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LINQ: Enhancing Distinct With The SelectorEqualityComparer

, 15 Apr 2010 CPOL 9.1K 4
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LINQ: Enhancing Distinct With The SelectorEqualityComparer
LINQ With C# (Portuguese)

In my last post, I introduced the PredicateEqualityComparer and a Distinct extension method that receives a predicate to internally create a PredicateEqualityComparer to filter elements.

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Using the predicate greatly improves readability, conciseness and expressiveness of the queries, but it can be even better. Most of the times, we don't want to provide a comparison method but just extract the comparison key for the elements.

So, I developed a SelectorEqualityComparer that takes a method that extracts the key value for each element. Something like this:

public class SelectorEqualityComparer<TSource, Tkey> : EqualityComparer<TSource>
    where Tkey : IEquatable<Tkey>
{
    private Func<TSource, Tkey> selector;

    public SelectorEqualityComparer(Func<TSource, Tkey> selector)
        : base()
    {
        this.selector = selector;
    }

    public override bool Equals(TSource x, TSource y)
    {
        Tkey xKey = this.GetKey(x);
        Tkey yKey = this.GetKey(y);

        if (xKey != null)
        {
            return ((yKey != null) && xKey.Equals(yKey));
        }

        return (yKey == null);
    }

    public override int GetHashCode(TSource obj)
    {
        Tkey key = this.GetKey(obj);

        return (key == null) ? 0 : key.GetHashCode();
    }

    public override bool Equals(object obj)
    {
        SelectorEqualityComparer<TSource, Tkey> comparer = 
			obj as SelectorEqualityComparer<TSource, Tkey>;
        return (comparer != null);
    }

    public override int GetHashCode()
    {
        return base.GetType().Name.GetHashCode();
    }

    private Tkey GetKey(TSource obj)
    {
        return (obj == null) ? (Tkey)(object)null : this.selector(obj);
    }
}

Now I can write code like this:

.Distinct(new SelectorEqualityComparer<Source, Key>(x => x.Field))

And, for improved readability, conciseness and expressiveness and support for anonymous types the corresponding Distinct extension method:

public static IEnumerable<TSource> Distinct<TSource, TKey>
	(this IEnumerable<TSource> source, Func<TSource, TKey> selector)
    where TKey : IEquatable<TKey>
{
    return source.Distinct(new SelectorEqualityComparer<TSource, TKey>(selector));
}

And the query is now written like this:

.Distinct(x => x.Field)

For most usages, it’s simpler than using a predicate.

License

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

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About the Author

Paulo Morgado
Software Developer (Senior) Paulo Morgado
Portugal Portugal

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Comments and Discussions

 
GeneralReadonly Pin
Alexander M. Batishchev23-Aug-10 5:12
memberAlexander M. Batishchev23-Aug-10 5:12 
GeneralRe: Readonly Pin
Paulo Morgado23-Aug-10 5:54
memberPaulo Morgado23-Aug-10 5:54 
GeneralSame as Pin
Partenon12-May-10 1:18
memberPartenon12-May-10 1:18 
GeneralRe: Same as Pin
Paulo Morgado12-May-10 12:20
memberPaulo Morgado12-May-10 12:20 
GeneralNice post Pin
J. Dunlap13-Apr-10 12:17
memberJ. Dunlap13-Apr-10 12:17 
GeneralRe: Nice post Pin
Paulo Morgado13-Apr-10 12:33
memberPaulo Morgado13-Apr-10 12:33 

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