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📄 chapter 1.mht

📁 C# Nuts and Bolt是学习C#的极好教程
💻 MHT
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only once.<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>{<o:p></o:p></P>
<P class=3DCprg>public static void<SPAN style=3D"mso-spacerun: =
yes">&nbsp;=20
</SPAN>Main(int i)<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 Warning</U><o:p></o:p></P>
<P class=3DCoutput>a.cs(3,21): warning CS0028: 'zzz.Main(int)' has the =
wrong=20
signature to be an entry point<o:p></o:p></P>
<P class=3DCoutput><![if =
!supportEmptyParas]><![endif]>&nbsp;<o:p></o:p></P>
<P class=3DCoutput><U>Compiler Error</U><o:p></o:p></P>
<P class=3DCoutput>error CS5001: Program 'a.exe' does not have an entry =
point=20
defined<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">Here, the compiler =
first displays=20
a warning signaling us that Main has not been created with the right =
parameters.=20
The error, following the warning, proclaims that we have forgotten to =
create a=20
function called Main. The signature includes the return type only in =
special=20
cases as entry point.<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>{<o:p></o:p></P>
<P class=3DCprg>public static long Main()<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>return 0;<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 Warning</U><o:p></o:p></P>
<P class=3DCoutput>a.cs(3,20): warning CS0028: 'zzz.Main()' has the =
wrong=20
signature to be an entry point<o:p></o:p></P>
<P class=3DCoutput><![if =
!supportEmptyParas]><![endif]>&nbsp;<o:p></o:p></P>
<P class=3DCoutput><U>Compiler Error</U><o:p></o:p></P>
<P class=3DCoutput>error CS5001: Program 'a.exe' does not have an entry =
point=20
defined<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 signature refers =
to the=20
parameters given to the function plus the return value. Main in the =
above=20
program returns 'long', hence we see 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>class zzz<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>public void<SPAN style=3D"mso-spacerun: yes">&nbsp;=20
</SPAN>Main()<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>error CS5001: Program 'a.exe' does not have an entry =
point=20
defined<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 signature also =
includes=20
modifiers like static etc. It just proves the importance of the function =
Main=20
and the way it has been defined.<o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><SPAN=20
style=3D"mso-spacerun: yes">&nbsp;</SPAN><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<SPAN style=3D"mso-spacerun: =
yes">&nbsp;=20
</SPAN>Main()<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 static void<SPAN style=3D"mso-spacerun: =
yes">&nbsp;=20
</SPAN>Main()<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><![if =
!supportEmptyParas]><![endif]>&nbsp;<o:p></o:p></P>
<P class=3DCoutput><U>Compiler Error</U><o:p></o:p></P>
<P class=3DCoutput>a.cs(2,21): error CS0017: Program 'a.exe' has more =
than one=20
entry point defined: 'zzz.Main()'<o:p></o:p></P>
<P class=3DCoutput>a.cs(8,21): error CS0017: Program 'a.exe' has more =
than one=20
entry point defined: 'yyy.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">The error proves the =
point that=20
you cannot have two different classes, zzz and yyy, containing Main. =
Only one=20
occurrence of Main is allowed. You always have only one chance of a =
lifetime.=20
Ditto with Main.<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>{<o:p></o:p></P>
<P class=3DCprg>private static void<SPAN style=3D"mso-spacerun: =
yes">&nbsp;=20
</SPAN>Main()<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>System.Console.WriteLine("hell");<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>hell<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 access modifiers =
are not=20
included within the signature for Main. Even though Main is made =
private, it is=20
considered as an entry point function, hence hell gets displayed. This =
function=20
is unique and works as a special case.<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 static void<SPAN style=3D"mso-spacerun: =
yes">&nbsp;=20
</SPAN>Main() {<o:p></o:p></P>
<P class=3DCprg>System.Console.WriteLine("hell");<o:p></o:p></P>
<P class=3DCprg>}<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>}<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(1,15): 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">You cannot create a =
function=20
outside a class or a structure. This rule has to be strictly adhered to =
even for=20
Main.<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 int Main()<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>return;<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(5,1): error CS0126: An object of a type =
convertible to=20
'int' is required<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">In this example, the =
function=20
Main has to return an int and we are not returning any value with the =
keyword=20
return. The compiler tries to convert a nothing into an int and hence =
the error.=20
This is a generic error that occurs whenever the compiler tries to =
convert a=20
data type into another and is not successful.<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>{<o:p></o:p></P>
<P class=3DCprg>public static int Main()<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(3,19): error CS0161: 'zzz.Main()': not all code =
paths=20
return a value<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 compiler's error =
messages=20
need the services of an editor. The function Main should return an int, =
but we=20
forgot to do so. This results in an error. Thus whenever an entity =
should return=20
a value, we should return such a data type or be prepared for an=20
error.<o:p></o:p></SPAN></P>
<P class=3DCbase><B><SPAN=20
style=3D"FONT-SIZE: 14pt; COLOR: windowtext; FONT-FAMILY: =
Tahoma">Scope</SPAN></B><B><SPAN=20
style=3D"FONT-SIZE: 18pt; COLOR: windowtext; FONT-FAMILY: Tahoma">=20
</SPAN></B><SPAN 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">Scope, as a concept, =
defines the=20
region within which a user-defined entity is visible and can be referred =
to.=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>int i;<o:p></o:p></P>
<P class=3DCprg>zzz i;<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(6,5): error CS0128: A local variable named 'i' =
is already=20
defined in this scope<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">Twins are always a =
problem. We=20
cannot have any two entities of the same name as the compiler will get =
confused=20
on whether we are referring to the i which is an int or a zzz. Thus =
never have=20
duplicate names even though at times they are =
allowed.<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>{<o:p></o:p></P>
<P class=3DCprg>public static void<SPAN style=3D"mso-spacerun: =
yes">&nbsp;=20
</SPAN>Main() <o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>int i =3D 8;<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>i =3D 90;<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(8,1): error CS0103: The name 'i' does not exist =
in the=20
class or namespace 'zzz'<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 variable is visible =
from the=20
position it is created to the first close bracket. As i has been created =
within=20
the inner brackets, it is not available to external code thereafter. =
Hence the=20
error.<o:p></o:p></SPAN></P>
<P class=3DCprg><U><![if =
!supportEmptyParas]><![endif]>&nbsp;<o:p></o:p></U></P>
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