HOW TO:初始化共享變量
2024-07-21 02:16:12
供稿：網(wǎng)友
“初始化共享變量”本不是一個(gè)how to問題。這里提出來，主要的再次認(rèn)識(shí)一下new。
在創(chuàng)建構(gòu)造函數(shù)時(shí)，new是很有趣的，sub new()有別于別的函數(shù)，因?yàn)樗梢猿霈F(xiàn)兩次。
public class testclass
shared sub new()
'...
end sub
sub new()
'...
end sub
'...
end class
當(dāng) dim t as new testclass時(shí)，它先執(zhí)行shared sub new()然后再執(zhí)行sub new()。shared sub new()只執(zhí)行一次，在實(shí)例類中，它在第一個(gè)實(shí)例產(chǎn)生時(shí)執(zhí)行。
運(yùn)用這個(gè)特性，shared sub new()在初始化類的共享變量時(shí)就會(huì)特別的方便。
下面作一個(gè)比較：
在靜態(tài)類中，
public class mycommand '使用shared sub new()
private shared cn as sqlclient.sqlconnection
shared sub new()
cn = new sqlclient.sqlconnection("persist security info=false;integrated security=sspi;database=northwind;server=(local)")
end sub
public shared function command(byval commandtext as string) as sqlclient.sqlcommand
return new sqlclient.sqlcommand(commandtext, cn)
end function
end class
public class mycommand0 '不使用shared sub new()
private shared cn as sqlclient.sqlconnection
public shared function command(byval commandtext as string) as sqlclient.sqlcommand
if cn is nothing then
cn = new sqlclient.sqlconnection("persist security info=false;integrated security=sspi;database=northwind;server=(local)")
end if
return new sqlclient.sqlcommand(commandtext, cn)
end function
end class
在實(shí)例類中，
public class mycommand1 '使用shared sub new()
private shared cn as sqlclient.sqlconnection
private _command as sqlclient.sqlcommand
shared sub new()
cn = new sqlclient.sqlconnection("persist security info=false;integrated security=sspi;database=northwind;server=(local)")
end sub
sub new()
_command = new sqlclient.sqlcommand("select * from orders", cn)
end sub
sub new(byval commandtext as string)
_command = new sqlclient.sqlcommand(commandtext, cn)
end sub
public readonly property command() as sqlclient.sqlcommand
get
return _command
end get
end property
'以下省略......
end class
public class mycommand2 '不使用shared sub new()
private shared cn as sqlclient.sqlconnection
private _command as sqlclient.sqlcommand
sub new()
if cn is nothing then
cn = new sqlclient.sqlconnection("persist security info=false;integrated security=sspi;database=northwind;server=(local)")
end if
_command = new sqlclient.sqlcommand("select * from orders", cn)
end sub
sub new(byval commandtext as string)
if cn is nothing then
cn = new sqlclient.sqlconnection("ersist security info=false;integrated security=sspi;database=northwind;server=(local)")
end if
_command = new sqlclient.sqlcommand(commandtext, cn)
end sub
public readonly property command() as sqlclient.sqlcommand
get
return _command
end get
end property
'以下省略......
end class
