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.
Assembly: System.Core (in System.Core.dll)
public static Dictionary<TKey, TSource> ToDictionary<TSource, TKey>( this IEnumerable<TSource> source, Func<TSource, TKey> keySelector )
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.
class Package { public string Company { get; set; } public double Weight { get; set; } public long TrackingNumber { get; set; } } public static void ToDictionaryEx1() { 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 = "Adventure Works", Weight = 33.8, TrackingNumber = 4665518773L } }; // Create a Dictionary of Package objects, // using TrackingNumber as the key. Dictionary<long, Package> dictionary = packages.ToDictionary(p => p.TrackingNumber); foreach (KeyValuePair<long, Package> kvp in dictionary) { Console.WriteLine( "Key {0}: {1}, {2} pounds", kvp.Key, kvp.Value.Company, kvp.Value.Weight); } } /* 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 */
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.