Question

I have this class (a partial listing):

class CiscoSwitch 
{
  private string _SwitchName = string.Empty;
  public SwitchName {get {return _SwitchName;} set{_SwitchName=value; }}
}

I have 2 lists of CiscoSwitch objects. I am trying to compare them to pick out the ones that are not duplicates. I only want the duplicates. I tried a Lambda expression but got a compiler error that CiscoSwitch was a non-delgate type.

I am now wondering about something like this – it would allow me to use the List.Except() method (I think):

static class SwitchComparer
{ 
  static bool CompareSwitchNames(CiscoSwitch s1, CiscoSwitch s2)
         {
            if (sw1.SwitchName == s2.SwitchName) {return true;}
             else {return false;}
         }
}

     // to find the differences 
 // this is a method of the CiscoSwitchClass
private List<CiscoSwitch> FindDifferences(List<CiscoSwitch> List1, List<CiscoSwitch> List2)
{
       return List1.Except(List2, SwitchComparer.CompareSwitchNames();
 }

this could also be done with a foreach but I think this way is a lot cleaner, if it is correct. I am also thinking there are other attributes of a CiscoSwitch I might want to compare some day so could add methods to the SwitchComparer class as I need them.

Answer 1

No, just having a single method like that won’t help you. You need to implement an IEqualityComparer<CiscoSwitch> to pass to Enumerable.Except – and even then your code would need to be:

return List1.Except(List2, new SwitchComparer()).ToList();

Overriding Equals and GetHashCode within CiscoSwitch will do the trick more naturally though – and ideally you should implement IEquatable<CiscoSwitch> too.

However, it’s worth noting that mutable types like this don’t play terribly nicely with things like Dictionary<,> – if you change an object in a way which affects its hash code after you’ve inserted it as a key into the dictionary, you won’t be able to get at it again. Consider making the type immutable if you can.

A couple of other points to note:

• Any time you write:

  if (condition)
  {
      return true;
  }
  else
  {
      return false;
  }
  

  you should instead write the far simpler:

  return condition;
  

  So your CompareSwitchNames method would be:

  static bool CompareSwitchNames(CiscoSwitch s1, CiscoSwitch s2)
  {
  return s1.SwitchName == s2.SwitchName;
  }

• Your parameter names for FindDifferences should follow .NET naming conventions (e.g. list1 and list2)

• Using Except will only find you the elements in the first list which aren’t in the second list; if you need to find the symmetric difference, consider using HashSet<T> explicitly.

EDIT: If you wanted to have multiple ways of comparing, you could have something like:

public static class SwitchComparers
{
    public static readonly IEqualityComparer<CiscoSwitch> ByName =
        new ByNameComparer();

    public static readonly IEqualityComparer<CiscoSwitch> ByCost =
        new ByCostComparer();


    private sealed class ByNameComparer : IEqualityComparer<CiscoSwitch>
    {
        // Implementation
    }

    private sealed class ByCostComparer : IEqualityComparer<CiscoSwitch>
    {
        // Implementation
    }
}