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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="17.7.htm"><img src="previous.gif" alt="previous" border="0" /></a><a href="17.7.2.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="17.6.htm"><img src="previous.gif" alt="previous at this level" border="0" /></a><a href="17.8.htm"><img src="next.gif" alt="next at this level" border="0" /></a> <span class="clause-number-link"><a href="17.7.htm">17.7</a></span><span class="clause-title-previous"> Events</span></span><span class="clause-depth"><a href="17.7.1.htm"><img src="previous.gif" alt="previous at this level" border="0" /></a><a href="17.7.2.htm"><img src="next.gif" alt="next at this level" border="0" /></a> <span class="clause-number">17.7.1</span><span class="clause-title"> Field-like events</span></span><span class="locator">
Paragraph 1</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P1S1"></a>Within the program text of the class or struct that contains the declaration of an event, certain events can be used like fields.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P1S2"></a>To be used in this way, an event must not be abstract or extern, and must not explicitly include <span class="non-terminal"><a href="17.7.htm#event-accessor-declarations">event-accessor-declarations</a></span>.</span> <span class="sentence"><span class="sentence-number">3</span> <a name="P1S3"></a>Such an event can be used in any context that permits a field.</span> <span class="sentence"><span class="sentence-number">4</span> <a name="P1S4"></a>The field contains a delegate (<a href="22.htm">§22</a>), which refers to the list of event handlers that have been added to the event.</span> <span class="sentence"><span class="sentence-number">5</span> <a name="P1S5"></a>If no event handlers have been added, the field contains null.</span> </span><span class="paragraph"><span class="example">[Example: In the example <pre class="code-example">
public delegate void EventHandler(object sender, EventArgs e);
public class Button: Control
{
public event EventHandler Click;
protected void OnClick(EventArgs e) {
if (Click != null) Click(this, e);
}
public void Reset() {
Click = null;
}
}
</pre></span></span><span class="paragraph"><span class="example">Click is used as a field within the Button class. As the example demonstrates, the field can be examined, modified, and used in delegate invocation expressions. The OnClick method in the Button class "raises" the Click event. The notion of raising an event is precisely equivalent to invoking the delegate represented by the event-thus, there are no special language constructs for raising events. Note that the delegate invocation is preceded by a check that ensures the delegate is non-null. </span></span><span class="paragraph"><span class="example">Outside the declaration of the Button class, the Clickmember can only be used on the left-hand side of the += and -= operators, as in <pre class="code-example">
b.Click += new EventHandler(...);
</pre>which appends a delegate to the invocation list of the Click event, and <pre class="code-example">
b.Click -= new EventHandler(...);
</pre>which removes a delegate from the invocation list of the Click event. 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>When compiling a field-like event, the compiler automatically creates storage to hold the delegate, and creates accessors for the event that add or remove event handlers to the delegate field.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P2S2"></a>In order to be thread-safe, the addition or removal operations are done while holding the lock (<a href="15.12.htm">§15.12</a>) on the containing object for an instance event, or the type object (<a href="14.5.11.htm">§14.5.11</a>) for a static event.</span> </span><span class="paragraph"><span class="note">[Note: Thus, an instance event declaration of the form: <pre class="code-example">
class X {
public event D Ev;
}
</pre>could be compiled to something equivalent to: <pre class="code-example">
class X {
private D __Ev; // field to hold the delegate
public event D Ev {
add {
lock(this) { __Ev = __Ev + value; }
}
remove {
lock(this) { __Ev = __Ev - value; }
}
}
}
</pre></span></span><span class="paragraph"><span class="note">Within the class X, references to Ev are compiled to reference the hidden field __Ev instead. The name "__Ev" is arbitrary; the hidden field could have any name or no name at all. </span></span><span class="paragraph"><span class="note">Similarly, a static event declaration of the form: <pre class="code-example">
class X {
public static event D Ev;
}
</pre>could be compiled to something equivalent to: <pre class="code-example">
class X {
private static D __Ev; // field to hold the delegate
public static event D Ev {
add {
lock(typeof(X)) { __Ev = __Ev + value; }
}
remove {
lock(typeof(X)) { __Ev = __Ev - value; }
}
}
}
</pre>end note]</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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