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Subject: Delegates and Events - C# Nuts and Bolt - Beta2
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<DIV class=3DSection1>
<P class=3DMsoNormal><IMG id=3D_x0000_i1025 height=3D48=20
src=3D"http://www.vijaymukhi.com/documents/books/csadv/bpb.jpg" =
width=3D64></P>
<P class=3DMsoNormal><![if =
!supportEmptyParas]><![endif]>&nbsp;<o:p></o:p></P>
<P class=3DMsoNormal><SPAN=20
style=3D"FONT-SIZE: 15pt; COLOR: black; FONT-FAMILY: Garamond">Chapter=20
5</SPAN></P>
<P class=3DMsoNormal><![if =
!supportEmptyParas]><![endif]>&nbsp;<o:p></o:p></P>
<P class=3DMsoNormal><SPAN=20
style=3D"FONT-SIZE: 26pt; COLOR: black; FONT-FAMILY: Garamond">Delegates =
and=20
Events</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">A delegate is =
extremely important=20
for C# as it is one of the four entities that can be placed in a =
namespace. This=20
makes it shareable among classes. Delegates are fully object oriented as =
they=20
entirely enclose or encapsulate an object instance and a method. A =
delegate=20
defines a class and extends System.Delegate. It can call any function as =
long as=20
the methods signature matches the delegates. This makes delegates ideal =
for=20
anonymous invocation. The methods signature only includes the return =
type and=20
the parameter list. If two delegates have the same parameter and return =
type,=20
that is, they share the same signature, we consider them as different =
delegates.=20
This chapter will feature these issues using simple programs.=20
<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 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>aa a =3D new aa();<o:p></o:p></P>
<P class=3DCprg>a.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 aa<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>public delegate void pqr();<o:p></o:p></P>
<P class=3DCprg>void<SPAN style=3D"mso-spacerun: yes">&nbsp; </SPAN>pqr1 =

()<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>System.Console.WriteLine("pqr1"); <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>pqr d<SPAN style=3D"mso-spacerun: yes">&nbsp; </SPAN>=3D =
new=20
pqr(pqr1);<o:p></o:p></P>
<P class=3DCprg>d();<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=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 is an object that =
looks like=20
aa. a.abc will call the abc function from this class. Earlier on, we =
specified=20
that new must be followed by the class. We stand corrected as usual =
here.=20
Likewise, we can also use a delegate name after new. A delegate is like =
a=20
property or an indexer. It is a first class member of a class. It looks =
like a=20
function, but has the keyword delegate in front of it along with the =
return type=20
of void.<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">When we use new to =
instantiate a=20
delegate, we pass it one parameter. This happens to be the name of a =
function,=20
namely, pqr1. This function is created in the same class, that is, aa, =
it takes=20
no parameters and returns a void. The keyword new obviously creates an =
object=20
that looks like a delegate as it was given a delegate as a datatype. d =
is now an=20
object that looks like pqr and stores the delegate object of pqr1. Then =
we use=20
the function invocation syntax and write d(). To our utter surprise, we =
have=20
actually executed the function pqr1 without writing pqr1().=20
<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 reasoning here is =
that d is a=20
delegate object for function pqr1 as we have passed it as a parameter at =
the=20
time of creation. Calling d calls function pqr1 instead. This is one =
level of=20
abstraction thus helping us write complicated =
code.<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 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>aa a =3D new aa();<o:p></o:p></P>
<P class=3DCprg>a.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 aa<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>public delegate void pqr();<o:p></o:p></P>
<P class=3DCprg>void<SPAN style=3D"mso-spacerun: yes">&nbsp; </SPAN>pqr1 =

()<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>System.Console.WriteLine("pqr"); <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>pqr d<SPAN style=3D"mso-spacerun: yes">&nbsp; </SPAN>=3D =
new=20
pqr(pqr1);<o:p></o:p></P>
<P class=3DCprg>d(100);<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>
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