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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="16.3.htm"><img src="previous.gif" alt="previous" border="0" /></a><a href="16.3.2.htm"><img src="next.gif" alt="next" border="0" /></a></span><span class="clause-depth"><a href="15.htm"><img src="previous.gif" alt="previous at this level" border="0" /></a><a href="17.htm"><img src="next.gif" alt="next at this level" border="0" /></a> <span class="clause-number-link"><a href="16.htm">16</a></span><span class="clause-title-previous"> Namespaces</span></span><span class="clause-depth"><a href="16.2.htm"><img src="previous.gif" alt="previous at this level" border="0" /></a><a href="16.4.htm"><img src="next.gif" alt="next at this level" border="0" /></a> <span class="clause-number-link"><a href="16.3.htm">16.3</a></span><span class="clause-title-previous"> Using directives</span></span><span class="clause-depth"><a href="16.3.1.htm"><img src="previous.gif" alt="previous at this level" border="0" /></a><a href="16.3.2.htm"><img src="next.gif" alt="next at this level" border="0" /></a> <span class="clause-number">16.3.1</span><span class="clause-title"> Using alias directives</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="16.3.1.htm#using-alias-directive">using-alias-directive</a></span> introduces an identifier that serves as an alias for a namespace or type within the immediately enclosing compilation unit or namespace body.</span> <span class="grammar-production"><span class="name"><a name="using-alias-directive"></a>using-alias-directive</span> : <span class="rhs"><span class="keyword">using</span> <span class="non-terminal"><a href="9.4.2.htm#identifier">identifier</a></span> <span class="terminal">=</span> <span class="non-terminal"><a href="10.8.htm#namespace-or-type-name">namespace-or-type-name</a></span> <span class="terminal">;</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>Within member declarations in a compilation unit or namespace body that contains a <span class="non-terminal"><a href="16.3.1.htm#using-alias-directive">using-alias-directive</a></span>, the identifier introduced by the <span class="non-terminal"><a href="16.3.1.htm#using-alias-directive">using-alias-directive</a></span> can be used to reference the given namespace or type.</span> </span><span class="paragraph"><span class="example">[Example: For example: <pre class="code-example">
namespace N1.N2
{
class A {}
}
namespace N3
{
using A = N1.N2.A;
class B: A {}
}
</pre></span></span><span class="paragraph"><span class="example">Above, within member declarations in the N3 namespace, A is an alias for N1.N2.A, and thus class N3.B derives from class N1.N2.A. The same effect can be obtained by creating an alias R for N1.N2 and then referencing R.A: <pre class="code-example">
namespace N3
{
using R = N1.N2;
class B: R.A {}
}
</pre>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>The identifier of a <span class="non-terminal"><a href="16.3.1.htm#using-alias-directive">using-alias-directive</a></span> must be unique within the declaration space of the compilation unit or namespace that immediately contains the <span class="non-terminal"><a href="16.3.1.htm#using-alias-directive">using-alias-directive</a></span>.</span> <span class="example">[Example: For example: <pre class="code-example">
namespace N3
{
class A {}
}
namespace N3
{
using A = N1.N2.A; // Error, A already exists
}
</pre></span></span><span class="paragraph"><span class="example">Above, N3 already contains a member A, so it is a compile-time error for a <span class="non-terminal"><a href="16.3.1.htm#using-alias-directive">using-alias-directive</a></span> to use that identifier. end example]</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P3S2"></a>Likewise, it is a compile-time error for two or more <span class="non-terminal"><a href="16.3.1.htm#using-alias-directive">using-alias-directive</a></span>s in the same compilation unit or namespace body to declare aliases by the same name.</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 <span class="non-terminal"><a href="16.3.1.htm#using-alias-directive">using-alias-directive</a></span> makes an alias available within a particular compilation unit or namespace body, but it does not contribute any new members to the underlying declaration space.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P4S2"></a>In other words, a <span class="non-terminal"><a href="16.3.1.htm#using-alias-directive">using-alias-directive</a></span> is not transitive, but, rather, affects only the compilation unit or namespace body in which it occurs.</span> </span><span class="paragraph"><span class="example">[Example: In the example <pre class="code-example">
namespace N3
{
using R = N1.N2;
}
namespace N3
{
class B: R.A {} // Error, R unknown
}
</pre>the scope of the <span class="non-terminal"><a href="16.3.1.htm#using-alias-directive">using-alias-directive</a></span> that introduces R only extends to member declarations in the namespace body in which it is contained, so R is unknown in the second namespace declaration. However, placing the <span class="non-terminal"><a href="16.3.1.htm#using-alias-directive">using-alias-directive</a></span> in the containing compilation unit causes the alias to become available within both namespace declarations: <pre class="code-example">
using R = N1.N2;
namespace N3
{
class B: R.A {}
}
namespace N3
{
class C: R.A {}
}
</pre>end example]</span> </span><span class="locator">
Paragraph 5</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P5S1"></a>Just like regular members, names introduced by <span class="non-terminal"><a href="16.3.1.htm#using-alias-directive">using-alias-directive</a></span>s are hidden by similarly named members in nested scopes.</span> <span class="example">[Example: In the example <pre class="code-example">
using R = N1.N2;
namespace N3
{
class R {}
class B: R.A {} // Error, R has no member A
}
</pre>the reference to R.A in the declaration of B causes a compile-time error because R refers to N3.R, not N1.N2. 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>The order in which <span class="non-terminal"><a href="16.3.1.htm#using-alias-directive">using-alias-directive</a></span>s are written has no significance, and resolution of the <span class="non-terminal"><a href="10.8.htm#namespace-or-type-name">namespace-or-type-name</a></span> referenced by a <span class="non-terminal"><a href="16.3.1.htm#using-alias-directive">using-alias-directive</a></span> is not affected by the <span class="non-terminal"><a href="16.3.1.htm#using-alias-directive">using-alias-directive</a></span> itself or by other <span class="non-terminal"><a href="16.3.htm#using-directive">using-directive</a></span>s in the immediately containing compilation unit or namespace body.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P6S2"></a>In other words, the <span class="non-terminal"><a href="10.8.htm#namespace-or-type-name">namespace-or-type-name</a></span> of a <span class="non-terminal"><a href="16.3.1.htm#using-alias-directive">using-alias-directive</a></span> is resolved as if the immediately containing compilation unit or namespace body had no <span class="non-terminal"><a href="16.3.htm#using-directive">using-directive</a></span>s.</span> <span class="example">[Example: In the example <pre class="code-example">
namespace N1.N2 {}
namespace N3
{
using R1 = N1; // OK
using R2 = N1.N2; // OK
using R3 = R1.N2; // Error, R1 unknown
}
</pre>the last <span class="non-terminal"><a href="16.3.1.htm#using-alias-directive">using-alias-directive</a></span> results in a compile-time error because it is not affected by the first <span class="non-terminal"><a href="16.3.1.htm#using-alias-directive">using-alias-directive</a></span>. 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>A <span class="non-terminal"><a href="16.3.1.htm#using-alias-directive">using-alias-directive</a></span> can create an alias for any namespace or type, including the namespace within which it appears and any namespace or type nested within that namespace.</span> </span><span class="locator">
Paragraph 8</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P8S1"></a>Accessing a namespace or type through an alias yields exactly the same result as accessing that namespace or type through its declared name.</span> <span class="example">[Example: For example, given <pre class="code-example">
namespace N1.N2
{
class A {}
}
namespace N3
{
using R1 = N1;
using R2 = N1.N2;
class B
{
N1.N2.A a; // refers to N1.N2.A
R1.N2.A b; // refers to N1.N2.A
R2.A c; // refers to N1.N2.A
}
}
</pre>the names N1.N2.A, R1.N2.A, and R2.A are equivalent and all refer to the class whose fully qualified name is N1.N2.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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