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Catch Errors in Your Code
by Charles Carroll

There are many tools to catch errors in your page

  • web.config settings that let a specific .aspx file catch every error on your site

  • page settings that let any given page specify a specific .aspx page that catches any errors on that page

  • error handling code in that page which is what we will discuss here

Error trapping in ASP.net is structured to solve two programming problems other error trapping schemes (ON ERROR RESUME NEXT notably) do not address:

  • If multiple errors occur on a page, provide an easy mechanism to intercept all errors

  • If a page calls an assembly that has NO or incomplete error handling the page should be able to catch the error the other code causes

The basic idea is:

TRY
    your code
CATCH
    here is error handling code
FINALLY
    here is cleanup code

in a  way that you can protect an entire page with a CATCH. You can also ensure any "cleanup" code is executed in the FINALLY block.

Here is a code sample that shows a modest error handler that should be the bare minimum most pages have!

   filename=\\experiments\\errorhandling\\catcherrors.aspx

<Test Script Below>

 

<%@ Import Namespace="System.Data.sqlclient" %> 
<script language="VB" runat="server"> 
Sub Page_Load(Src As Object, E As EventArgs)
TRY
   Dim strConn as string
   strconn ="server=xxx;uid=xxx;pwd=xxx" 
   DIM strSQL as string
   strSQL="select * from junk"
   Dim Conn as New SQLConnection(strConn) 
   Dim Cmd as New SQLCommand(strSQL,Conn) 
   Conn.Open() 
   myDataGrid.DataSource = Cmd.ExecuteReader(system.data.CommandBehavior.CloseConnection) 
   myDataGrid.DataBind() 
CATCH sqlerr as SQLException
   status.text="<br>SQLError# =" & sqlerr.Number & ", SQL Msg=" & sqlerr.Message & "<br>"
CATCH exc As Exception
   status.text=status.text & "<br>Exception #" & exc.ToString() & "<br>"
END TRY
 
End Sub 
</script> 
<html><head> 
<title>Basic Error Trapping Demo</title> 
</head> 
<body bgcolor="#FFFFFF"> 
<font face="Verdana"><h3>Error Trapping Demo</h3></font><br>
Query Results<br>
<asp:literal id="status" runat="server"/>
<br>
<ASP:DataGrid id="MyDataGrid" runat="server" 
Width="100%" 
BackColor="white" 
BorderColor="black" 
ShowFooter="false" 
CellPadding=3 
CellSpacing="0" 
Font-Name="Verdana" 
Font-Size="8pt" 
Headerstyle-BackColor="lightblue" 
Headerstyle-Font-Size="10pt" 
Headerstyle-Font-Style="bold" 
MaintainState="false" 
/> 
</body></html>

The beauty of this system is the nesting support, i.e. if you want to handle the error f

TRY
    ...your code for task1...
    TRY
    ... your code for task2..
    CATCH
    ... your error handling for task2 ...
    FINALLY
    ... your cleanup code for task2...

    TRY
    ... your error code for task3...
    CATCH
    ... your code for task3...
    FINALLY
    ... your error code for task3...

    TRY
    ... your error code for task4...
    CATCH
    ... your code for task4...
    FINALLY
    ... your error code for task4...

    ... task 5 code ...
    
CATCH
    here is error code for task1,task5,task6 and any ASSEMBLY unhandled exceptions
FINALLY
    here is cleanup code for task1,task5,task6 and any ASSEMBLY unhandled exceptions

As long as you make at least one handler for all your code on a page you control the error message for any unexpected errors.

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