Click here to Skip to main content
Click here to Skip to main content

LINQ Extension Method to Return a Unique List Based on a Key

, 5 Dec 2008 CPOL
Rate this:
Please Sign up or sign in to vote.
LINQ Extension method to return a Unique List based on a key


This article is about a handy LINQ extension method that will take a List<T> and a key selector, and return a unique List<T> based on that key.


I had a need to use an in-code way of refining a List<T> so there was no duplication on a key. I developed this handy LINQ Extension to do it for me.

Using the Code

Use this method on any List<T>. This is an extension method, so it has to be put in a static class.


List<MyClass> classList; 

Assuming classList is populated with values...

List<MyClass> filteredList = classList.Unique(cl => cl.SomeKeyProperty);
/// <summary>
/// Takes a List of type <typeparamref name="T"/> 
/// and a function as the key selector and returns a unique list of type 
/// <typeparamref name="T"/>
/// from that list
/// </summary>
/// <typeparam name="KEY">The type of the KEY.</typeparam>
/// <typeparam name="T"></typeparam>
/// <param name="InputList">The input list.</param>
/// <param name="func">The func.</param>
/// <returns></returns>
/// <example><code>List&lt;T&gt; uniqueList = 
/// 	nonUniqueList.Unique(key=>key.ID);</code></example>
public static List<T> Unique<KEY, T>(this List<T> InputList, Func<T, KEY> func)
    if (func == null)
        throw new ArgumentNullException("Key selector function cannot be null");

    if (InputList == null)
    { return null; }

    if (InputList.Count == 0)
    { return InputList; }

    // Convert the inputList to a dictionary based on the key selector provided
    Dictionary<KEY, T> uniqueDictionary = new Dictionary<KEY, T>();
    InputList.ForEach(item =>
        // Use the key selector function to retrieve the key
        KEY k = func.Invoke(item);

        // Check the dictionary for that key
        if (!uniqueDictionary.ContainsKey(k))
            // Add that item to the dictionary 
            uniqueDictionary.Add(k, item);

    // Get the enumerator of the dictionary
    Dictionary<KEY, T>.Enumerator e = uniqueDictionary.GetEnumerator();

    List<T> uniqueList = new List<T>();
    while (e.MoveNext())
        // Enumerate through the dictionary keys and pull out 
        // the values into a unique list

    // return the unique list
    return uniqueList;

Points of Interest  

While LINQ has a .ToDictionary() extension method, if you have a List<T> that contains items that aren't unique, .ToDictionary() will throw an exception indicating that a key already exists in the dictionary.  So I had to write the code above to only add items to the dictionary if they didn't already exist.


  • v1.0 12/05/2008


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Stephen Inglish
Software Developer (Senior) Harland Financial Solutions
United States United States
No Biography provided

Comments and Discussions

GeneralSuggestions Pinmembertonyt5-Dec-08 16:56 
GeneralI give up Pinmembertonyt5-Dec-08 17:02 
GeneralRe: I give up PinmvpColin Angus Mackay6-Dec-08 1:02 
GeneralRe: I give up Pinmembertonyt6-Dec-08 16:26 
GeneralRe: I give up PinmvpColin Angus Mackay7-Dec-08 1:37 
GeneralRe: I give up Pinmembertonyt8-Dec-08 22:43 
GeneralRe: I give up PinmemberGünther M. FOIDL6-Dec-08 3:12 
GeneralRe: I give up Pinmembertonyt6-Dec-08 16:27 
Answer&lt; and &gt; PinmemberTobiasP16-Dec-08 1:25 
GeneralRe: Suggestions PinmemberGünther M. FOIDL6-Dec-08 3:15 
GeneralRe: Suggestions [modified] Pinmembertonyt6-Dec-08 16:34 
GeneralRe: Suggestions PinmemberGünther M. FOIDL7-Dec-08 2:49 

General General    News News    Suggestion Suggestion    Question Question    Bug Bug    Answer Answer    Joke Joke    Rant Rant    Admin Admin   

Use Ctrl+Left/Right to switch messages, Ctrl+Up/Down to switch threads, Ctrl+Shift+Left/Right to switch pages.

| Advertise | Privacy | Terms of Use | Mobile
Web03 | 2.8.150331.1 | Last Updated 5 Dec 2008
Article Copyright 2008 by Stephen Inglish
Everything else Copyright © CodeProject, 1999-2015
Layout: fixed | fluid