LINQ's Distinct() on a particular property

Question

I am playing with LINQ to learn about it, but I can\'t figure out how to use Distinct when I do not have a simple list (a simple list of integers is pretty easy

Answer 1

Personally I use the following class:

public class LambdaEqualityComparer<TSource, TDest> : 
    IEqualityComparer<TSource>
{
    private Func<TSource, TDest> _selector;

    public LambdaEqualityComparer(Func<TSource, TDest> selector)
    {
        _selector = selector;
    }

    public bool Equals(TSource obj, TSource other)
    {
        return _selector(obj).Equals(_selector(other));
    }

    public int GetHashCode(TSource obj)
    {
        return _selector(obj).GetHashCode();
    }
}

Then, an extension method:

public static IEnumerable<TSource> Distinct<TSource, TCompare>(
    this IEnumerable<TSource> source, Func<TSource, TCompare> selector)
{
    return source.Distinct(new LambdaEqualityComparer<TSource, TCompare>(selector));
}

Finally, the intended usage:

var dates = new List<DateTime>() { /* ... */ }
var distinctYears = dates.Distinct(date => date.Year);

The advantage I found using this approach is the re-usage of LambdaEqualityComparer class for other methods that accept an IEqualityComparer. (Oh, and I leave the yield stuff to the original LINQ implementation...)

Answer 2

In case you need a Distinct method on multiple properties, you can check out my PowerfulExtensions library. Currently it's in a very young stage, but already you can use methods like Distinct, Union, Intersect, Except on any number of properties;

This is how you use it:

using PowerfulExtensions.Linq;
...
var distinct = myArray.Distinct(x => x.A, x => x.B);