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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
Assemblies:   System.Linq (in System.Linq.dll)
  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: 4.6, 4.5, 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

XNA Framework

Supported in: 3.0

Portable Class Library

Supported in: Portable Class Library

Supported in: Windows Phone 8.1

Supported in: Windows Phone Silverlight 8.1

Supported in: Windows Phone Silverlight 8