Distinct list of objects based on an arbitrary key in LINQ

Question

I have some objects:

class Foo {
    public Guid id;
    public string description;
}

var list = new List();
list.Add(new Foo() { id = Guid.Empt         


        

Answer 1

Using the Distinct() method is about 4x faster than using GroupBy() in my informal tests. For 1 million Foo's my test has Distinct() at about 0.89 seconds to make a unique array out of a non-unique array where GroupBy() takes about 3.4 seconds.

My Distinct() call looks like,

var unique = list.Distinct(FooComparer.Instance).ToArray();

and FooComparer looks like,

class FooComparer : IEqualityComparer {
    public static readonly FooComparer Instance = new FooComparer();

    public bool Equals(Foo x, Foo y) {
        return x.id.Equals(y.id);
    }

    public int GetHashCode(Foo obj) {
        return obj.id.GetHashCode();
    }
}

and my GroupBy() version looks like,

var unique = (from l in list group l by l.id into g select g.First()).ToArray();