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

Microsoft Silverlight will reach end of support after October 2021. Learn more.

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

  • 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.