📄 chapter 3.mht
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!supportEmptyParas]><![endif]> <o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext">Enums cannot derive =
from other=20
enums and can only have the above mentioned types as the underlying data =
types.<o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]> <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()<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>enum yyy <o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>a1,black,hell<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>class xxx : yyy<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]> <o:p></o:p></SPAN></P>
<P class=3DCoutput><U>Compiler Error</U><o:p></o:p></P>
<P class=3DCoutput>a.cs(11,7): error CS0509: 'xxx' : cannot inherit from =
sealed=20
class 'yyy'<o:p></o:p></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]> <o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext">Internally, an enum =
is treated as=20
a sealed class and thus follows all the rules pertaining to sealed=20
classes.<o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]> <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>}<o:p></o:p></P>
<P class=3DCprg>enum yyy <o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>a1,black,hell,a1<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]> <o:p></o:p></SPAN></P>
<P class=3DCoutput><U>Compiler Error</U><o:p></o:p></P>
<P class=3DCoutput>a.cs(6,15): error CS0102: The class 'yyy' already =
contains a=20
definition for 'a1'<o:p></o:p></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]> <o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext">An enum cannot have =
two members=20
with the same name. Every enum member has a value which if not specified =
starts=20
with 0 and the next enum member increments it by =
1.<o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]> <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()<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>System.Console.WriteLine((int)yyy.a1 + " " + =
(int)yyy.black + " "=20
+ (int)yyy.hell + " " + (int)yyy.a2);<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>enum yyy <o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>a1 =3D 4,black,hell =3D 9,a2<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]> <o:p></o:p></SPAN></P>
<P class=3DCoutput><U>Output</U><o:p></o:p></P>
<P class=3DCoutput>4 5 9 10<o:p></o:p></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]> <o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext">We can, however, =
specify the=20
constant values for the enum members. a1 starts at 4 and as we have not=20
specified a value for black, it is 5. Hell, though, starts at 9 and a2, =
the next=20
member, increases it by one thus getting a value of =
10.<o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]> <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()<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>enum yyy : byte<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>a1 =3D 400,black,hell =3D 9,a2<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]> <o:p></o:p></SPAN></P>
<P class=3DCoutput><U>Compiler Error</U><o:p></o:p></P>
<P class=3DCoutput>a.cs(9,6): error CS0031: Constant value '400' cannot =
be=20
converted to a 'byte'<o:p></o:p></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]> <o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext">If we try and go =
beyond the range=20
of values supported by the underlying data type, we receive an error =
similar to=20
the one we'd get if we initialize a byte to a value beyond its=20
capacity.<o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]> <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()<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>System.Console.WriteLine((int)yyy.a1 + " " + =
(int)yyy.black + " "=20
+ (int)yyy.hell + " " + (int)yyy.a2);<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>enum yyy <o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>a1 =3D 4,black,hell =3D 9,a2=3Da1<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]> <o:p></o:p></SPAN></P>
<P class=3DCoutput><U>Output</U><o:p></o:p></P>
<P class=3DCoutput>4 5 9 4<o:p></o:p></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext">Nothing stops us from =
initializing more than one enum member to the same constant=20
value.<o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]> <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()<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>yyy.a1 =3D 10;<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>enum yyy <o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>a1 =3D 4,black,hell =3D 9,a2=3Da1<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]> <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 CS0131: The left-hand side of an =
assignment=20
must be a variable, property or indexer<o:p></o:p></P>
<P class=3DCoutput>a.cs(5,10): error CS0029: Cannot implicitly convert =
type 'int'=20
to 'yyy'<o:p></o:p></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]> <o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext">An enum member =
behaves like a=20
constant whose value cannot be changed.<o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]> <o:p></o:p></SPAN></P>
<P class=3DCprg><U>a.cs</U><o:p></o:p></P>
<P class=3DCprg>using System;<o:p></o:p></P>
<P class=3DCprg>enum yyy<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>red,green,blue =3D 3<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>public static void Main() {<o:p></o:p></P>
<P class=3DCprg>Console.WriteLine(xxx(yyy.blue));<o:p></o:p></P>
<P class=3DCprg>Console.WriteLine(xxx(yyy.green));<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCprg>static string xxx(yyy c) {<o:p></o:p></P>
<P class=3DCprg>switch (c) {<o:p></o:p></P>
<P class=3DCprg><SPAN=20
style=3D"mso-tab-count: =
1"> =20
</SPAN>case yyy.red: <o:p></o:p></P>
<P class=3DCprg><SPAN=20
style=3D"mso-tab-count: =
1"> =20
</SPAN>return "red";<o:p></o:p></P>
<P class=3DCprg><SPAN=20
style=3D"mso-tab-count: =
1"> =20
</SPAN>case yyy.blue: <o:p></o:p></P>
<P class=3DCprg><SPAN=20
style=3D"mso-tab-count: =
1"> =20
</SPAN>return "blue";<o:p></o:p></P>
<P class=3DCprg><SPAN=20
style=3D"mso-tab-count: =
1"> =20
</SPAN>case yyy.green: <o:p></o:p></P>
<P class=3DCprg><SPAN=20
style=3D"mso-tab-count: =
1"> =20
</SPAN>return "green";<o:p></o:p></P>
<P class=3DCprg><SPAN=20
style=3D"mso-tab-count: =
1"> =20
</SPAN>default:<o:p></o:p></P>
<P class=3DCprg><SPAN=20
style=3D"mso-tab-count: =
1"> =20
</SPAN>return "invalid";<o:p></o:p></P>
<P class=3DCprg><SPAN=20
style=3D"mso-tab-count: =
2"> &nbs=
p; =20
</SPAN>}<o:p></o:p></P>
<P class=3DCprg><SPAN=20
style=3D"mso-tab-count: =
1"> =20
</SPAN>}<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]> <o:p></o:p></SPAN></P>
<P class=3DCoutput><U>Output</U><o:p></o:p></P>
<P class=3DCoutput>blue<o:p></o:p></P>
<P class=3DCoutput>green<o:p></o:p></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]> <o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext">Here we have created =
an enum that=20
has three members. The switch statement can take any data type and the =
case=20
statement uses the member names rather than numbers to compare for a =
match. This=20
makes our program more readable and easier to =
document.<o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]> <o:p></o:p></SPAN></P>
<P class=3DCprg><U>a.cs</U><o:p></o:p></P>
<P class=3DCprg>enum yyy {<o:p></o:p></P>
<P class=3DCprg>red =3D green ,green<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>public static void 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]> <o:p></o:p></SPAN></P>
<P class=3DCoutput><U>Compiler Error</U><o:p></o:p></P>
<P class=3DCoutput>a.cs(3,1): error CS0110: The evaluation of the =
constant value=20
for 'yyy.red' involves a circular definition<o:p></o:p></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]> <o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext">Similar to constants, =
we cannot=20
have a circular definition. Here red depends on green and the value of =
green is=20
one more than the value of red. This circular definition causes an=20
error.<o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]> <o:p></o:p></SPAN></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext">An enum refers to its =
own members=20
without a qualifier whereas any external identifier<SPAN=20
style=3D"mso-spacerun: yes"> </SPAN>must precede it<SPAN=20
style=3D"mso-spacerun: yes"> </SPAN>with the name of the enum. By =
default,=20
an enum and its members are public. We can explicitly use the public =
modifier if=20
we so choose to. The same accessibility rules of classes apply here=20
also.<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() <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>enum yyy : System.Enum<o:p></o:p></P>
<P class=3DCprg>{<o:p></o:p></P>
<P class=3DCprg>red ,green<o:p></o:p></P>
<P class=3DCprg>}<o:p></o:p></P>
<P class=3DCbase><SPAN style=3D"COLOR: windowtext"><![if =
!supportEmptyParas]><![endif]> <o:p></o:p></SPAN></P>
<P class=3DCoutput><U>Compiler Error</U><o:p></o:p></P>
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