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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)

Public Shared Function ToDictionary(Of TSource, TKey) (
	source As IEnumerable(Of TSource),
	keySelector As Func(Of TSource, TKey)
) As Dictionary(Of TKey, TSource)


Type: System.Collections.Generic.IEnumerable<TSource>

An IEnumerable<T> to create a Dictionary<TKey, TValue> from.

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.

Type Parameters


The type of the elements of source.


The type of the key returned by keySelector.

Exception Condition

source or keySelector is null.


keySelector produces a key that is null.


keySelector produces duplicate keys for two elements.

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

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")
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

Universal Windows Platform
Available since 8
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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