Using Distinct with LINQ and Objects

Question

Until recently, I was using a Distinct in LINQ to select a distinct category (an enum) from a table. This was working fine.

I now need to have it distinct on a class

Answer 1

For explanation, take a look at other answers. I'm just providing one way to handle this issue.

You might like this:

public class LambdaComparer:IEqualityComparer{
  private readonly Func _comparer;
  private readonly Func _hash;
  public LambdaComparer(Func comparer):
    this(comparer,o=>0) {}
  public LambdaComparer(Func comparer,Func hash){
    if(comparer==null) throw new ArgumentNullException("comparer");
    if(hash==null) throw new ArgumentNullException("hash");
    _comparer=comparer;
    _hash=hash;
  }
  public bool Equals(T x,T y){
    return _comparer(x,y);
  }
  public int GetHashCode(T obj){
    return _hash(obj);
  }
}

Usage:

public void Foo{
  public string Fizz{get;set;}
  public BarEnum Bar{get;set;}
}

public enum BarEnum {One,Two,Three}

var lst=new List();
lst.Distinct(new LambdaComparer(
  (x1,x2)=>x1.Fizz==x2.Fizz&&
           x1.Bar==x2.Bar));

You can even wrap it around to avoid writing noisy new LambdaComparer(...) thing:

public static class EnumerableExtensions{
 public static IEnumerable SmartDistinct
  (this IEnumerable lst, Func pred){
   return lst.Distinct(new LambdaComparer(pred));
 }
}

Usage:

lst.SmartDistinct((x1,x2)=>x1.Fizz==x2.Fizz&&x1.Bar==x2.Bar);

NB: works reliably only for Linq2Objects