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copied!<p>Because there is already an implicit conversion from <code>Derived</code> to <code>BaseClass</code>, and the converse does not make any sense.</p> <p>Regarding the latter: if your <code>Base</code> objects are meant to be implicitly convertible to <code>Derived</code> -- why aren't they <code>Derived</code> objects in the first place?</p> <p>Obligatory quotes from the <a href="http://www.ecma-international.org/publications/standards/Ecma-334.htm">standard</a>:</p> <blockquote> <h2>6.1.6 Implicit reference conversions</h2> <p>The implicit reference conversions are:</p> <ul> <li>[...]</li> <li>From any class-type S to any class-type T, provided S is derived from T.</li> </ul> </blockquote> <p>This says there's an implicit conversion <code>Derived</code> => <code>Base</code>, as we all know.</p> <blockquote> <h2>6.2.4 Explicit reference conversions</h2> <p>The explicit reference conversions are:</p> <ul> <li>[...]</li> <li>From any class-type S to any class-type T, provided S is a base class of T.</li> <li>[...]</li> </ul> </blockquote> <p>This says there's already an explicit conversion <code>Base</code> => <code>Derived</code> (which is what allows you to try downcasting at runtime).</p> <blockquote> <h2>6.4.1 Permitted user-defined conversions</h2> <p>C# permits only certain user-defined conversions to be declared. <strong>In particular, it is not possible to redefine an already existing implicit or explicit conversion.</strong></p> </blockquote> <p>And this says that since the two conversions of interest are already defined by the language, you can't redefine them.</p>

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