Question

This is a question for the generic collection gurus.

I’m shocked to find that TList does not override equals. Take a look at this example:

list1:=TList<String>.Create;       
list2:=TList<String>.Create;

list1.Add('Test');
list2.Add('Test');

Result:=list1.Equals(list2);

“Result” is false, even though the two Lists contain the same data. It is using the default equals() (which just compares the two references for equality).

Looking at the code, it looks like the same is true for all the other generic collection types too.

Is this right, or am I missing something??

It seems like a big problem if trying to use TLists in practice. How do I get around this? Do I create my own TBetterList that extends TList and overrides equals to do something useful?
Or will I run into further complications with Delphi generics…… ?

[edit: I have one answer so far, with a lot of upvotes, but it doesn’t really tell me what I want to know. I’ll try to rephrase the question]

In Java, I can do this:

List<Person> list1=new ArrayList<Person>();
List<Person> list2=new ArrayList<Person>();
list1.add(person1);
list2.add(person1);
boolean result=list1.equals(list2);

result will be true. I don’t have to subclass anything, it just works.

How can I do the equivalent in Delphi?

If I write the same code in Delphi, result will end up false.

If there is a solution that only works with TObjects but not Strings or Integers then that would be very useful too.

Answer 1

I looked around and found a solution in DeHL (an open source Delphi library). DeHL has a Collections library, with its own alternative List implementation. After asking the developer about this, the ability to compare generic TLists was added to the current unstable version of DeHL.

So this code will now give me the results I’m looking for (in Delphi):

list1:=TList<Person>.Create([Person.Create('Test')]);
list2:=TList<Person>.Create([Person.Create('Test')]);

PersonsEqual:=list1.Equals(list2); // equals true

It works for all types, including String and Integer types

stringList1:=TList<string>.Create(['Test']);
stringList2:=TList<string>.Create(['Test']);

StringsEqual:=stringList1.Equals(stringList2); // also equals true

Sweet!

You will need to check out the latest unstable version of DeHL (r497) to get this working. The current stable release (0.8.4) has the same behaviour as the standard Delphi TList.

Be warned, this is a recent change and may not make it into the final API of DeHL (I certainly hope it does).

So perhaps I will use DeHL instead of the standard Delphi collections? Which is a shame, as I prefer using standard platform libraries whenever I can. I will look further into DeHL.