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📁 C# Nuts and Bolt是学习C#的极好教程
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Subject: Concepts Revisited - C# Nuts and Bolt - Beta2
Date: Wed, 29 Aug 2007 13:27:37 +0800
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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
1</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">Concepts=20
Revisited</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">Entry=20
Point</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">The first function to =
be called=20
in any C# program is Main. Obviously someone at Microsoft tossed a coin =
and as=20
it fell 'heads', they chose Main. Had it been 'tails', one wonders what =
the=20
first function would have been called. As everything starts from Main, =
it is=20
also called the entry point of the program. It is the fountainhead of =
all=20
knowledge. Incidentally, the C programming language also calls Main as =
its first=20
function for all C-language programs.<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">We can have as many =
as four=20
different ways to declare Main in our program. They are as=20
follows:<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>static void Main() {...}<o:p></o:p></P>
<P class=3DCprg>static int Main() {...}<o:p></o:p></P>
<P class=3DCprg>static void Main(string[] a) {...}<o:p></o:p></P>
<P class=3DCprg>static int Main(string[] args) {...}<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 operating system =
calls Main=20
and waits for it to return a value. This value denotes the success or =
failure of=20
the program.<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">Main can either =
return a number=20
or result in no return value, that is, void. If it returns an int, by=20
convention, a value of zero means success and any other value indicates =
an=20
error. No international authority can standardize the error numbers, it =
largely=20
depends on the programmer himself. <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">Copy as a command =
takes two=20
parameters; the source and destination files. Similarly, a C# program =
can also=20
accept command line parameters at runtime. The executable<SPAN=20
style=3D"mso-spacerun: yes">&nbsp; </SPAN>a.exe can be entered=20
as<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>&gt;a one two three<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=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 Main(string[] a)<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>for ( i=3D0; i&lt; a.Length; i++)<o:p></o:p></P>
<P class=3DCprg>System.Console.WriteLine(a[i]);<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>one<o:p></o:p></P>
<P class=3DCoutput>two<o:p></o:p></P>
<P class=3DCoutput>three<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">one, two, three are =
called=20
command line parameters. The program accepts them in an array of =
strings. As the=20
array is a parameter to the function, we are free to decide on its name. =
Every=20
array has a member called Length, which tells us the size of the array. =
In our=20
case, it is three. Thus, a[0] will contain the first word one and not =
the name=20
of the program, a[1] will contain two and a[3] - three. Main now behaves =
like=20
any other function. What we do next with the command line arguments =
depends=20
entirely upon us. <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 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>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=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(3,20): error CS0017: Program 'a.exe' has more =
than one=20
entry point defined: 'zzz.Main(string[])'<o:p></o:p></P>
<P class=3DCoutput>a.cs(6,19): error CS0017: Program 'a.exe' has more =
than one=20
entry point defined: '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">You can have one and =
only one=20
function called Main in any C# program. Even though you can call it with =

different parameters, with the name changing, Main as a function must be =
given=20
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