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    <p>Delphi generics do not support covariance and contravariance so what you are attempting to do is not possible with the language's current syntax. I suggest you have a read of the following blog articles that cover the matter in more detail.</p> <ul> <li><a href="http://blogs.teamb.com/craigstuntz/2009/10/01/38465/" rel="nofollow noreferrer">Craig Stuntz: Comparing C#, C++, and Delphi (Win32) Generics</a></li> <li><a href="http://tech.turbu-rpg.com/149/generics-and-the-covariance-problem" rel="nofollow noreferrer">Mason Wheeler: Generics and the Covariance Problem</a></li> </ul> <p>Fundamentally what you are attempting to do is this:</p> <pre><code>type TBase = class; TDerived = class(TBase); TBaseList = TList&lt;TBase&gt;; TDerivedList = TList&lt;TDerived&gt;; var BaseList: TBaseList; DerivedList: TDerivedList; ... BaseList := TDerivedList;//does not compile </code></pre> <p>The designers have not stopped you doing this out of spite. There is a good reason. Consider the following standard example:</p> <pre><code>type TAnimal = class; TCat = class(TAnimal); TPenguin = class(TAnimal); var AnimalList: TList&lt;TAnimal&gt;; CatList: TList&lt;TCat&gt;; Penguin: TPenguin; ... AnimalList := CatList;//does not compile because... AnimalList.Add(Penguin);//...of the danger of this </code></pre> <p>Whilst it is reasonable to add a <code>TPenguin</code> to a <code>TList&lt;TAnimal&gt;</code>, the actual list that <code>AnimalList</code> refers to is a <code>TList&lt;TCat&gt;</code> and a penguin is not a cat.</p> <p>And, if you want to think of it in the context of your example hierarchy, here's an illustration of code that justifies the language design.</p> <pre><code>aList := aCustomerList;//does not compile aList.Add(aCustomPerson); //this would add a TCustomPerson instance to a list containing //TCustomer instances, but a TCustomPerson is not a TCustomer </code></pre>
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    1. USDavid Heffernan
    1. USDavid Heffernan
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    1. COThanks for giving words to my intuition. I "knew" it had to be something like this, but couldn't come up with the words or the examples.
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      1. USMarjan Venema
    2. COHaving read the links you provided - I must say ... THANKS. This information should be included in the help for generics - TObjectList especially!.
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      1. USBimmer_R
 

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