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Enumerable.ToLookup(Of TSource, TKey, TElement) Method (IEnumerable(Of TSource), Func(Of TSource, TKey), Func(Of TSource, TElement))

Creates a Lookup(Of TKey, TElement) from an IEnumerable(Of T) according to specified key selector and element selector functions.

Namespace:  System.Linq
Assembly:  System.Core (in System.Core.dll)

<ExtensionAttribute> _
Public Shared Function ToLookup(Of TSource, TKey, TElement) ( _
	source As IEnumerable(Of TSource), _
	keySelector As Func(Of TSource, TKey), _
	elementSelector As Func(Of TSource, TElement) _
) As ILookup(Of TKey, TElement)
Dim source As IEnumerable(Of TSource)
Dim keySelector As Func(Of TSource, TKey)
Dim elementSelector As Func(Of TSource, TElement)
Dim returnValue As ILookup(Of TKey, TElement)

returnValue = source.ToLookup(keySelector, _

Type Parameters


The type of the elements of source.


The type of the key returned by keySelector.


The type of the value returned by elementSelector.


Type: System.Collections.Generic.IEnumerable(Of TSource)

The IEnumerable(Of T) to create a Lookup(Of TKey, TElement) from.

Type: System.Func(Of TSource, TKey)

A function to extract a key from each element.

Type: System.Func(Of TSource, TElement)

A transform function to produce a result element value from each element.

Return Value

Type: System.Linq.ILookup(Of TKey, TElement)
A Lookup(Of TKey, TElement) that contains values of type TElement selected from the input sequence.

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type IEnumerable(Of TSource). When you use instance method syntax to call this method, omit the first parameter. For more information, see Extension Methods (Visual Basic) or Extension Methods (C# Programming Guide).


source or keySelector or elementSelector is Nothing.

The ToLookup(Of TSource, TKey, TElement)(IEnumerable(Of TSource), Func(Of TSource, TKey), Func(Of TSource, TElement)) method returns a Lookup(Of TKey, TElement), a one-to-many dictionary that maps keys to collections of values. A Lookup(Of TKey, TElement) differs from a Dictionary(Of TKey, TValue), which performs a one-to-one mapping of keys to single values.

The default equality comparer Default is used to compare keys.

The following code example demonstrates how to use ToLookup(Of TSource, TKey, TElement)(IEnumerable(Of TSource), Func(Of TSource, TKey), Func(Of TSource, TElement)) to create a Lookup(Of TKey, TElement) by using a key selector function and an element selector function.

Structure Package
    Public Company As String 
    Public Weight As Double 
    Public TrackingNumber As Long
End Structure 

Sub ToLookupEx1()
    ' Create a list of Packages. 
    Dim packages As New List(Of Package)(New Package() _
         {New Package With _
          {.Company = "Coho Vineyard", .Weight = 25.2, .TrackingNumber = 89453312L}, _
          New Package With _
          {.Company = "Lucerne Publishing", .Weight = 18.7, .TrackingNumber = 89112755L}, _
          New Package With _
          {.Company = "Wingtip Toys", .Weight = 6.0, .TrackingNumber = 299456122L}, _
          New Package With _
          {.Company = "Contoso Pharmaceuticals", .Weight = 9.3, .TrackingNumber = 670053128L}, _
          New Package With _
          {.Company = "Wide World Importers", .Weight = 33.8, .TrackingNumber = 4665518773L}})

    ' Create a Lookup to organize the packages.  
    ' Use the first character of Company as the key value. 
    ' Select Company appended to TrackingNumber  
    ' as the element values of the Lookup. 
    Dim lookup As ILookup(Of Char, String) = _
        packages.ToLookup(Function(p) _
                              Convert.ToChar(p.Company.Substring(0, 1)), _
                          Function(p) _
                              p.Company & " " & p.TrackingNumber)

    Dim output As New System.Text.StringBuilder

    ' Iterate through each IGrouping in the Lookup. 
    For Each packageGroup As IGrouping(Of Char, String) In lookup
        ' Print the key value of the IGrouping.
        ' Iterate through each value in the IGrouping and print its value. 
        For Each str As String In packageGroup
            output.AppendLine("   " & str)

    ' Select a group of packages by indexing directly into the Lookup. 
    Dim cgroup As IEnumerable(Of String) = lookup("C"c)

    output.AppendLine(vbCrLf & "Packages from Company names that start with 'C':")
    For Each str As String In cgroup

    ' Display the output.
End Sub 

' This code produces the following output: 

' C 
'    Coho Vineyard 89453312 
'    Contoso Pharmaceuticals 670053128 
' L 
'    Lucerne Publishing 89112755 
' W 
'    Wingtip Toys 299456122 
'    Wide World Importers 4665518773 

' Packages from Company names that start with 'C': 
' Coho Vineyard 89453312 
' Contoso Pharmaceuticals 670053128

Windows 7, Windows Vista, Windows XP SP2, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5

.NET Compact Framework

Supported in: 3.5

XNA Framework

Supported in: 3.0