chapter 1.mht

C# Nuts and Bolt是学习C#的极好教程

</SPAN>at=20
zzz.Main()<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">All exceptions thrown =
at run time=20
can be caught using try-catch. In this way, any unforeseen application =
error can=20
be caught at run time gracefully. <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>class zzz {<o:p></o:p></P>
<P class=3DCprg>public static void Main() {<o:p></o:p></P>
<P class=3DCprg>yyy a;<o:p></o:p></P>
<P class=3DCprg>a=3Dnew yyy();<o:p></o:p></P>
<P class=3DCprg>try<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>a.abc();<o:p></o:p></P>
<P class=3DCprg>try<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>a.pqr();<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>catch (System.Exception e)<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>System.Console.WriteLine("In inner try" +=20
e.ToString());<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>finally<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>System.Console.WriteLine("In inner =
finally");<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>System.Console.WriteLine("Bye");<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>catch (System.Exception e)<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>System.Console.WriteLine("In Exception" +=20
e.ToString());<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>finally<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>System.Console.WriteLine("In finally" );<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>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>//throw new System.Exception();<o:p></o:p></P>
<P class=3DCprg>}<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>throw new System.Exception();<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<o:p></o:p></U></P>
<P class=3DCoutput>In inner trySystem.Exception: Exception of type=20
System.Exception was thrown.<o:p></o:p></P>
<P class=3DCoutput><SPAN style=3D"mso-spacerun: yes">&nbsp;&nbsp; =
</SPAN>at=20
yyy.pqr()<o:p></o:p></P>
<P class=3DCoutput><SPAN style=3D"mso-spacerun: yes">&nbsp;&nbsp; =
</SPAN>at=20
zzz.Main()<o:p></o:p></P>
<P class=3DCoutput>In inner finally<o:p></o:p></P>
<P class=3DCoutput>Bye<o:p></o:p></P>
<P class=3DCoutput>In finally<o:p></o:p></P>
<P class=3DCoutput><![if =
!supportEmptyParas]><![endif]>&nbsp;<o:p></o:p></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext">Interesting !!!=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">Basically, you are =
allowed to=20
have as many try-catch blocks in your code as your heart's desire. Every =
time=20
you execute a function, that function may throw an exception. So you can =
place=20
the function call in a try-catch block. Moreover, within a catch block, =
you can=20
have another try-catch which should be considered only when there is a =
function=20
call in the catch. <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">In the program, abc =
does not=20
throw any exception but pqr does. The catch that catches it is the inner =
catch=20
and not the outer catch. Hence, the outer catch is not called. =
Conversely, the=20
finally for both the trys is called, the order is from the inner to the =
outer=20
resulting into seeing bye before last. Finally waits for the try-catch =
to=20
complete its task.<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">Namespaces</SPAN></B><SPAN=20
style=3D"COLOR: windowtext"><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">C# programs are =
organized using=20
namespaces which provide an organizational system for presenting =
programs. A=20
source file is defined as a compilation unit. A C# program can consist =
of one or=20
more compilation units. When we try and compile a C# program, all =
compilation=20
units are compiled together and hence can depend on each=20
other.<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 namespace vijay<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(1,8): error CS1518: Expected class, delegate, =
enum,=20
interface, or struct<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 namespace is =
explicitly public=20
and we are not allowed to specify any access modifiers to=20
it.<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(string[] a)<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>using System;<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P=20
class=3DCoutput><U><![if =
!supportEmptyParas]><![endif]>&nbsp;<o:p></o:p></U></P>
<P class=3DCoutput><U>Compiler Error</U><o:p></o:p></P>
<P class=3DCoutput>a.cs(5,7): error CS1003: Syntax error, '('=20
expected<o:p></o:p></P>
<P class=3DCoutput>a.cs(5,13): error CS1026: ) expected<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 using directive =
can only be=20
placed at the beginning of the program. It cannot be used anywhere else. =
The=20
scope of using is the current source file and it does not influence any =
other=20
programs included while compiling. <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(string[] a)<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>namespace n1<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>class yyy<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>namespace n1<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>class yy<o:p></o:p></P>
<P class=3DCprg>{<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=3DCbase><SPAN style=3D"COLOR: windowtext">Namespaces are =
open-ended, the=20
net result here will be that the namespace n1 will contain both classes =
-- yy=20
and yyy. However, we cannot change the name of the second class to yyy =
as we=20
cannot have the same name in the same namespace.<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>using z =3D a1.a2.yyy;<o:p></o:p></P>
<P class=3DCprg>using z1 =3D a1.a2;<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(string[] a)<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>z b;<o:p></o:p></P>
<P class=3DCprg>z1.yyy<SPAN style=3D"mso-spacerun: yes">&nbsp;=20
</SPAN>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>namespace a1.a2<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>class yyy {}<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=3DCbase><SPAN style=3D"COLOR: windowtext">Using nested =
namespaces can make=20
our names too large. We are provided with some help in the form of an =
alias in=20
the using directive. In the program, we have created two aliases -- z =
and z1.=20
Wherever we write a1.a2.yyy, the full name of the class yyy, it can now =
be=20
replaced with z. z1 stands for only the namespace name and thus we have =
to=20
specify the name of the class. An alias is conceptually very simple. =
Wherever C#=20
sees a z, it will blindly replace it by a1.a2.yyy.<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>using z =3D a1.a2.yyy;<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(string[] a)<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>namespace a1.a2<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>class yyy {}<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>class z<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P=20
class=3DCoutput><U><![if =
!supportEmptyParas]><![endif]>&nbsp;<o:p></o:p></U></P>
<P class=3DCoutput><U>Compiler Error</U><o:p></o:p></P>
<P class=3DCoutput>a.cs(1,7): error CS0576: Namespace '' already =
contains an alias=20
definition for 'z'<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">You cannot have a =
class name and=20
an alias with similar names as this will confuse the compiler. Hence the =

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=3DCprg><U>a.cs</U><o:p></o:p></P>
<P class=3DCprg>namespace a1.a2<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>using z =3D a1.a2.yyy;<o:p></o:p></P>
<P class=3DCprg>class yyy {}<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>namespace a3<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>class ddd : z<o:p></o:p></P>
<P class=3DCprg>{<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(8,13): error CS0246: The type or namespace name =
'z' could=20
not be found (are you missing a using directive or an assembly=20
reference?)<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 class zzz remains =
the same as=20
before. An alias created in one namespace or compilation unit is simply =
not=20
available to another. The scope of z is only limited to namespace=20
a1.a2.<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>using z =3D a1.a2.yyy;<o:p></o:p></P>
<P class=3DCprg>namespace a1.a2<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p><