Article
11/18/2011

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

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Creates a Dictionary<TKey, TValue> from an IEnumerable<T> according to a specified key selector function.

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

Syntax

'Declaration
<ExtensionAttribute> _
Public Shared Function ToDictionary(Of TSource, TKey) ( _
    source As IEnumerable(Of TSource), _
    keySelector As Func(Of TSource, TKey) _
) As Dictionary(Of TKey, TSource)

public static Dictionary<TKey, TSource> ToDictionary<TSource, TKey>(
    this 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
Type: System.Collections.Generic.IEnumerable<TSource>
An IEnumerable<T> to create a Dictionary<TKey, TValue> from.

keySelector
Type: System.Func<TSource, TKey>
A function to extract a key from each element.

Return Value

Type: System.Collections.Generic.Dictionary<TKey, TSource>
A Dictionary<TKey, TValue> that contains keys and values.

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 is nulla null reference (Nothing in Visual Basic). -or- keySelector produces a key that is nulla null reference (Nothing in Visual Basic).
ArgumentException	keySelector produces duplicate keys for two elements.

ArgumentNullException

source or keySelector is nulla null reference (Nothing in Visual Basic).

-or-

keySelector produces a key that is nulla null reference (Nothing in Visual Basic).

ArgumentException

keySelector produces duplicate keys for two elements.

Remarks

The ToDictionary<TSource, TKey>(IEnumerable<TSource>, Func<TSource, TKey>) method uses the default equality comparer Default to compare keys.

Examples

The following code example demonstrates how to use ToDictionary<TSource, TKey>(IEnumerable<TSource>, Func<TSource, TKey>) to create a Dictionary<TKey, TValue> by using a key selector.

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

   Sub ToDictionaryEx1()
      ' Create a list of Package values.
      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 = "Adventure Works", .Weight = 33.8, .TrackingNumber = 4665518773L}})

      ' Create a Dictionary that contains Package values, 
      ' using TrackingNumber as the key.
      Dim dict As Dictionary(Of Long, Package) = _
          packages.ToDictionary(Function(p) p.TrackingNumber)

      ' Display the results.
      Dim output As New System.Text.StringBuilder
      For Each kvp As KeyValuePair(Of Long, Package) In dict
         output.AppendLine("Key " & kvp.Key & ": " & _
                           kvp.Value.Company & ", " & _
                           kvp.Value.Weight & " pounds")
      Next
      outputBlock.Text &= output.ToString() & vbCrLf
   End Sub

   ' This code produces the following output:
   '
   ' Key 89453312: Coho Vineyard, 25.2 pounds
   ' Key 89112755: Lucerne Publishing, 18.7 pounds
   ' Key 299456122: Wingtip Toys, 6 pounds
   ' Key 4665518773: Adventure Works, 33.8 pounds

      class Package
      {
         public string Company { get; set; }
         public double Weight { get; set; }
         public long TrackingNumber { get; set; }
      }

      public static void ToDictionaryEx1()
      {
         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 = "Adventure Works", Weight = 33.8, TrackingNumber = 4665518773L } };

         // Create a Dictionary of Package objects, 
         // using TrackingNumber as the key.
         Dictionary<long, Package> dictionary =
             packages.ToDictionary(p => p.TrackingNumber);

         foreach (KeyValuePair<long, Package> kvp in dictionary)
         {
            outputBlock.Text += String.Format(
                "Key {0}: {1}, {2} pounds",
                kvp.Key,
                kvp.Value.Company,
                kvp.Value.Weight) + "\n";
         }
      }

      /*
       This code produces the following output:

       Key 89453312: Coho Vineyard, 25.2 pounds
       Key 89112755: Lucerne Publishing, 18.7 pounds
       Key 299456122: Wingtip Toys, 6 pounds
       Key 4665518773: Adventure Works, 33.8 pounds
      */

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.

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