How do I use a custom comparer with the Linq Distinct method?

Question

I was reading a book about Linq, and saw that the Distinct method has an overload that takes a comparer. This would be a good solution to a problem I have where I want to get the distinct entities from a collection, but want the comparison to be on the entity ID, even if the other properties are different.

According to the book, if I have a Gribulator entity, I should be able to create a comparer like this...

private class GribulatorComparer : IComparer<Gribulator> {
  public int Compare(Gribulator g1, Gribulator g2) {
    return g1.ID.CompareTo(g2.ID);
  }
}

...and then use it like this...

List<Gribulator> distinctGribulators
  = myGribulators.Distinct(new GribulatorComparer()).ToList();

However, this gives the following compiler errors...

'System.Collections.Generic.List' does not contain a definition for 'Distinct' and the best extension method overload 'System.Linq.Enumerable.Distinct(System.Collections.Generic.IEnumerable, System.Collections.Generic.IEqualityComparer)' has some invalid arguments

Argument 2: cannot convert from 'LinqPlayground.Program.GribulatorComparer' to 'System.Collections.Generic.IEqualityComparer'

I've searched around a bit, and have seen plenty of examples that use code like this, but no complaints about compiler errors.

What am I doing wrong? Also, is this the best way of doing this? I want a one-off solution here, so don't want to start changing the code for the entity itself. I want the entity to remain as normal, but just in this one place, compare by ID only.

Thanks for any help.

Cythiacyto

Forgot to add, I'm using VS2012, and so .NET 4.0 in C# – Cythiacyto

Answer 1

You're implementing your comparer as an IComparer<T>, the LINQ method overload requires an implementation of IEqualityComparer:

private class GribulatorComparer : IEqualityComparer<Gribulator> {
  public bool Equals(Gribulator g1, Gribulator g2) {
    return g1.ID == g2.ID;
  }
}

edit:

For clarification, the IComparer interface can be used for sorting, as that's basically what the Compare() method does.

Like this:

items.OrderBy(x => new ItemComparer());

private class ItemComparer : IComparer<Item>
{
    public int Compare(Item x, Item y)
    {
        return x.Id.CompareTo(y.Id)
    }
}

Which will sort your collection using that comparer, however LINQ provides a way to do that for simple fields (like an int Id).

items.OrderBy(x => x.Id);

Nebulose

Thanks Peter, that explains it! The example I was using in the book was for OrderBy, but he had referred to it from where he discussed Distinct, and I hadn't picked up on the fact that I needed a different interface. Wasn't very clear in the book actually :( – Cythiacyto

By the way, do you know if there is a way to do this with an anonymous function, instead of having to create a separate class for the comparer? Seems like overkill – Cythiacyto

As far as I know, there isn't. I've actually been thinking about overloading linq operators with stuff like that. – Nebulose

Shame, because for a one-off like my situation, it's overkill to create a whole new class. I've got used to throwing anonymous functions around! Thanks again. – Cythiacyto

I think it's worth mentioning that writing a custom IEqualityComparer also requires an implementation of GetHashCode() – Baht