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.
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 )
Parameters
- source
-
Type:
System.Collections.Generic.IEnumerable<TSource>
The IEnumerable<T> to create a Lookup<TKey, TElement> from.
- keySelector
-
Type:
System.Func<TSource, TKey>
A function to extract a key from each element.
- elementSelector
-
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.
Type Parameters
- TSource
The type of the elements of source.
- TKey
The type of the key returned by keySelector.
- TElement
The type of the value returned by elementSelector.
| Exception | Condition |
|---|---|
| ArgumentNullException | 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 = packages .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. Console.WriteLine(packageGroup.Key); // 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: C Coho Vineyard 89453312 Contoso Pharmaceuticals 670053128 L Lucerne Publishing 89112755 W Wingtip Toys 299456122 Wide World Importers 4665518773 */
Available since 8
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1