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    <p>OK, let’s examine the first one. The error is:</p> <blockquote> <p>The type 'V' cannot be used as type parameter 'V' in the generic type or method 'Test.Generic_Route'. There is no implicit reference conversion from 'V' to 'Test.Generic_Visit'.</p> </blockquote> <p>It complains about this declaration:</p> <pre><code>public abstract class Generic_TSPRoute&lt;V, E&gt; : Generic_Route&lt;V, E&gt; where V : Concrete_TSPVisit where E : Concrete_TSPNode </code></pre> <p>This establishes two definitions:</p> <ul> <li><p><code>V</code> is a <code>Concrete_TSPVisit</code> (or a descendent of it)</p></li> <li><p><code>E</code> is a <code>Concrete_TSPNode</code> (or a descendent of it)</p></li> </ul> <p>Now let’s see what <code>Generic_Route&lt;V, E&gt;</code> lets us put in:</p> <pre><code>public class Generic_Route&lt;V, E&gt; where V : Generic_Visit&lt;E&gt; where E : GenericElement </code></pre> <p>The second constraint is fine because <code>Concrete_TSPNode</code> is a <code>GenericElement</code>. The first one is problematic: Remember that <code>E</code> is a <code>Concrete_TSPNode</code> <em>or a descendent of it</em>, therefore <code>Generic_Visit&lt;E&gt;</code> could be:</p> <ul> <li><p><code>Generic_Visit&lt;Concrete_TSPNode&gt;</code>, <strong>or</strong></p></li> <li><p><code>Generic_Visit&lt;some subclass of Concrete_TSPNode&gt;</code></p></li> </ul> <p>However, we also know from earlier that <code>V</code> is a <code>Concrete_TSPVisit</code> (or a descendent of it).</p> <ul> <li><p><code>Concrete_TSPVisit</code> inherits from <code>Generic_TSPVisit&lt;Concrete_TSPNode&gt;</code></p></li> <li><p><code>Generic_TSPVisit&lt;Concrete_TSPNode&gt;</code> inherits from <code>Generic_Visit&lt;Concrete_TSPNode&gt;</code></p></li> </ul> <p>Notice something? This requires it to be a <code>Generic_Visit&lt;Concrete_TSPNode&gt;</code>. It is emphatically not allowed to be <code>Generic_Visit&lt;some subclass of Concrete_TSPNode&gt;</code>.</p> <p><strong>In other words,</strong> imagine I write this:</p> <pre><code>var route = new Generic_TSPRoute&lt;Concrete_TSPVisit, Concrete_TSPNode_Subclass&gt;(); </code></pre> <p>According to your hierarchy, <code>Concrete_TSPVisit</code> is a <code>Generic_Visit&lt;Concrete_TSPNode&gt;</code> and therefore has a property that looks like</p> <pre><code>public Concrete_TSPNode Element { get; set; } </code></pre> <p>If I retrieve a value from this property, it is only guaranteed to be a <code>Concrete_TSPNode</code> but not necessarily a <code>Concrete_TSPNode_Subclass</code>.</p> <h2>EDIT:</h2> <p>I’ll leave this answer because it explains the reason for the compiler error, but Enigmativity’s answer actually provides the solution to the problem.</p>
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    1. POHow to use Inheritance when using Generic Constraints
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    1. CO@Timwi : Thanks for the comprehensive answer. That does help me see the problem (I've never quite understood the flavours of variance, I should read Eric Lipperts blog more closely). Could you explain the 'setter/getter' problem about being read only? I might be missing something obvious... but can't see why using an interface will limit me to being read only?
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    2. CO@Ian: Using an interface doesn’t limit you to being read-only. Using *co-variance* does. For example, an `IEnumerable<string>` can double as an `IEnumerable<object>` because `IEnumerable<T>` is co-variant in `T`, which it can be because it is read-only (`T` only occurs as a return type). An `IComparer<object>` can double as an `IComparer<string>` because it is contra-variant in `T`, which it can be because it is write-only (`T` only occurs as a parameter type).
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    3. CO@Ian: P.S. I just added some more to my answer to explain the problem when `Generic_Visit<E>` has both a setter and a getter.
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