In Delphi how to have a class that implements IComparable and inherits from TEqualityComparer?

Question

To explain my question I'll use something very similar to the TDictionary example in Embarcadero's website, but my class implements IComparable.

In Delphi (XE7):

TCity = class(TInterfacedObject, IComparable)
  Country: String;
  Latitude: Double;
  Longitude: Double;

  function CompareTo(Obj: TObject): Integer;
end;

I want a TDictionary with cities as keys:

Dictionary := TDictionary<TCity, string>.Create;

CityA := TCity.Create;
CityA.Country := 'United Kingdom';
CityA.Latitude := 51.5;
CityA.Longitude := -0.17;
Dictionary.Add(CityA, 'London');

Now, if I create a new city with the same values as before where CityA.CompareTo(CityB) returns 0, like:

CityB := TCity.Create;
CityB.Country := 'United Kingdom';
CityB.Latitude := 51.5;
CityB.Longitude := -0.17;

And before adding CityB to the dictionary, I was expecting that:

Dictionary.ContainsKey(CityB)

would use my CompareTo implementation and ContainsKey would return True, but that is not the case. It seems that my class will also need to inherit from TEqualityComparer.

So how do I add an equality comparer—used by ContainsKey—to my class, additional to the CompareTo already there?

I know I could use a TDelegatedEqualityComparer and anonymous methods to create a custom comparer to be used when creating the dictionary. Something like:

Dictionary := TDictionary<TCity, string>.Create(CityComparer)

But is there a way to include the equality comparer in TCity, so that I don't need to have one when creating the dictionary?

Answer 1

You are mixing two things here. IComparable is meant for sorting.

IEqualityComparer<T> is to check if two items are equal or not but it also has the GetHashCode method that the dictionary is using internally.

For objects the default implementation for the IEqualityComparer<T> uses the virtual GetHashCode and Equals methods from TObject. So if you want to treat two different instances as equal you have two options:

1. override these two methods in your class

2. provide a custom implementation of IEqualityComparer<T>

Answer 2

Not as far as I know. If you don't provide an IEqualityComparer in then constructor TDictionary wil use the default one for the key.

Look in the constructor :

constructor TDictionary<TKey,TValue>.Create(ACapacity: Integer; const AComparer: IEqualityComparer<TKey>);
var
  cap: Integer;
begin
  inherited Create;
  if ACapacity < 0 then
    raise EArgumentOutOfRangeException.CreateRes(@SArgumentOutOfRange);
  FComparer := AComparer;
  if FComparer = nil then
    FComparer := TEqualityComparer<TKey>.Default; // <-- HERE!
  SetCapacity(ACapacity);
end;

Her here in GetBucketIndex it is used

    function TDictionary<TKey,TValue>.GetBucketIndex(const Key: TKey; HashCode: Integer): Integer;
begin    
 ...
        // Found: return location.
        if (hc = HashCode) and FComparer.Equals(FItems[Result].Key, Key) then
...
end;

So no, you'll have to give the IComparable in the constructor