Enumerable.ToLookup<TSource, TKey, TElement> Method (IEnumerable<TSource>, Func<TSource, TKey>, Func<TSource, TElement>)
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Creates a Lookup<TKey, TElement> from an IEnumerable<T> according to specified key selector and element selector functions.
Namespace: System.Linq
Assembly: System.Core (in System.Core.dll)
Syntax
'Declaration
<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)
public static ILookup<TKey, TElement> ToLookup<TSource, TKey, TElement>(
this 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 value returned by elementSelector.
Parameters
- source
Type: System.Collections.Generic.IEnumerable<TSource>
The IEnumerable<T> to create a Lookup<TKey, TElement> from.
- keySelector
Type: System.Func<TSource, TKey>
A function to extract a key from each element.
- elementSelector
Type: System.Func<TSource, TElement>
A transform function to produce a result element value from each element.
Return Value
Type: System.Linq.ILookup<TKey, TElement>
A Lookup<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<TSource>. When you use instance method syntax to call this method, omit the first parameter.
Exceptions
| Exception | Condition |
|---|---|
| ArgumentNullException | source or keySelector or elementSelector is nulla null reference (Nothing in Visual Basic). |
Remarks
The ToLookup<TSource, TKey, TElement>(IEnumerable<TSource>, Func<TSource, TKey>, Func<TSource, TElement>) method returns a Lookup<TKey, TElement>, a one-to-many dictionary that maps keys to collections of values. A Lookup<TKey, TElement> differs from a Dictionary<TKey, TValue>, which performs a one-to-one mapping of keys to single values.
The default equality comparer Default is used to compare keys.
Examples
The following code example demonstrates how to use ToLookup<TSource, TKey, TElement>(IEnumerable<TSource>, Func<TSource, TKey>, Func<TSource, TElement>) to create a Lookup<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.
output.AppendLine(packageGroup.Key)
' Iterate through each value in the IGrouping and print its value.
For Each str As String In packageGroup
output.AppendLine(" " & str)
Next
Next
' 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
output.AppendLine(str)
Next
' Display the output.
outputBlock.Text &= output.ToString() & vbCrLf
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
class Package
{
public string Company { get; set; }
public double Weight { get; set; }
public long TrackingNumber { get; set; }
}
public static void ToLookupEx1()
{
// Create a list of Packages.
List<Package> packages =
new List<Package>
{ new Package { Company = "Coho Vineyard",
Weight = 25.2, TrackingNumber = 89453312L },
new Package { Company = "Lucerne Publishing",
Weight = 18.7, TrackingNumber = 89112755L },
new Package { Company = "Wingtip Toys",
Weight = 6.0, TrackingNumber = 299456122L },
new Package { Company = "Contoso Pharmaceuticals",
Weight = 9.3, TrackingNumber = 670053128L },
new Package { 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.
ILookup<char, string> lookup =
packages
.ToLookup(p => Convert.ToChar(p.Company.Substring(0, 1)),
p => p.Company + " " + p.TrackingNumber);
// Iterate through each IGrouping in the Lookup.
foreach (IGrouping<char, string> packageGroup in lookup)
{
// Print the key value of the IGrouping.
outputBlock.Text += packageGroup.Key + "\n";
// Iterate through each value in the
// IGrouping and print its value.
foreach (string str in packageGroup)
outputBlock.Text += String.Format(" {0}", str) + "\n";
}
}
/*
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
*/
Version Information
Silverlight
Supported in: 5, 4, 3
Silverlight for Windows Phone
Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0
XNA Framework
Supported in: Xbox 360, Windows Phone OS 7.0
Platforms
For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.