📄 14.15.htm
字号:
<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="14.14.htm"><img src="previous.gif" alt="previous" border="0" /></a><a href="14.16.htm"><img src="next.gif" alt="next" border="0" /></a></span><span class="clause-depth"><a href="13.htm"><img src="previous.gif" alt="previous at this level" border="0" /></a><a href="15.htm"><img src="next.gif" alt="next at this level" border="0" /></a> <span class="clause-number-link"><a href="14.htm">14</a></span><span class="clause-title-previous"> Expressions</span></span><span class="clause-depth"><a href="14.14.htm"><img src="previous.gif" alt="previous at this level" border="0" /></a><a href="14.16.htm"><img src="next.gif" alt="next at this level" border="0" /></a> <span class="clause-number">14.15</span><span class="clause-title"> Constant expressions</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 <span class="non-terminal"><a href="14.15.htm#constant-expression">constant-expression</a></span> is an expression that can be fully evaluated at compile-time.</span> <span class="grammar-production"><span class="name"><a name="constant-expression"></a>constant-expression</span> : <span class="rhs"><span class="non-terminal"><a href="14.14.htm#expression">expression</a></span> </span></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 type of a constant expression can be one of the following: <span class="keyword">sbyte</span>, <span class="keyword">byte</span>, <span class="keyword">short</span>, <span class="keyword">ushort</span>, <span class="keyword">int</span>, <span class="keyword">uint</span>, <span class="keyword">long</span>, <span class="keyword">ulong</span>, <span class="keyword">char</span>, <span class="keyword">float</span>, <span class="keyword">double</span>, <span class="keyword">decimal</span>, <span class="keyword">bool</span>, string, any enumeration type, or the null type.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P2S2"></a>The following constructs are permitted in constant expressions: </span><ul><li><span class="sentence"><span class="sentence-number">3</span> <a name="P2S3"></a> Literals (including the null literal).</span> </li><li><span class="sentence"><span class="sentence-number">4</span> <a name="P2S4"></a> References to const members of class and struct types.</span> </li><li><span class="sentence"><span class="sentence-number">5</span> <a name="P2S5"></a> References to members of enumeration types.</span> </li><li><span class="sentence"><span class="sentence-number">6</span> <a name="P2S6"></a> Parenthesized sub-expressions, which are themselves constant expressions.</span> </li><li><span class="sentence"><span class="sentence-number">7</span> <a name="P2S7"></a> Cast expressions, provided the target type is one of the types listed above.</span> </li><li><span class="sentence"><span class="sentence-number">8</span> <a name="P2S8"></a> The predefined +, -, !, and ~ unary operators.</span> </li><li><span class="sentence"><span class="sentence-number">9</span> <a name="P2S9"></a> The predefined +, -, *, /, %, <<, >>, &, |, ^, &&, ||, ==, !=, <, >, <=, and >= binary operators, provided each operand is of a type listed above.</span> </li><li><span class="sentence"><span class="sentence-number">10</span> <a name="P2S10"></a> The ?: conditional operator.</span> </li></ul></span><span class="locator">
Paragraph 3</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P3S1"></a>Whenever an expression is of one of the types listed above and contains only the constructs listed above, the expression is evaluated at compile-time.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P3S2"></a>This is true even if the expression is a sub-expression of a larger expression that contains non-constant constructs.</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 compile-time evaluation of constant expressions uses the same rules as run-time evaluation of non-constant expressions, except that where run-time evaluation would have thrown an exception, compile-time evaluation causes a compile-time error to occur.</span> </span><span class="locator">
Paragraph 5</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P5S1"></a>Unless a constant expression is explicitly placed in an unchecked context, overflows that occur in <span class="non-terminal"><a href="11.1.htm#integral-type">integral-type</a></span> arithmetic operations and conversions during the compile-time evaluation of the expression always cause compile-time errors (<a href="14.5.12.htm">§14.5.12</a>).</span> </span><span class="locator">
Paragraph 6</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P6S1"></a>Constant expressions occur in the contexts listed below.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P6S2"></a>In these contexts, a compile-time error occurs if an expression cannot be fully evaluated at compile-time.</span> <ul><li><span class="sentence"><span class="sentence-number">3</span> <a name="P6S3"></a> Constant declarations (<a href="17.3.htm">§17.3</a>).</span> </li><li><span class="sentence"><span class="sentence-number">4</span> <a name="P6S4"></a> Enumeration member declarations (<a href="21.30.htm">§21.30</a>).</span> </li><li><span class="sentence"><span class="sentence-number">5</span> <a name="P6S5"></a> case labels of a switch statement (<a href="15.7.2.htm">§15.7.2</a>).</span> </li><li><span class="sentence"><span class="sentence-number">6</span> <a name="P6S6"></a> goto case statements (<a href="15.9.3.htm">§15.9.3</a>).</span> </li><li><span class="sentence"><span class="sentence-number">7</span> <a name="P6S7"></a> Dimension lengths in an array creation expression (<a href="14.5.10.2.htm">§14.5.10.2</a>) that includes an initializer.</span> </li><li><span class="sentence"><span class="sentence-number">8</span> <a name="P6S8"></a> Attributes (<a href="24.htm">§24</a>).</span> </li></ul></span><span class="locator">
Paragraph 7</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P7S1"></a>An implicit constant expression conversion (<a href="13.1.6.htm">§13.1.6</a>) permits a constant expression of type <span class="keyword">int</span> to be converted to <span class="keyword">sbyte</span>, <span class="keyword">byte</span>, <span class="keyword">short</span>, <span class="keyword">ushort</span>, <span class="keyword">uint</span>, or <span class="keyword">ulong</span>, provided the value of the constant expression is within the range of the destination type.</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
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -