Enumerable.GroupBy Method

Definition

Overloads

GroupBy<TSource,TKey,TElement,TResult>(IEnumerable<TSource>, Func<TSource,TKey>, Func<TSource,TElement>, Func<TKey,IEnumerable<TElement>,TResult>)

Groups the elements of a sequence according to a specified key selector function and creates a result value from each group and its key. The elements of each group are projected by using a specified function.

GroupBy<TSource,TKey,TElement,TResult>(IEnumerable<TSource>, Func<TSource,TKey>, Func<TSource,TElement>, Func<TKey,IEnumerable<TElement>,TResult>, IEqualityComparer<TKey>)

Groups the elements of a sequence according to a specified key selector function and creates a result value from each group and its key. Key values are compared by using a specified comparer, and the elements of each group are projected by using a specified function.

GroupBy<TSource,TKey,TElement>(IEnumerable<TSource>, Func<TSource,TKey>, Func<TSource,TElement>)

Groups the elements of a sequence according to a specified key selector function and projects the elements for each group by using a specified function.

GroupBy<TSource,TKey,TElement>(IEnumerable<TSource>, Func<TSource,TKey>, Func<TSource,TElement>, IEqualityComparer<TKey>)

Groups the elements of a sequence according to a key selector function. The keys are compared by using a comparer and each group's elements are projected by using a specified function.

GroupBy<TSource,TKey,TResult>(IEnumerable<TSource>, Func<TSource,TKey>, Func<TKey,IEnumerable<TSource>,TResult>)

Groups the elements of a sequence according to a specified key selector function and creates a result value from each group and its key.

GroupBy<TSource,TKey,TResult>(IEnumerable<TSource>, Func<TSource,TKey>, Func<TKey,IEnumerable<TSource>,TResult>, IEqualityComparer<TKey>)

Groups the elements of a sequence according to a specified key selector function and creates a result value from each group and its key. The keys are compared by using a specified comparer.

GroupBy<TSource,TKey>(IEnumerable<TSource>, Func<TSource,TKey>)

Groups the elements of a sequence according to a specified key selector function.

GroupBy<TSource,TKey>(IEnumerable<TSource>, Func<TSource,TKey>, IEqualityComparer<TKey>)

Groups the elements of a sequence according to a specified key selector function and compares the keys by using a specified comparer.

GroupBy(IEnumerable, Func, Func, Func<>,TResult>)

Groups the elements of a sequence according to a specified key selector function and creates a result value from each group and its key. The elements of each group are projected by using a specified function.

public static System.Collections.Generic.IEnumerable<TResult> GroupBy<TSource,TKey,TElement,TResult> (this System.Collections.Generic.IEnumerable<TSource> source, Func<TSource,TKey> keySelector, Func<TSource,TElement> elementSelector, Func<TKey,System.Collections.Generic.IEnumerable<TElement>,TResult> resultSelector);
Type Parameters
TSource

The type of the elements of source.

TKey

The type of the key returned by keySelector.

TElement

The type of the elements in each IGrouping<TKey,TElement>.

TResult

The type of the result value returned by resultSelector.

Parameters
source
IEnumerable<TSource>

An IEnumerable<T> whose elements to group.

keySelector
Func<TSource,TKey>

A function to extract the key for each element.

elementSelector
Func<TSource,TElement>

A function to map each source element to an element in an IGrouping<TKey,TElement>.

resultSelector
Func<TKey,IEnumerable<TElement>,TResult>

A function to create a result value from each group.

Returns
IEnumerable<TResult>

A collection of elements of type TResult where each element represents a projection over a group and its key.

Examples

The following code example demonstrates how to use GroupBy<TSource,TKey,TElement,TResult>(IEnumerable<TSource>, Func<TSource,TKey>, Func<TSource,TElement>, Func<TKey,IEnumerable<TElement>,TResult>) to group the projected elements of a sequence and then project a sequence of results of type TResult.

class Pet
{
    public string Name { get; set; }
    public double Age { get; set; }
}

public static void GroupByEx4()
{
    // Create a list of pets.
    List<Pet> petsList =
        new List<Pet>{ new Pet { Name="Barley", Age=8.3 },
                       new Pet { Name="Boots", Age=4.9 },
                       new Pet { Name="Whiskers", Age=1.5 },
                       new Pet { Name="Daisy", Age=4.3 } };

    // Group Pet.Age values by the Math.Floor of the age.
    // Then project an anonymous type from each group
    // that consists of the key, the count of the group's
    // elements, and the minimum and maximum age in the group.
    var query = petsList.GroupBy(
        pet => Math.Floor(pet.Age),
        pet => pet.Age,
        (baseAge, ages) => new
        {
            Key = baseAge,
            Count = ages.Count(),
            Min = ages.Min(),
            Max = ages.Max()
        });

    // Iterate over each anonymous type.
    foreach (var result in query)
    {
        Console.WriteLine("
Age group: " + result.Key); Console.WriteLine("Number of pets in this age group: " + result.Count); Console.WriteLine("Minimum age: " + result.Min); Console.WriteLine("Maximum age: " + result.Max); } /* This code produces the following output: Age group: 8 Number of pets in this age group: 1 Minimum age: 8.3 Maximum age: 8.3 Age group: 4 Number of pets in this age group: 2 Minimum age: 4.3 Maximum age: 4.9 Age group: 1 Number of pets in this age group: 1 Minimum age: 1.5 Maximum age: 1.5 */ }
Structure Pet
    Public Name As String
    Public Age As Double
End Structure

Public Sub GroupByEx4()
    ' Create a list of pets.
    Dim petsList As New List(Of Pet)(New Pet() _
                         {New Pet With {.Name = "Barley", .Age = 8.3},
                          New Pet With {.Name = "Boots", .Age = 4.9},
                          New Pet With {.Name = "Whiskers", .Age = 1.5},
                          New Pet With {.Name = "Daisy", .Age = 4.3}})

    ' Group Pet.Age values by the Math.Floor of the age.
    ' Then project an anonymous type from each group
    ' that consists of the key, the count of the group's
    ' elements, and the minimum and maximum age in the group.
    Dim query = petsList.GroupBy(
Function(pet) Math.Floor(pet.Age),
Function(pet) pet.Age,
Function(baseAge, ages) New With
    {.Key = baseAge,
    .Count = ages.Count(),
    .Min = ages.Min(),
    .Max = ages.Max()}
)

    Dim output As New System.Text.StringBuilder
    ' Iterate over each anonymous type.
    For Each result In query
        output.AppendLine(vbCrLf & "Age group: " & result.Key)
        output.AppendLine("Number of pets in this age group: " & result.Count)
        output.AppendLine("Minimum age: " & result.Min)
        output.AppendLine("Maximum age: " & result.Max)
    Next

    ' Display the output.
    MsgBox(output.ToString)
End Sub

' This code produces the following output:

' Age group: 8
' Number of pets in this age group: 1
' Minimum age: 8.3
' Maximum age: 8.3
'
' Age group: 4
' Number of pets in this age group: 2
' Minimum age: 4.3
' Maximum age: 4.9
'
' Age group: 1
' Number of pets in this age group: 1
' Minimum age: 1.5
' Maximum age: 1.5

Remarks

In query expression syntax, a group by (Visual C#) or Group By Into (Visual Basic) clause translates to an invocation of GroupBy.

GroupBy(IEnumerable, Func, Func, Func<>,TResult>, IEqualityComparer)

Groups the elements of a sequence according to a specified key selector function and creates a result value from each group and its key. Key values are compared by using a specified comparer, and the elements of each group are projected by using a specified function.

public static System.Collections.Generic.IEnumerable<TResult> GroupBy<TSource,TKey,TElement,TResult> (this System.Collections.Generic.IEnumerable<TSource> source, Func<TSource,TKey> keySelector, Func<TSource,TElement> elementSelector, Func<TKey,System.Collections.Generic.IEnumerable<TElement>,TResult> resultSelector, System.Collections.Generic.IEqualityComparer<TKey> comparer);
Type Parameters
TSource

The type of the elements of source.

TKey

The type of the key returned by keySelector.

TElement

The type of the elements in each IGrouping<TKey,TElement>.

TResult

The type of the result value returned by resultSelector.

Parameters
source
IEnumerable<TSource>

An IEnumerable<T> whose elements to group.

keySelector
Func<TSource,TKey>

A function to extract the key for each element.

elementSelector
Func<TSource,TElement>

A function to map each source element to an element in an IGrouping<TKey,TElement>.

resultSelector
Func<TKey,IEnumerable<TElement>,TResult>

A function to create a result value from each group.

comparer
IEqualityComparer<TKey>

An IEqualityComparer<T> to compare keys with.

Returns
IEnumerable<TResult>

A collection of elements of type TResult where each element represents a projection over a group and its key.

GroupBy(IEnumerable, Func, Func)

Groups the elements of a sequence according to a specified key selector function and projects the elements for each group by using a specified function.

public static System.Collections.Generic.IEnumerable<System.Linq.IGrouping<TKey,TElement>> GroupBy<TSource,TKey,TElement> (this System.Collections.Generic.IEnumerable<TSource> source, Func<TSource,TKey> keySelector, Func<TSource,TElement> elementSelector);
Type Parameters
TSource

The type of the elements of source.

TKey

The type of the key returned by keySelector.

TElement

The type of the elements in the IGrouping<TKey,TElement>.

Parameters
source
IEnumerable<TSource>

An IEnumerable<T> whose elements to group.

keySelector
Func<TSource,TKey>

A function to extract the key for each element.

elementSelector
Func<TSource,TElement>

A function to map each source element to an element in the IGrouping<TKey,TElement>.

Returns
IEnumerable<IGrouping<TKey,TElement>>

An IEnumerable<IGrouping<TKey, TElement>> in C# or IEnumerable(Of IGrouping(Of TKey, TElement)) in Visual Basic where each IGrouping<TKey,TElement> object contains a collection of objects of type TElement and a key.

Exceptions

source or keySelector or elementSelector is null.

Examples

The following code example demonstrates how to use GroupBy<TSource,TKey,TElement>(IEnumerable<TSource>, Func<TSource,TKey>, Func<TSource,TElement>) to group the elements of a sequence.

class Pet
{
    public string Name { get; set; }
    public int Age { get; set; }
}

// Uses method-based query syntax.
public static void GroupByEx1()
{
    // Create a list of pets.
    List<Pet> pets =
        new List<Pet>{ new Pet { Name="Barley", Age=8 },
                       new Pet { Name="Boots", Age=4 },
                       new Pet { Name="Whiskers", Age=1 },
                       new Pet { Name="Daisy", Age=4 } };

    // Group the pets using Age as the key value 
    // and selecting only the pet's Name for each value.
    IEnumerable<IGrouping<int, string>> query =
        pets.GroupBy(pet => pet.Age, pet => pet.Name);

    // Iterate over each IGrouping in the collection.
    foreach (IGrouping<int, string> petGroup in query)
    {
        // Print the key value of the IGrouping.
        Console.WriteLine(petGroup.Key);
        // Iterate over each value in the 
        // IGrouping and print the value.
        foreach (string name in petGroup)
            Console.WriteLine("  {0}", name);
    }
}

/*
 This code produces the following output:

 8
   Barley
 4
   Boots
   Daisy
 1
   Whiskers
*/

Structure Pet
    Public Name As String
    Public Age As Integer
End Structure

Sub GroupByEx1()
    'Create a list of Pet objects.
    Dim pets As New List(Of Pet)(New Pet() _
                         {New Pet With {.Name = "Barley", .Age = 8},
                          New Pet With {.Name = "Boots", .Age = 4},
                          New Pet With {.Name = "Whiskers", .Age = 1},
                          New Pet With {.Name = "Daisy", .Age = 4}})

    ' Group the pets using Age as the key 
    ' and selecting only the pet's Name for each value.
    Dim query As IEnumerable(Of IGrouping(Of Integer, String)) =
pets.GroupBy(Function(pet) pet.Age,
             Function(pet) pet.Name)

    Dim output As New System.Text.StringBuilder
    ' Iterate over each IGrouping in the collection.
    For Each petGroup As IGrouping(Of Integer, String) In query
        ' Print the key value of the IGrouping.
        output.AppendLine(petGroup.Key)
        ' Iterate over each value in the IGrouping and print the value.
        For Each name As String In petGroup
            output.AppendLine("  " & name)
        Next
    Next

    ' Display the output.
    MsgBox(output.ToString)
End Sub

' This code produces the following output:
'
' 8
'   Barley
' 4
'   Boots
'   Daisy
' 1
'   Whiskers

In query expression syntax, a group by (Visual C#) or Group By Into (Visual Basic) clause translates to an invocation of GroupBy. The translation of the query expression in the following example is equivalent to the query in the example above.

IEnumerable<IGrouping<int, string>> query =
    from pet in pets
    group pet.Name by pet.Age;
    Dim query =
From pet In pets
Group pet.Name By Age = pet.Age Into ageGroup = Group
Note

In a Visual C# or Visual Basic query expression, the element and key selection expressions occur in the reverse order from their argument positions in a call to the GroupBy<TSource,TKey,TElement>(IEnumerable<TSource>, Func<TSource,TKey>, Func<TSource,TElement>) method.

Remarks

This method is implemented by using deferred execution. The immediate return value is an object that stores all the information that is required to perform the action. The query represented by this method is not executed until the object is enumerated either by calling its GetEnumerator method directly or by using foreach in Visual C# or For Each in Visual Basic.

The GroupBy<TSource,TKey,TElement>(IEnumerable<TSource>, Func<TSource,TKey>, Func<TSource,TElement>) method returns a collection of IGrouping<TKey,TElement> objects, one for each distinct key that was encountered. An IGrouping<TKey,TElement> is an IEnumerable<T> that also has a key associated with its elements.

The IGrouping<TKey,TElement> objects are yielded in an order based on the order of the elements in source that produced the first key of each IGrouping<TKey,TElement>. Elements in a grouping are yielded in the order that the elements that produced them appear in source.

The default equality comparer Default is used to compare keys.

GroupBy(IEnumerable, Func, Func, IEqualityComparer)

Groups the elements of a sequence according to a key selector function. The keys are compared by using a comparer and each group's elements are projected by using a specified function.

public static System.Collections.Generic.IEnumerable<System.Linq.IGrouping<TKey,TElement>> GroupBy<TSource,TKey,TElement> (this System.Collections.Generic.IEnumerable<TSource> source, Func<TSource,TKey> keySelector, Func<TSource,TElement> elementSelector, System.Collections.Generic.IEqualityComparer<TKey> comparer);
Type Parameters
TSource

The type of the elements of source.

TKey

The type of the key returned by keySelector.

TElement

The type of the elements in the IGrouping<TKey,TElement>.

Parameters
source
IEnumerable<TSource>

An IEnumerable<T> whose elements to group.

keySelector
Func<TSource,TKey>

A function to extract the key for each element.

elementSelector
Func<TSource,TElement>

A function to map each source element to an element in an IGrouping<TKey,TElement>.

comparer
IEqualityComparer<TKey>

An IEqualityComparer<T> to compare keys.

Returns
IEnumerable<IGrouping<TKey,TElement>>

An IEnumerable<IGrouping<TKey, TElement>> in C# or IEnumerable(Of IGrouping(Of TKey, TElement)) in Visual Basic where each IGrouping<TKey,TElement> object contains a collection of objects of type TElement and a key.

Exceptions

source or keySelector or elementSelector is null.

Remarks

This method is implemented by using deferred execution. The immediate return value is an object that stores all the information that is required to perform the action. The query represented by this method is not executed until the object is enumerated either by calling its GetEnumerator method directly or by using foreach in Visual C# or For Each in Visual Basic.

The GroupBy<TSource,TKey,TElement>(IEnumerable<TSource>, Func<TSource,TKey>, Func<TSource,TElement>, IEqualityComparer<TKey>) method returns a collection of IGrouping<TKey,TElement> objects, one for each distinct key that was encountered. An IGrouping<TKey,TElement> is an IEnumerable<T> that also has a key associated with its elements.

The IGrouping<TKey,TElement> objects are yielded in an order based on the order of the elements in source that produced the first key of each IGrouping<TKey,TElement>. Elements in a grouping are yielded in the order that the elements that produced them appear in source.

If comparer is null, the default equality comparer Default is used to compare keys.

If two keys are considered equal according to comparer, the first key is chosen as the key for that grouping.

In query expression syntax, a group by (Visual C#) or Group By Into (Visual Basic) clause translates to an invocation of GroupBy. For more information and usage examples, see group clause and Group By Clause.

GroupBy(IEnumerable, Func, Func<>,TResult>)

Groups the elements of a sequence according to a specified key selector function and creates a result value from each group and its key.

public static System.Collections.Generic.IEnumerable<TResult> GroupBy<TSource,TKey,TResult> (this System.Collections.Generic.IEnumerable<TSource> source, Func<TSource,TKey> keySelector, Func<TKey,System.Collections.Generic.IEnumerable<TSource>,TResult> resultSelector);
Type Parameters
TSource

The type of the elements of source.

TKey

The type of the key returned by keySelector.

TResult

The type of the result value returned by resultSelector.

Parameters
source
IEnumerable<TSource>

An IEnumerable<T> whose elements to group.

keySelector
Func<TSource,TKey>

A function to extract the key for each element.

resultSelector
Func<TKey,IEnumerable<TSource>,TResult>

A function to create a result value from each group.

Returns
IEnumerable<TResult>

A collection of elements of type TResult where each element represents a projection over a group and its key.

Examples

The following code example demonstrates how to use GroupBy<TSource,TKey,TResult>(IEnumerable<TSource>, Func<TSource,TKey>, Func<TKey,IEnumerable<TSource>,TResult>) to group the elements of a sequence and project a sequence of results of type TResult.

class Pet
{
    public string Name { get; set; }
    public double Age { get; set; }
}

public static void GroupByEx3()
{
    // Create a list of pets.
    List<Pet> petsList =
        new List<Pet>{ new Pet { Name="Barley", Age=8.3 },
                       new Pet { Name="Boots", Age=4.9 },
                       new Pet { Name="Whiskers", Age=1.5 },
                       new Pet { Name="Daisy", Age=4.3 } };

    // Group Pet objects by the Math.Floor of their age.
    // Then project an anonymous type from each group
    // that consists of the key, the count of the group's
    // elements, and the minimum and maximum age in the group.
    var query = petsList.GroupBy(
        pet => Math.Floor(pet.Age),
        (age, pets) => new
        {
            Key = age,
            Count = pets.Count(),
            Min = pets.Min(pet => pet.Age),
            Max = pets.Max(pet => pet.Age)
        });

    // Iterate over each anonymous type.
    foreach (var result in query)
    {
        Console.WriteLine("
Age group: " + result.Key); Console.WriteLine("Number of pets in this age group: " + result.Count); Console.WriteLine("Minimum age: " + result.Min); Console.WriteLine("Maximum age: " + result.Max); } /* This code produces the following output: Age group: 8 Number of pets in this age group: 1 Minimum age: 8.3 Maximum age: 8.3 Age group: 4 Number of pets in this age group: 2 Minimum age: 4.3 Maximum age: 4.9 Age group: 1 Number of pets in this age group: 1 Minimum age: 1.5 Maximum age: 1.5 */ }
Structure Pet
    Public Name As String
    Public Age As Double
End Structure

Public Sub GroupByEx3()
    ' Create a list of pets.
    Dim petsList As New List(Of Pet)(New Pet() _
                         {New Pet With {.Name = "Barley", .Age = 8.3},
                          New Pet With {.Name = "Boots", .Age = 4.9},
                          New Pet With {.Name = "Whiskers", .Age = 1.5},
                          New Pet With {.Name = "Daisy", .Age = 4.3}})

    ' Group Pet objects by the Math.Floor of their age.
    ' Then project an anonymous type from each group
    ' that consists of the key, the count of the group's
    ' elements, and the minimum and maximum age in the group.
    Dim query = petsList.GroupBy(
Function(pet) Math.Floor(pet.Age),
Function(age, pets) New With
    {.Key = age,
    .Count = pets.Count(),
    .Min = pets.Min(Function(pet) pet.Age),
    .Max = pets.Max(Function(Pet) Pet.Age)}
)

    Dim output As New System.Text.StringBuilder
    ' Iterate over each anonymous type.
    For Each result In query
        output.AppendLine(vbCrLf & "Age group: " & result.Key)
        output.AppendLine("Number of pets in this age group: " & result.Count)
        output.AppendLine("Minimum age: " & result.Min)
        output.AppendLine("Maximum age: " & result.Max)
    Next

    ' Display the output.
    MsgBox(output.ToString)
End Sub

' This code produces the following output:

' Age group: 8
' Number of pets in this age group: 1
' Minimum age: 8.3
' Maximum age: 8.3
'
' Age group: 4
' Number of pets in this age group: 2
' Minimum age: 4.3
' Maximum age: 4.9
'
' Age group: 1
' Number of pets in this age group: 1
' Minimum age: 1.5
' Maximum age: 1.5

Remarks

In query expression syntax, a group by (Visual C#) or Group By Into (Visual Basic) clause translates to an invocation of GroupBy.

GroupBy(IEnumerable, Func, Func<>,TResult>, IEqualityComparer)

Groups the elements of a sequence according to a specified key selector function and creates a result value from each group and its key. The keys are compared by using a specified comparer.

public static System.Collections.Generic.IEnumerable<TResult> GroupBy<TSource,TKey,TResult> (this System.Collections.Generic.IEnumerable<TSource> source, Func<TSource,TKey> keySelector, Func<TKey,System.Collections.Generic.IEnumerable<TSource>,TResult> resultSelector, System.Collections.Generic.IEqualityComparer<TKey> comparer);
Type Parameters
TSource

The type of the elements of source.

TKey

The type of the key returned by keySelector.

TResult

The type of the result value returned by resultSelector.

Parameters
source
IEnumerable<TSource>

An IEnumerable<T> whose elements to group.

keySelector
Func<TSource,TKey>

A function to extract the key for each element.

resultSelector
Func<TKey,IEnumerable<TSource>,TResult>

A function to create a result value from each group.

comparer
IEqualityComparer<TKey>

An IEqualityComparer<T> to compare keys with.

Returns
IEnumerable<TResult>

A collection of elements of type TResult where each element represents a projection over a group and its key.

GroupBy(IEnumerable, Func)

Groups the elements of a sequence according to a specified key selector function.

public static System.Collections.Generic.IEnumerable<System.Linq.IGrouping<TKey,TSource>> GroupBy<TSource,TKey> (this System.Collections.Generic.IEnumerable<TSource> source, Func<TSource,TKey> keySelector);
Type Parameters
TSource

The type of the elements of source.

TKey

The type of the key returned by keySelector.

Parameters
source
IEnumerable<TSource>

An IEnumerable<T> whose elements to group.

keySelector
Func<TSource,TKey>

A function to extract the key for each element.

Returns
IEnumerable<IGrouping<TKey,TSource>>

An IEnumerable<IGrouping<TKey, TSource>> in C# or IEnumerable(Of IGrouping(Of TKey, TSource)) in Visual Basic where each IGrouping<TKey,TElement> object contains a sequence of objects and a key.

Exceptions

source or keySelector is null.

Remarks

This method is implemented by using deferred execution. The immediate return value is an object that stores all the information that is required to perform the action. The query represented by this method is not executed until the object is enumerated either by calling its GetEnumerator method directly or by using foreach in Visual C# or For Each in Visual Basic.

The GroupBy<TSource,TKey>(IEnumerable<TSource>, Func<TSource,TKey>) method returns a collection of IGrouping<TKey,TElement> objects, one for each distinct key that was encountered. An IGrouping<TKey,TElement> is an IEnumerable<T> that also has a key associated with its elements.

The IGrouping<TKey,TElement> objects are yielded in an order based on the order of the elements in source that produced the first key of each IGrouping<TKey,TElement>. Elements in a grouping are yielded in the order they appear in source.

The default equality comparer Default is used to compare keys.

In query expression syntax, a group by (Visual C#) or Group By Into (Visual Basic) clause translates to an invocation of GroupBy. For more information and usage examples, see group clause and Group By Clause.

GroupBy(IEnumerable, Func, IEqualityComparer)

Groups the elements of a sequence according to a specified key selector function and compares the keys by using a specified comparer.

public static System.Collections.Generic.IEnumerable<System.Linq.IGrouping<TKey,TSource>> GroupBy<TSource,TKey> (this System.Collections.Generic.IEnumerable<TSource> source, Func<TSource,TKey> keySelector, System.Collections.Generic.IEqualityComparer<TKey> comparer);
Type Parameters
TSource

The type of the elements of source.

TKey

The type of the key returned by keySelector.

Parameters
source
IEnumerable<TSource>

An IEnumerable<T> whose elements to group.

keySelector
Func<TSource,TKey>

A function to extract the key for each element.

comparer
IEqualityComparer<TKey>

An IEqualityComparer<T> to compare keys.

Returns
IEnumerable<IGrouping<TKey,TSource>>

An IEnumerable<IGrouping<TKey, TSource>> in C# or IEnumerable(Of IGrouping(Of TKey, TSource)) in Visual Basic where each IGrouping<TKey,TElement> object contains a collection of objects and a key.

Exceptions

source or keySelector is null.

Remarks

This method is implemented by using deferred execution. The immediate return value is an object that stores all the information that is required to perform the action. The query represented by this method is not executed until the object is enumerated either by calling its GetEnumerator method directly or by using foreach in Visual C# or For Each in Visual Basic.

The GroupBy<TSource,TKey>(IEnumerable<TSource>, Func<TSource,TKey>, IEqualityComparer<TKey>) method returns a collection of IGrouping<TKey,TElement> objects, one for each distinct key that was encountered. An IGrouping<TKey,TElement> is an IEnumerable<T> that also has a key associated with its elements.

The IGrouping<TKey,TElement> objects are yielded in an order based on the order of the elements in source that produced the first key of each IGrouping<TKey,TElement>. Elements in a grouping are yielded in the order they appear in source.

If comparer is null, the default equality comparer Default is used to compare keys.

If two keys are considered equal according to comparer, the first key is chosen as the key for that grouping.

In query expression syntax, a group by (Visual C#) or Group By Into (Visual Basic) clause translates to an invocation of GroupBy. For more information and usage examples, see group clause and Group By Clause.