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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8" /><title>Hyperlinked ECMA C# Language Specification</title><meta name="author" content="Jon Jagger" /><link rel="stylesheet" href="ecma334.css"></link></head><body><div align="right"><em><a href="http://www.jaggersoft.com">Jon Jagger</a></em></div><div align="right"><a href="mailto:jon@jaggersoft.com">jon@jaggersoft.com</a></div><form method="get" action="http://search.atomz.com/search/"><input size="30" name="sp-q"></input><input type="submit" value="Search C# Spec"></input><input type="hidden" name="sp-a" value="sp10024177"></input><input type="hidden" name="sp-f" value="ISO-8859-1"></input></form><a href="toc.htm">Table of Contents</a> <a href="1.htm">1</a> <a href="2.htm">2</a> <a href="3.htm">3</a> <a href="4.htm">4</a> <a href="5.htm">5</a> <a href="6.htm">6</a> <a href="7.htm">7</a> <a href="8.htm">8</a> <a href="9.htm">9</a> <a href="10.htm">10</a> <a href="11.htm">11</a> <a href="12.htm">12</a> <a href="13.htm">13</a> <a href="14.htm">14</a> <a href="15.htm">15</a> <a href="16.htm">16</a> <a href="17.htm">17</a> <a href="18.htm">18</a> <a href="19.htm">19</a> <a href="20.htm">20</a> <a href="21.htm">21</a> <a href="22.htm">22</a> <a href="23.htm">23</a> <a href="24.htm">24</a> <a href="25.htm">25</a> <a href="notes.htm">Notes</a> <a href="HyperlinkedCSharpECMA.zip">Download</a><span class="ruler"></span><span class="heading">ECMA-334 C# Language Specification</span><span class="navigate"><a href="17.1.2.htm"><img src="previous.gif" alt="previous" border="0" /></a><a href="17.1.2.2.htm"><img src="next.gif" alt="next" border="0" /></a></span><span class="clause-depth"><a href="16.htm"><img src="previous.gif" alt="previous at this level" border="0" /></a><a href="18.htm"><img src="next.gif" alt="next at this level" border="0" /></a> <span class="clause-number-link"><a href="17.htm">17</a></span><span class="clause-title-previous"> Classes</span></span><span class="clause-depth"><a href="17.1.htm"><img src="previous.gif" alt="previous at this level" border="0" /></a><a href="17.2.htm"><img src="next.gif" alt="next at this level" border="0" /></a> <span class="clause-number-link"><a href="17.1.htm">17.1</a></span><span class="clause-title-previous"> Class declarations</span></span><span class="clause-depth"><a href="17.1.1.htm"><img src="previous.gif" alt="previous at this level" border="0" /></a><a href="17.1.3.htm"><img src="next.gif" alt="next at this level" border="0" /></a> <span class="clause-number-link"><a href="17.1.2.htm">17.1.2</a></span><span class="clause-title-previous"> Class base specification</span></span><span class="clause-depth"><a href="17.1.2.1.htm"><img src="previous.gif" alt="previous at this level" border="0" /></a><a href="17.1.2.2.htm"><img src="next.gif" alt="next at this level" border="0" /></a> <span class="clause-number">17.1.2.1</span><span class="clause-title"> Base classes</span></span><span class="locator">
Paragraph 1</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P1S1"></a>When a <span class="non-terminal"><a href="11.2.htm#class-type">class-type</a></span> is included in the <span class="non-terminal"><a href="17.1.2.htm#class-base">class-base</a></span>, it specifies the direct base class of the class being declared.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P1S2"></a>If a class declaration has no <span class="non-terminal"><a href="17.1.2.htm#class-base">class-base</a></span>, or if the <span class="non-terminal"><a href="17.1.2.htm#class-base">class-base</a></span> lists only interface types, the direct base class is assumed to be object.</span> <span class="sentence"><span class="sentence-number">3</span> <a name="P1S3"></a>A class inherits members from its direct base class, as described in <a href="17.2.1.htm">§17.2.1</a>.</span> </span><span class="paragraph"><span class="example">[Example: In the example <pre class="code-example">
class A {}
class B: A {}
</pre>class A is said to be the direct base class of B, and B is said to be derived from A. Since A does not explicitly specify a direct base class, its direct base class is implicitly object. end example]</span> </span><span class="locator">
Paragraph 2</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P2S1"></a>The direct base class of a class type must be at least as accessible as the class type itself (<a href="10.5.4.htm">§10.5.4</a>).</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P2S2"></a>For example, it is a compile-time error for a public class to derive from a private or internal class.</span> </span><span class="locator">
Paragraph 3</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P3S1"></a>The direct base class of a class type must not be any of the following types: System.Array, System.Delegate, System.Enum, or System.ValueType.</span> </span><span class="locator">
Paragraph 4</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P4S1"></a>The base classes of a class are the direct base class and its base classes.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P4S2"></a>In other words, the set of base classes is the transitive closure of the direct base class relationship.</span> <span class="note">[Note: Referring to the example above, the base classes of B are A and object. end note]</span> </span><span class="locator">
Paragraph 5</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P5S1"></a>Except for class object, every class has exactly one direct base class.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P5S2"></a>The object class has no direct base class and is the ultimate base class of all other classes.</span> </span><span class="locator">
Paragraph 6</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P6S1"></a>When a class B derives from a class A, it is a compile-time error for A to depend on B.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P6S2"></a>A class directly depends on its direct base class (if any) and directly depends on the class within which it is immediately nested (if any).</span> <span class="sentence"><span class="sentence-number">3</span> <a name="P6S3"></a>Given this definition, the complete set of classes upon which a class depends is the transitive closure of the directly depends on relationship.</span> </span><span class="paragraph"><span class="example">[Example: The example <pre class="code-example">
class A: B {}
class B: C {}
class C: A {}
</pre>is in error because the classes circularly depend on themselves. Likewise, the example <pre class="code-example">
class A: B.C {}
class B: A
{
public class C {}
}
</pre>results in a compile-time error because A depends on B.C (its direct base class), which depends on B (its immediately enclosing class), which circularly depends on A. end example]</span> </span><span class="locator">
Paragraph 7</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P7S1"></a>Note that a class does not depend on the classes that are nested within it.</span> <span class="example">[Example: In the example <pre class="code-example">
class A
{
class B: A {}
}
</pre></span></span><span class="paragraph"><span class="example">B depends on A (because A is both its direct base class and its immediately enclosing class), but A does not depend on B (since B is neither a base class nor an enclosing class of A). Thus, the example is valid. end example]</span> </span><span class="locator">
Paragraph 8</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P8S1"></a>It is not possible to derive from a sealed class.</span> <span class="example">[Example: In the example <pre class="code-example">
sealed class A {}
class B: A {} // Error, cannot derive from a sealed class
</pre>class B results in a compile-time error because it attempts to derive from the sealed class A. end example]</span> </span><span class="ruler"></span><table><tr><td><table align="left" bgcolor="navy"><tr bgcolor="navy"><td><font face="Arial,sans-serif" size="6" color="yellow"><strong>{ JSL }</strong></font></td></tr></table></td></tr><tr><td><font face="Arial,sans-serif" size="2" color="navy"><strong>Jagger Software Ltd</strong></font></td></tr><tr><td><font face="Arial,sans-serif" size="2" color="navy"><strong>Company # 4070126</strong></font></td></tr><tr><td><font face="Arial,sans-serif" size="2" color="navy"><strong>VAT # 762 5213 42</strong></font></td></tr></table><img src="valid-html401.png" align="left" height="31" width="88" alt="Valid HTML 4.01" /><img src="vcss.gif" align="left" height="31" width="88" alt="Valid CSS" /></body></html>⌨️ 快捷键说明
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