How to use Distinct() or Groupby() in a list that returns an anonymous type?

Question

I’m doing a select on the bench to bring the IdNavio and Navio, but I don’t want to bring the repeated.

I’m trying to use the methods Distinct() and GroupBy(), but they’re not working, I’m using the namespace System.Linq.

Example:

var listaDeNavios = Context.VWProgramacaoEmbarque
                           .Select(e => new { IdNavio = e.IdNavio, Navio = e.Navio });

Can you do it using these methods? Or is there another way for this type?

Answer 1

If you want to make a distinct by property NavioId, you can group the results by this property and then use the method Distinct.

Usually I prefer to do an extension method for this

public static class QueryableExtensions
{
    public static IQueryable<TSource> DistinctBy<TSource, TKey>(this IQueryable<TSource> source, Expression<Func<TSource, TKey>> keySelector)
    {
        return source.GroupBy(keySelector).Select(x => x.FirstOrDefault());
    }
}

The use would be like this

var listaDeNavios = Context.VWProgramacaoEmbarque
                       .Select(e => new { IdNavio = e.IdNavio, Navio = e.Navio })
                       .DistinctBy(e => e.IdNavio);

Answer 2

The problem is your Idnavio is a unique value. The Enumerable.Distinct() uses Object methods to do its job. In the case of anonymous type the compiler will override the Object.Equals() and the Object.GetHashCode() and will compare all values of your type. Since your Idnavio will always have different values among themselves, the same will always be different.

The simple solution would be a Iequalitycomparer, but he is not an employee since you have an anonymous type and only using reflection you could know which properties will participate in the comparison, something similar to the one exemplified here:

https://www.codeproject.com/Articles/94272/A-Generic-IEqualityComparer-for-Linq-Distinct

So the best solution, in my humble opinion, is for you to create a Command class and overwrite Object.Equals() and Object.GetHashCode(), so implementing Distinctby would be much simpler:

public class NavioComparer : IEqualityComparer<NavioCommand>
{
    public bool Equals(NavioCommand first, NavioCommand second)
    {
        return first.Navio.Equals(second.Navio);
    }

    public int GetHashCode(NavioCommand obj)
    {
        return obj.Navio.GetHashCode();
    }
}

Other addenda:

• Morelinq’s Distinctby employee, however he works with Ienumerable, not Iqueryable, so his Distinct would run in memory, not in the database.
• You could also implement your own Distinctby that accepts a Iqueryable (LINQ example).