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

, 5 Dec 2008 CPOL
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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

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Comments and Discussions

GeneralDistinct and Intersect Pin
Mark Nischalke5-Dec-08 14:50
memberMark Nischalke5-Dec-08 14:50 
GeneralRe: Distinct and Intersect Pin
Günther M. FOIDL6-Dec-08 3:21
memberGünther M. FOIDL6-Dec-08 3:21 
GeneralRe: Distinct and Intersect Pin
Mark Nischalke6-Dec-08 4:30
memberMark Nischalke6-Dec-08 4:30 
GeneralRe: Distinct and Intersect Pin
Günther M. FOIDL6-Dec-08 5:32
memberGünther M. FOIDL6-Dec-08 5:32 
OK, you intersect the (unique) keys with the original list to return the unique list. Nice!

GeneralImprovement [modified] Pin
Günther M. FOIDL5-Dec-08 11:52
memberGünther M. FOIDL5-Dec-08 11:52 
GeneralRe: Improvement Pin
Jean-Paul Mikkers5-Dec-08 13:04
memberJean-Paul Mikkers5-Dec-08 13:04 
GeneralConvert extension from List to IEnumerable&lt;T&gt; Pin
Michael Lee Yohe19-Dec-08 7:13
memberMichael Lee Yohe19-Dec-08 7:13 
GeneralRe: Convert extension from List to IEnumerable<T> Pin
Partenon11-May-10 0:21
memberPartenon11-May-10 0:21 

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