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.


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.
            outputBlock.Text += packageGroup.Key + "\n";
            // Iterate through each value in the 
            // IGrouping and print its value.
            foreach (string str in packageGroup)
               outputBlock.Text += String.Format("    {0}", str) + "\n";

       This code produces the following output:

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


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

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

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