Net as shown below.

Example: GroupBy clause in VB.Net
Dim groupQuery = From s In studentList
                 Group By s.Age Into Group

For Each group In groupQuery
    Console.WriteLine("Age Group: {0}", group.Age) // Each group has key property name
    
    For Each student In group.Group // Each group has inner collection
        Console.WriteLine("Student Name: {0}", student.StudentName)
    Next
Next
Try it 

Notice that each group will have a property name on which group is performed. In the above example, we have used Age to form a group so each group will have "Age" property name instead of "Key" as a property name.

Output:

AgeGroup: 18 StudentName: John StudentName: Bill AgeGroup: 21 StudentName: Steve StudentName: Abram AgeGroup: 20 StudentName: Ram

GroupBy in Method Syntax

The GroupBy() extension method works the same way in the method syntax. Specify the lambda expression for key selector field name in GroupBy extension method.

Example: GroupBy in method syntax C#
IList<Student> studentList = new List<Student>() { 
        new Student() { StudentID = 1, StudentName = "John", Age = 18 } ,
        new Student() { StudentID = 2, StudentName = "Steve",  Age = 21 } ,
        new Student() { StudentID = 3, StudentName = "Bill",  Age = 18 } ,
        new Student() { StudentID = 4, StudentName = "Ram" , Age = 20 } ,
        new Student() { StudentID = 5, StudentName = "Abram" , Age = 21 } 
    };

var groupedResult = studentList.GroupBy(s => s.Age);

foreach (var ageGroup in groupedResult)
{
    Console.WriteLine("Age Group: {0}", ageGroup.Key);  //Each group has a key 
             
    foreach(Student s in ageGroup)  //Each group has a inner collection  
        Console.WriteLine("Student Name: {0}", s.StudentName);
}
Try it 

Example: GroupBy in method syntax VB.Net 
Dim groupQuery = studentList.GroupBy(Function(s) s.Age)
        
For Each ageGroup In groupQuery
            
    Console.WriteLine("Age Group: {0}", ageGroup.Key)  //Each group has a key 
            
    For Each student In ageGroup.AsEnumerable()  //Each group has a inner collection
        Console.WriteLine("Student Name: {0}", student.StudentName)
    Next
Next
        
Try it 

Output:

AgeGroup: 18 StudentName: John StudentName: Bill AgeGroup: 21 StudentName: Steve StudentName: Abram AgeGroup: 20 StudentName: Ram

ToLookup

ToLookup is the same as GroupBy; the only difference is GroupBy execution is deferred, whereas ToLookup execution is immediate. Also, ToLookup is only applicable in Method syntax. ToLookup is not supported in the query syntax.

Example: ToLookup in method syntax C#
IList<Student> studentList = new List<Student>() { 
        new Student() { StudentID = 1, StudentName = "John", Age = 18 } ,
        new Student() { StudentID = 2, StudentName = "Steve",  Age = 21 } ,
        new Student() { StudentID = 3, StudentName = "Bill",  Age = 18 } ,
        new Student() { StudentID = 4, StudentName = "Ram" , Age = 20 } ,
        new Student() { StudentID = 5, StudentName = "Abram" , Age = 21 } 
    };

var lookupResult = studentList.ToLookup(s => s.age);

foreach (var group in lookupResult)
{
    Console.WriteLine("Age Group: {0}", group.Key);  //Each group has a key 
             
    foreach(Student s in group)  //Each group has a inner collection  
        Console.WriteLine("Student Name: {0}", s.StudentName);
}
        
        
Try it 

Example: ToLookup in method syntax VB.Net
Dim loopupResult = studentList.ToLookup(Function(s) s.Age)

Points to Remember :

GroupBy & ToLookup return a collection that has a key and an inner collection based on a key field value.
The execution of GroupBy is deferred whereas that of ToLookup is immediate.
A LINQ query syntax can be end with the GroupBy or Select clause.