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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="19.4.htm"><img src="previous.gif" alt="previous" border="0" /></a><a href="19.6.htm"><img src="next.gif" alt="next" border="0" /></a></span><span class="clause-depth"><a href="18.htm"><img src="previous.gif" alt="previous at this level" border="0" /></a><a href="20.htm"><img src="next.gif" alt="next at this level" border="0" /></a> <span class="clause-number-link"><a href="19.htm">19</a></span><span class="clause-title-previous"> Arrays</span></span><span class="clause-depth"><a href="19.4.htm"><img src="previous.gif" alt="previous at this level" border="0" /></a><a href="19.6.htm"><img src="next.gif" alt="next at this level" border="0" /></a> <span class="clause-number">19.5</span><span class="clause-title"> Array covariance</span></span><span class="locator">
Paragraph 1</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P1S1"></a>For any two <span class="non-terminal"><a href="11.2.htm#reference-type">reference-type</a></span>s A and B, if an implicit reference conversion (<a href="13.1.4.htm">§13.1.4</a>) or explicit reference conversion (<a href="13.2.3.htm">§13.2.3</a>) exists from A to B, then the same reference conversion also exists from the array type A[R] to the array type B[R], where R is any given <span class="non-terminal"><a href="19.1.htm#rank-specifier">rank-specifier</a></span> (but the same for both array types).</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P1S2"></a>This relationship is known as array covariance.</span> <span class="sentence"><span class="sentence-number">3</span> <a name="P1S3"></a>Array covariance, in particular, means that a value of an array type A[R] may actually be a reference to an instance of an array type B[R], provided an implicit reference conversion exists from B to A.</span> </span><span class="locator">
Paragraph 2</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P2S1"></a>Because of array covariance, assignments to elements of reference type arrays include a run-time check which ensures that the value being assigned to the array element is actually of a permitted type (<a href="14.13.1.htm">§14.13.1</a>).</span> </span><span class="paragraph"><span class="example">[Example: For example: <pre class="code-example">
class Test
{
static void Fill(object[] array, int index, int count, object value) {
for (int i = index; i < index + count; i++) array[i] = value;
}
static void Main() {
string[] strings = new string[100];
Fill(strings, 0, 100, "Undefined");
Fill(strings, 0, 10, null);
Fill(strings, 90, 10, 0);
}
}
</pre></span></span><span class="paragraph"><span class="example">The assignment to array[i] in the Fill method implicitly includes a run-time check, which ensures that the object referenced by value is either null or an instance of a type that is compatible with the actual element type of array. In Main, the first two invocations of Fill succeed, but the third invocation causes a System.ArrayTypeMismatchException to be thrown upon executing the first assignment to array[i]. The exception occurs because a boxed <span class="keyword">int</span> cannot be stored in a string array. end example]</span> </span><span class="locator">
Paragraph 3</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P3S1"></a>Array covariance specifically does not extend to arrays of <span class="non-terminal"><a href="11.1.htm#value-type">value-type</a></span>s.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P3S2"></a>For example, no conversion exists that permits an int[] to be treated as an object[].</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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