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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="17.9.2.htm"><img src="previous.gif" alt="previous" border="0" /></a><a href="17.10.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="17.8.htm"><img src="previous.gif" alt="previous at this level" border="0" /></a><a href="17.10.htm"><img src="next.gif" alt="next at this level" border="0" /></a> <span class="clause-number-link"><a href="17.9.htm">17.9</a></span><span class="clause-title-previous"> Operators</span></span><span class="clause-depth"><a href="17.9.2.htm"><img src="previous.gif" alt="previous at this level" border="0" /></a><a href="17.9.3.htm"><img src="next.gif" alt="next at this level" border="0" /></a> <span class="clause-number">17.9.3</span><span class="clause-title"> Conversion operators</span></span><span class="locator">
Paragraph 1</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P1S1"></a>A conversion operator declaration introduces a user-defined conversion (<a href="13.4.htm">§13.4</a>), which augments the pre-defined implicit and explicit conversions.</span> </span><span class="locator">
Paragraph 2</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P2S1"></a>A conversion operator declaration that includes the implicit keyword introduces a user-defined implicit conversion.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P2S2"></a>Implicit conversions can occur in a variety of situations, including function member invocations, cast expressions, and assignments.</span> <span class="sentence"><span class="sentence-number">3</span> <a name="P2S3"></a>This is described further in <a href="13.1.htm">§13.1</a>.</span> </span><span class="locator">
Paragraph 3</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P3S1"></a>A conversion operator declaration that includes the explicit keyword introduces a user-defined explicit conversion.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P3S2"></a>Explicit conversions can occur in cast expressions, and are described further in <a href="13.2.htm">§13.2</a>.</span> </span><span class="locator">
Paragraph 4</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P4S1"></a>A conversion operator converts from a source type, indicated by the parameter type of the conversion operator, to a target type, indicated by the return type of the conversion operator.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P4S2"></a>A class or struct is permitted to declare a conversion from a source type S to a target type T provided all of the following are true: </span><ul><li><span class="sentence"><span class="sentence-number">3</span> <a name="P4S3"></a> S and T are different types.</span> </li><li><span class="sentence"><span class="sentence-number">4</span> <a name="P4S4"></a> Either S or T is the class or struct type in which the operator declaration takes place.</span> </li><li><span class="sentence"><span class="sentence-number">5</span> <a name="P4S5"></a> Neither S nor T is object or an <span class="non-terminal"><a href="11.2.htm#interface-type">interface-type</a></span>.</span> </li><li><span class="sentence"><span class="sentence-number">6</span> <a name="P4S6"></a> T is not a base class of S, and S is not a base class of T.</span> </li></ul></span><span class="locator">
Paragraph 5</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P5S1"></a>From the second rule it follows that a conversion operator must convert either to or from the class or struct type in which the operator is declared.</span> <span class="example">[Example: For example, it is possible for a class or struct type C to define a conversion from C to <span class="keyword">int</span> and from <span class="keyword">int</span> to C, but not from <span class="keyword">int</span> to <span class="keyword">bool</span>. end example]</span> </span><span class="locator">
Paragraph 6</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P6S1"></a>It is not possible to redefine a pre-defined conversion.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P6S2"></a>Thus, conversion operators are not allowed to convert from or to object because implicit and explicit conversions already exist between object and all other types.</span> <span class="sentence"><span class="sentence-number">3</span> <a name="P6S3"></a>Likewise, neither the source nor the target types of a conversion can be a base type of the other, since a conversion would then already exist.</span> </span><span class="locator">
Paragraph 7</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P7S1"></a>User-defined conversions are not allowed to convert from or to <span class="non-terminal"><a href="11.2.htm#interface-type">interface-type</a></span>s.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P7S2"></a>In particular, this restriction ensures that no user-defined transformations occur when converting to an <span class="non-terminal"><a href="11.2.htm#interface-type">interface-type</a></span>, and that a conversion to an <span class="non-terminal"><a href="11.2.htm#interface-type">interface-type</a></span> succeeds only if the object being converted actually implements the specified <span class="non-terminal"><a href="11.2.htm#interface-type">interface-type</a></span>.</span> </span><span class="locator">
Paragraph 8</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P8S1"></a>The signature of a conversion operator consists of the source type and the target type.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P8S2"></a>(Note that this is the only form of member for which the return type participates in the signature.)</span> <span class="sentence"><span class="sentence-number">3</span> <a name="P8S3"></a>The implicit or explicit classification of a conversion operator is not part of the operator's signature.</span> <span class="sentence"><span class="sentence-number">4</span> <a name="P8S4"></a>Thus, a class or struct cannot declare both an implicit and an explicit conversion operator with the same source and target types.</span> </span><span class="paragraph"><span class="note">[Note: In general, user-defined implicit conversions should be designed to never throw exceptions and never lose information. If a user-defined conversion can give rise to exceptions (for example, because the source argument is out of range) or loss of information (such as discarding high-order bits), then that conversion should be defined as an explicit conversion. end note]</span> </span><span class="paragraph"><span class="example">[Example: In the example <pre class="code-example">
using System;
public struct Digit
{
byte value;
public Digit(byte value) {
if (value < 0 || value > 9) throw new ArgumentException();
this.value = value;
}
public static implicit operator byte(Digit d) {
return d.value;
}
public static explicit operator Digit(byte b) {
return new Digit(b);
}
}
</pre>the conversion from Digit to <span class="keyword">byte</span> is implicit because it never throws exceptions or loses information, but the conversion from <span class="keyword">byte</span> to Digit is explicit since Digit can only represent a subset of the possible values of a <span class="keyword">byte</span>. 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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