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POGenerics, Polymorphism, Interfaces: what is the solution?
 

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  1. POGenerics, Polymorphism, Interfaces: what is the solution?
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    <p>I know the title is Very wide - spanning over a lot !</p> <p>And I hope that this question might evolve to a bigger "info wiki thingy" on the subjects.</p> <p><strong>What I have learned - so far:</strong></p> <ul> <li>When using Generics - understand the concepts (<a href="http://tech.turbu-rpg.com/149/generics-and-the-covariance-problem" rel="nofollow noreferrer">covariance and contravariance</a>).</li> <li><a href="https://stackoverflow.com/a/9142020/696574">Do NOT "mis-use"</a> the concept of generics combined with inheritance. I did and it could make you head directly into covariance problems! Make sure you "break off" the generic at the correct point in you inheritance - if you are combining the two.</li> </ul> <p><em>(please correct me - if you think i'm wrong, missing or have misunderstood anything).</em></p> <p><strong>My problem was:</strong></p> <p>But by now I've spend countless hours, trying to figure out, how to solve this "big puzzle" I have on my desk. And I've gotten some good answers from several of you SO users already - but now its time to get something working in a bigger scale.</p> <p>I ventured into Generics with this one: <a href="https://stackoverflow.com/a/7872779/696574">Generics and Polymorphism working together</a></p> <p>And now I'm kinda stuck on this one: <a href="https://stackoverflow.com/a/9142020/696574">Situations where Generics won't work</a></p> <p>Why I end up with covariance problems - is because of my class procedure in my hierarchy.</p> <p>So I'm wondering if Interfaces is my next bold move in this "saga". How do one "step over" a covariance problem. One thing is to find out that you actually have this problem - another thing is "how to work around it".</p> <p>So IF any of you good people "out there" has any opinions on this - I'm all ears. Basically : Tell me to go for Interfaces (I have never done one from scratch myself). Or .. throw me a bone in the direction you would suggest.</p> <p>My current source pool is as stated in the second link - from the top.</p> <p>Here is a small snippet from my earlier <a href="https://stackoverflow.com/q/9140485/696574">post</a> that shows my covariance problem. <a href="https://stackoverflow.com/users/505088/david-heffernan">David</a> kindly explained - Why I ran into the bush.. But now I need info on - How to run around it.</p> <pre><code>var aList : TBaseList&lt;TBaseObject&gt;; // used as a list parameter for methods aPersonList : TPersonList&lt;TPerson&gt;; aCustomerList : TCustomerList&lt;TCustomer&gt;; begin aPersonList := TPersonList&lt;TPerson&gt;.Create; aCustomerList := TCustomerList&lt;TCustomer&gt;.Create; aList := aCustomerList; &lt;-- this FAILS !! types not equal .. end; </code></pre> <p>Regards</p>
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    1. COIt would help if we actually knew what your actual problem is. In my experience, covariance is seldom a problem, if the design is not too complex. IOW, tell us the real problem, so we have something concrete to solve.
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      1. USRudy Velthuis
    2. CO@RudyVelthuis Well... as you mention - it is very complex and would require a lot of rewrite from former [post](http://stackoverflow.com/q/9140485/696574). But basically its the same problem - only the question has changed. Since I now know what I have done wrong - I merely hope that someone can point me in the direction of a solution. If it's still unclear - let me know and I'll try and explain as good as I can.
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    3. COHow to work around the problem depends on what the problem *is*. Once you assign a value to `aList`, what did you expect to *do* with that variable? (Also, why are TPersonList and TCustomerList generic? What *else* can you have a TPersonList of?)
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