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

Creates a Lookup<TKey, TElement> from an IEnumerable<T> according to specified key selector and element selector functions.

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

public static ILookup<TKey, TElement> ToLookup<TSource, TKey, TElement>(
	this IEnumerable<TSource> source,
	Func<TSource, TKey> keySelector,
	Func<TSource, TElement> elementSelector

Type Parameters


The type of the elements of source.


The type of the key returned by keySelector.


The type of the value returned by elementSelector.


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

The IEnumerable<T> to create a Lookup<TKey, TElement> from.

Type: System.Func<TSource, TKey>

A function to extract a key from each element.

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.

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).


source or keySelector or elementSelector is null.

The ToLookup<TSource, TKey, TElement>(IEnumerable<TSource>, Func<TSource, TKey>, Func<TSource, TElement>) method returns a Lookup<TKey, TElement>, a one-to-many dictionary that maps keys to collections of values. A Lookup<TKey, TElement> differs from a Dictionary<TKey, TValue>, which performs a one-to-one mapping of keys to single values.

The default equality comparer Default is used to compare keys.

The following code example demonstrates how to use ToLookup<TSource, TKey, TElement>(IEnumerable<TSource>, Func<TSource, TKey>, Func<TSource, TElement>) to create a Lookup<TKey, TElement> by using a key selector function and an element selector function.

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 =
        .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.
        // Iterate through each value in the  
        // IGrouping and print its value. 
        foreach (string str in packageGroup)
            Console.WriteLine("    {0}", str);

 This code produces the following output:

     Coho Vineyard 89453312
     Contoso Pharmaceuticals 670053128
     Lucerne Publishing 89112755
     Wingtip Toys 299456122
     Wide World Importers 4665518773

Windows 7, Windows Vista, Windows XP SP2, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

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

.NET Framework

Supported in: 3.5

.NET Compact Framework

Supported in: 3.5

XNA Framework

Supported in: 3.0