精通ASP.NET（基于VB.NET)( 三）VB.NET 异常处理
（1）结构化异常处理:：
try
...........
catch
............
flnally
................
end try
其中Catch的结构有三种：Catch、Catch...As 和 Catch...When。其中Catch...As 和 Catch...When 子句捕捉特定的异常，并允许相关的语句块指定应用程序如何处理。
例：（登录界面部分代码）
Dim sqlconn as New sqlcliet.sqlconnection(configurationsettings.appsettings("connectionstring"))
Dim select_string as string="select * from user where user_name='" & trim(user_name.text) & "' and user_pwd='" & trim(user_pwd.text) & "' "
Dim select_comm as New sqlclient.sqlcommand(select_string,sqlconn)
Dim select_reader as sqlclient.sqldatareader
try
sqlconn.open()
select_reader=selec_comm.executereader(commandbehaviorcloseconnection)
if select_reader.read()=true then
dim username as string
session("username")=trim(user_name.text)
response.redirect("mian.aspx")
select_reader.close()
sqlconn.close()
else
response.write("
1<script langusge="javascript" runat="server">window.alert('用户名或密码错误错误!')</script>
")
end if
catch sqlexc as execption
response.write("错误发生!"&sqlexc.tostring())
finally
if not select_reader is nothing then
select_reader.close()
end if
(2)非结构化异常处理：通过Error对象及On Error、Resume和Error三种语句实现。
例：
** On Error ** Resume Next**Err.Clear****Err.Raise** （33333）** Err.Description ** = "您没有输入数字！"
MsgBox（ ** Err.Number ** ）
MsgBox（ ** Err.Description ** ）