chapter 5.mht

来自「C# Nuts and Bolt是学习C#的极好教程」· MHT 代码 · 共 1,476 行 · 第 1/5 页

<P class=3DCprg>public class bb<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>public void xyz()<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>System.Console.WriteLine("xyz"); <o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]>&nbsp;<o:p></o:p></SPAN></P>
<P class=3DCoutput><U>Output</U><o:p></o:p></P>
<P class=3DCoutput>pqr1<o:p></o:p></P>
<P class=3DCoutput>xyz<o:p></o:p></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]>&nbsp;<o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext">A delegate is not =
confined to=20
executing code in its own class as its definition can be placed outside =
a class.=20
Here the delegate invocation list invokes two functions, one from the =
same class=20
pqr1 and the other function xyz from the class bb. Thus, the delegate =
not only=20
ignores the function it is calling but also the class from which the =
function is=20
called. It believes in equality thereby does not differentiate code from =
one=20
class to another.<o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]>&nbsp;<o:p></o:p></SPAN></P>
<P class=3DCbase><B><SPAN=20
style=3D"FONT-SIZE: 14pt; COLOR: windowtext; FONT-FAMILY: =
Tahoma">Events<o:p></o:p></SPAN></B></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]>&nbsp;<o:p></o:p></SPAN></P>
<P class=3DCprg><U>a.cs</U><o:p></o:p></P>
<P class=3DCprg>class zzz {<o:p></o:p></P>
<P class=3DCprg>public static void Main() {<o:p></o:p></P>
<P class=3DCprg>yyy l =3D new yyy();<o:p></o:p></P>
<P class=3DCprg>l.abc();<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>public class yyy<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>public void abc() <o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>System.Console.WriteLine("hi");<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCoutput><U>Output</U><o:p></o:p></P>
<P class=3DCoutput>hi<o:p></o:p></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]>&nbsp;<o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext">We start with a very =
simple=20
program. In function Main we create an object </SPAN><SPAN=20
style=3D"COLOR: windowtext; FONT-FAMILY: 'Trebuchet MS'">l</SPAN><SPAN=20
style=3D"COLOR: windowtext"> that looks like class yyy. Then we simply =
call a=20
function abc from it. In this case, we are calling the function abc =
directly.=20
There is no entity in between. Lets see how we can call this function=20
indirectly.<o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]>&nbsp;<o:p></o:p></SPAN></P>
<P class=3DCprg><U>a.cs</U><o:p></o:p></P>
<P class=3DCprg>public delegate void ddd();<o:p></o:p></P>
<P class=3DCprg>class zzz<SPAN style=3D"mso-spacerun: yes">&nbsp;=20
</SPAN>{<o:p></o:p></P>
<P class=3DCprg>public static void Main()<SPAN style=3D"mso-spacerun: =
yes">&nbsp;=20
</SPAN>{<o:p></o:p></P>
<P class=3DCprg>yyy l =3D new yyy();<o:p></o:p></P>
<P class=3DCprg>l.c +=3D new ddd(l.abc);<o:p></o:p></P>
<P class=3DCprg>l.a1();<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>public class yyy<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>public event ddd c;<o:p></o:p></P>
<P class=3DCprg>public void a1() <o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>if (c !=3D null)<o:p></o:p></P>
<P class=3DCprg>abc();<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>public void abc() <o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>System.Console.WriteLine("hi");<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]>&nbsp;<o:p></o:p></SPAN></P>
<P class=3DCoutput><U>Output</U><o:p></o:p></P>
<P class=3DCoutput>hi<o:p></o:p></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]>&nbsp;<o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext">We are doing exactly =
the same=20
thing as we did earlier but now its with a lot more fanfare. We started =
out by=20
creating a delegate ddd. We then instantiated an object that looks like =
the=20
delegate ddd and passed it one parameter, the name of the function. In =
our=20
specific case it is abc. As abc resides in a class yyy and we are =
operating in=20
function Main within class zzz, we have no choice but to give the full =
name,=20
</SPAN><SPAN=20
style=3D"COLOR: windowtext; FONT-FAMILY: 'Trebuchet MS'">l</SPAN><SPAN=20
style=3D"COLOR: windowtext">.abc. The object c on the left hand side is =
not a=20
delegate but an event. Also the syntax is +=3D and not =3D as it would =
give us the=20
following error.<o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]>&nbsp;<o:p></o:p></SPAN></P>
<P class=3DCoutput><U>Compiler Error</U><o:p></o:p></P>
<P class=3DCoutput>a.cs(5,3): error CS0070: The event 'yyy.c' can only =
appear on=20
the left hand side of +=3D or -=3D (except when used from within the =
type=20
'yyy')<o:p></o:p></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]>&nbsp;<o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext">The object c has an =
odd-looking=20
definition. Earlier we stored the return value of the instantiation of a =

delegate in a delegate object, here we have added the keyword event to =
the=20
object. Now we call function a1 from Main. In a1, we are checking the =
value of=20
the object c before calling abc. If it is not null, then we call the =
function=20
abc, otherwise we don't.<o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]>&nbsp;<o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext">If we comment out the =
line new=20
ddd, then the object c does not get initialized. It will retain its =
value of=20
null and the function abc will not be executed.<o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]>&nbsp;<o:p></o:p></SPAN></P>
<P class=3DCprg><U>a.cs</U><o:p></o:p></P>
<P class=3DCprg>public delegate void ddd();<o:p></o:p></P>
<P class=3DCprg>class zzz<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>public static void Main() <o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>yyy l =3D new yyy();<o:p></o:p></P>
<P class=3DCprg>l.c +=3D new ddd(l.abc);<o:p></o:p></P>
<P class=3DCprg>l.a1();<o:p></o:p></P>
<P class=3DCprg>l.c -=3D new ddd(l.abc);<o:p></o:p></P>
<P class=3DCprg>l.a1();<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>public class yyy<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>public event ddd c;<o:p></o:p></P>
<P class=3DCprg>public void a1() <o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>if (c !=3D null)<o:p></o:p></P>
<P class=3DCprg>abc();<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>public void abc() <o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>System.Console.WriteLine("hi");<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]>&nbsp;<o:p></o:p></SPAN></P>
<P class=3DCoutput><U>Output</U><o:p></o:p></P>
<P class=3DCoutput>hi<o:p></o:p></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]>&nbsp;<o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext">The first time we =
execute=20
</SPAN><SPAN=20
style=3D"COLOR: windowtext; FONT-FAMILY: 'Trebuchet MS'">l.</SPAN><SPAN=20
style=3D"COLOR: windowtext">a1, the object c has a value, it is not =
null. On the=20
next line we have -=3D where we are subtracting the delegate ddd from =
the event c.=20
The delegate stands for the function abc in each case. On subtraction, =
this=20
common function is eliminated and the event object c now has a value =
null. Thus=20
we added the function abc in the first round and removed it in the =
second=20
round.<o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]>&nbsp;<o:p></o:p></SPAN></P>
<P class=3DCprg><U>a.cs</U><o:p></o:p></P>
<P class=3DCprg>public delegate void ddd();<o:p></o:p></P>
<P class=3DCprg>class zzz<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>public static void Main() <o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>yyy l =3D new yyy();<o:p></o:p></P>
<P class=3DCprg>l.c +=3D new ddd(l.abc);<o:p></o:p></P>
<P class=3DCprg>l.c();<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>public class yyy<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>public event ddd c;<o:p></o:p></P>
<P class=3DCprg>public void abc() <o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>System.Console.WriteLine("hi");<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]>&nbsp;<o:p></o:p></SPAN></P>
<P class=3DCoutput><U>Compiler Error</U><o:p></o:p></P>
<P class=3DCoutput>a.cs(8,3): error CS0070: The event 'yyy.c' can only =
appear on=20
the left hand side of +=3D or -=3D (except when used from within the =
type=20
'yyy')<o:p></o:p></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]>&nbsp;<o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext">Unlike a delegate, an =
event=20
object cannot be used directly. Earlier, we used to execute a function =
through=20
the delegate object. That is not permitted with events. Events are =
employed for=20
notification purposes only.<o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]>&nbsp;<o:p></o:p></SPAN></P>
<P class=3DCprg><U>a.cs</U><o:p></o:p></P>
<P class=3DCprg>public delegate void ddd();<o:p></o:p></P>
<P class=3DCprg>class zzz<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>public static void Main() <o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>yyy l =3D new yyy();<o:p></o:p></P>
<P class=3DCprg>l.c +=3D new ddd(l.abc);<o:p></o:p></P>
<P class=3DCprg>l.pqr();<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>public class yyy<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>public event ddd c;<o:p></o:p></P>
<P class=3DCprg>public void pqr()<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>c();<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>public void abc() <o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>System.Console.WriteLine("abc");<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]>&nbsp;<o:p></o:p></SPAN></P>
<P class=3DCoutput><U>Output</U><o:p></o:p></P>
<P class=3DCoutput>abc<o:p></o:p></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]>&nbsp;<o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext">If you are still =
ignorant about=20
the use of an event, you are totally blameless. Whatever we have done so =
far=20
could have easily been done without events. In the above case, we have=20
associated an event c with a function abc in class yyy. We then execute =
function=20
pqr by giving c(). This executes the event object c. Earlier, a similar =
thing=20
was not possible as we were not in the same class. Now as the event =
object c=20
stands for a function abc, it calls that function. =
<o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]>&nbsp;<o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]>&nbsp;<o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext">The next program =
demonstrates=20
that we can call as many functions we want.<o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]>&nbsp;<o:p></o:p></SPAN></P>
<P class=3DCprg><U>a.cs</U><o:p></o:p></P>
<P class=3DCprg>public delegate void ddd(); <o:p></o:p></P>
<P class=3DCprg>class zzz {<o:p></o:p></P>
<P class=3DCprg>public static void Main() <o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>yyy l =3D new yyy();<o:p></o:p></P>
<P class=3DCprg>l.c +=3D new ddd(l.abc);<o:p></o:p></P>
<P class=3DCprg>l.c +=3D new ddd(l.xyz);<o:p></o:p></P>
<P class=3DCprg>l.pqr();<o:p></o:p></P>