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📁 C# Nuts and Bolt是学习C#的极好教程
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Subject: Types - C# Nuts and Bolt - Beta2
Date: Wed, 29 Aug 2007 13:28:22 +0800
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<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
4</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">Types</SPAN></P>
<P class=3DMsoNormal><![if =
!supportEmptyParas]><![endif]>&nbsp;<o:p></o:p></P>
<P class=3DCbase><B><SPAN=20
style=3D"FONT-SIZE: 14pt; COLOR: windowtext; FONT-FAMILY: Tahoma">Type=20
Declaration</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">A declaration creates =
a name in a=20
declaration space. The only declaration that can have the same name is a =

constructor, method, indexer or an operator. These are the only entities =
that=20
can be overloaded in a type. We cannot have a field and a method with =
the same=20
name in a declaration space. Each time we create a new class, struct or=20
interface, a new declaration space is created.<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 only entities in =
a=20
class/struct that can share the same name as the class/struct is a =
instance=20
constructor or a static constructor. Base class and derived classes live =
in=20
separate declaration space, hence the derived class can hide names =
created in=20
the base class. An enum also creates its own declaration space.=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">A local variable =
declaration=20
space is created by the {} braces or a block. The textual order of names =
created=20
is of no importance at all except in certain situations. The =
initialization of=20
the variables is carried out in the order they have been created. Local=20
variables must be defined prior to using them in the remaining program. =
The=20
enums declaration order is only important when constant expression =
values are=20
not used.<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">There are five =
different ways to=20
create our own types. These are class, struct, interface, enum or a =
delegate.=20
These type declarations can occur in three places only. The usual =
suspects are=20
namespaces, struct and the obvious classes. <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 compiler, =
basically,=20
understands only two types, viz, value types and reference types. It can =

seamlessly convert any value type into a reference type or an object. =
This=20
conversion of value to and from reference types is given a name, boxing =
and=20
unboxing.<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">A value type can be =
one of the=20
two, either a structure or an enum. The simple data types are predefined =

structures further divided into numeric, integer or floating point =
types. We=20
have nine integral types sbyte, byte, short, ushort, int, uint, long, =
ulong and=20
char and two floating point types float and =
double.<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 : int<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(1,7): error CS0509: 'zzz' : cannot inherit from =
sealed=20
class 'int'<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 cannot derive from =
any of the=20
value types as they are explicitly sealed. This deterrence restricts you =
from=20
tinkering around with the guts of the compiler unnecessarily.<SPAN=20
style=3D"mso-spacerun: yes">&nbsp; </SPAN><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">There is one =
fundamental=20
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