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.NET Framework Class Library

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

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

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. For more information, see Extension Methods (Visual Basic) or Extension Methods (C# Programming Guide).
Exception Condition
ArgumentNullException

source or keySelector is null.

-or-

keySelector produces a key that is null.

ArgumentException

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")
        Next
        MsgBox(output.ToString())
    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

.NET Framework

Supported in: 4.5.2, 4.5.1, 4.5, 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone 8, Silverlight 8.1

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.