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PO.Net generics -- implementation inheritance from interface
 

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    <p>I have two sequences of objects 'A' and 'B'. Comparing the sequences should produce a third sequence 'C' of elements that indicate whether:</p> <ul> <li>the objects were "deleted" from 'A' or </li> <li>"inserted" from 'B'. </li> </ul> <p>All remaining elements are considered as "matched".</p> <h2>What I would like to do:</h2> <p>Declare <code>Inserted&lt;T&gt;</code>, <code>Deleted&lt;T&gt;</code>, and <code>Matched&lt;T&gt;</code> generic classes that inherit all their properties from the <code>T</code> base class. The generic class must be able to instantiate itself from the object it inherits. </p> <h2>The code:</h2> <pre><code>public interface IInstantiable&lt;T&gt; { void CopyFrom(T o); } [Serializable] public class Inserted&lt;T&gt; : T where T : IInstantiable&lt;T&gt; { public Inserted() { } public Inserted(T t) { this.CopyFrom(t); } } </code></pre> <h2>The error:</h2> <pre><code>'MyNamespace.Inserted&lt;T&gt;' does not contain a definition for 'CopyFrom' and no extension method 'CopyFrom' accepting a first argument of type 'MyNamespace.Inserted&lt;T&gt;' could be found (are you missing a using directive or an assembly reference?) </code></pre> <h2>Further discussion:</h2> <p>I define my own <code>IInstantiable</code> interface to enforce the existence of a <code>CopyFrom</code> method. I cannot use the standard <code>ICloneable</code> interface, because it only defines a method that copies the object to a new instance, whereas I need the object to copy its members in the constructor.</p> <p>The error goes away if the generic defines its own implementation of the <code>CopyFrom</code> method; however, this does not achieve the desired goal of specializing the <code>CopyFrom</code> method to handle the specific needs of the base class. Only the base class could know what properties should be copied. (Or am I missing something?)</p> <p><strong>Note</strong>: The final object should have the same public members as its base class, as the object should be capable of serialization.</p> <p>Is this possible in .NET?</p> <h2>The answer:</h2> <p>What I am attempting to do is impossible, simply because the generic class cannot be an extension of the template base class. Visual Studio complains "Cannot derive from 'T' because it is a type parameter." (I hadn't noticed this error yet because I had not implemented the <code>CopyFrom</code> method in the generic class yet.)</p> <p>If I were to change the interface into a class and supply a stub implementation in that class, I could inherit from it as suggested below; however, this introduces a new base class into my inheritance hierarchy.</p> <pre><code>public class IInstantiable&lt;T&gt; { public virtual void CopyFrom(T o) { } } [Serializable] public class Inserted&lt;T&gt; : IInstantiable&lt;T&gt; where T : IInstantiable&lt;T&gt; { public Inserted() { } public Inserted(T t) { base.CopyFrom(t); } } </code></pre> <p>Unfortunately, I cannot use this new base class in its templatized form because I must introduce it at the root of my inheritance hierarchy. It works only if I remove the template and make it as generic as possible.</p> <pre><code>public class IInstantiable { public virtual void CopyFrom(Object o) { } } </code></pre> <p>However, this still does not make my <code>Inserted&lt;T&gt;</code> generic look like the object it is initialized from, and since I cannot inherit from the same type as the type parameter, it does not suit my initial purpose. </p> <p>Moving away from "fancy generics" based on the type system to more (ahem) generic annotated structures might prove to be the best solution; however, the default behavior of my selected serialization approach (XmlSerialization) does not have the automatic support that would make this configuration a viable solution. Generics will not work; use hard-coded class definitions instead. </p>
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    1. COFirstly `Inserted<T> : T` isn't allowed.
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    2. COIt looks like you just want: `public class Inserted<T> : IInstantiable<T>`. Does that not work for you?
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    3. COWell I did notice that you didn't inherit from `IInstantiable<T>`, you just created a templated type based on it. Have you tried `public class Inserted<T> : IInstantiable<T> : where T IInstantiable<T>`? (actually since you didn't template the where wouldn't compile so leave that off)
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