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

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.

   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
      */