Response.Write is not a good thing to use on ASP.NET pages.

[MMock]

http://www.vbforums.com/archive/index.php/t-395604.html
Why not?

When I hit an exception in my code behind, can I put up a windows-style message box? I'd like to pass the variable ex.message to it, but none of these is working:

            ' Response.Write("<script language='javascript'>alert(ex.message);</script>")
            ' Response.Write("<script language='javascript'>alert(" + ex.Message + ");</script>")
            ' Response.Write("<script language=javascript>alert('This is a dynamic alert.');</script>")

[mendhak]

Funnily enough, this was once discussed in Chit Chat of all places. I will paste what I said here (with some amendments to make me appear smarter than I am)

Using Response.Write is bad. Using Response.Write to write javascript is worse. Using Response.Write to write "cool" AJAX is shameful, something I'd never admit to in public.

The reason is, Response.Write occurs when the response stream is being sent and so the output can be sent at any point in the page rather than when it should be sent. Additionally, if there is a problem with the Response.Write, you could actually mess up the stream, end it, or just mangle the page. If you create a script block of a function you've made, it's always best to use Page.RegisterClientScript (or its equivalent) so that the script is output to the correct place in the page. Incorrect placement of scripts (hence, Response.Write) can cause problems especially when you're working with the DOM. ASP.NET when used correctly effectively negates the purpose of Response.Write. In fact, Response.Write is very much like <% inline ASP.NET code %>; quick and dirty and good for tutorials.


For your specific case, you can use

Page.RegisterStartupScript("mymsg","<script type='text/JavaScript'>alert('" & ex.Message & "');")

[MMock]

Funnily enough, this was once discussed in Chit Chat of all places. I will paste what I said here ...

Yes, I thought it amusing that even just googling stuff led me to one of your brilliant explanations!

Thanks.

[mendhak]

You actually found that thread via google?

What keywords did you use?

[MMock]

Can't remember but I'll see if I can recreate it tomorrow, have to leave and pick up children...

[RobDog888]

It would be easier to just search vbforums first.

[MMock]

.


For your specific case, you can use

Page.RegisterStartupScript("mymsg","<script type='text/JavaScript'>alert('" & ex.Message & "');")

My compiler says that is obsolete (probably asp.net 1.1 to 2.0 difference). So this is what I tried:

        Try
            adapter.Fill(dsInvoices)
        Catch ex As Exception
            ' Response.Write("<script language='javascript'>alert(ex.message);</script>")
            ' Page.RegisterStartupScript("mymsg", "<script type='text/JavaScript'>alert('" & ex.Message & "');")
            Dim t As Type = Me.[GetType]()
            If Not ClientScript.IsClientScriptBlockRegistered(t, "ExceptionMessage") Then
                ClientScript.RegisterClientScriptBlock(t, "ExceptionMessage", "<script type='text/JavaScript'>alert('" & ex.Message & "');")
            End If
            conn.Close()
            Exit Sub
        End Try

This is probably a stupid question, but when would I actually see the alert? I forced the exception by trying to execute a non-existent sp, and all I saw was a blank IE window.

Thanks for your help.

[mendhak]

Yes, in ASP.NET 2.0+, it's ClientScript instad of Page.Register....

Note that ClientScript.RegisterClientScriptBlock != Page.RegisterStartupScript. The equivalent is ClientScript.RegisterStartupScript.

RegisterStartupScript means "execute this as soon as the page loads", while RegisterClientScriptBlock means "here's some new JavaScript. Put it on the page, I might use it later."


Also, move your conn.Close() to the Finally block, not the Catch.


Also also, you need to close your script tag.

"<script type='text/JavaScript'>alert('" & ex.Message & "');</script>"

[MMock]

Nag, nag, nag... (just kidding).

There seems to be a missing end paren somewhere - I get error expected ')' (though I sure don't see it). But you're right that it's not right the way I had it. I just displayed a blank screen again.

I have conn.Close where it is because I bail out next instruction. If I have Finally, how do I know if I'm there and things are good or bad? I usually only have Finally's at the end of my sub, not in the middle...("bad programmer!" you are probably going to say...)

[mendhak]

Try it simpler first.

        Try
            adapter.Fill(dsInvoices)
        Catch ex As Exception
           
                ClientScript.RegisterClientScriptBlock(Me.GetType, "ExceptionMessage", "<script type='text/JavaScript'>alert('Hello World!');</script>")
      
            Exit Sub
        Finally
            conn.Close()
        End Try

The finally gets executed regardless of whether there is an exception or not. Since you logically would want to close the connection after the operation, the close should go into finally. Even if it's in a loop of some sort, it'll still close it properly.

<NagMode>
Also, if the dataadapter is using the conn, you don't need to open it and therefore you don't need to close it for that matter.
</NagMode>

[MMock]

Try it simpler first.

        Try
            adapter.Fill(dsInvoices)
        Catch ex As Exception
           
                ClientScript.RegisterClientScriptBlock(Me.GetType, "ExceptionMessage", "<script type='text/JavaScript'>alert('Hello World!');</script>")
      
            Exit Sub
        Finally
            conn.Close()
        End Try

Okay, this "simpler" way worked. I don't know what I was doing wrong before. I have a question. I replaced 'hello world' with 'Program Error in RowDataBound event of grid'. Can I concatenate ex.message to this? (Assuming the answer is yes, I guess I should ask how?) Also, what is "ExceptionMessage"?

Now to get *really* picky, the reason I resurrected this thread is because in my other thread More Advanced Gridview Updating I was having an error processing my even-numbered rows. I took me a while to find because I didn't have a proper exception handler. So now that I do, how would I bail out of a row edit in the midst of a RowDataBound event? I'd like to put the entire grid back in view mode because I am getting an exception, there is no current data displayed for the row, yet the user is in edit mode and could really mess things up (not that this bug would ever make its way into production... )

Thanks.

[MMock]

Okay, this "simpler" way worked. I don't know what I was doing wrong before. I have a question. I replaced 'hello world' with 'Program Error in RowDataBound event of grid'. Can I concatenate ex.message to this? (Assuming the answer is yes, I guess I should ask how?) Also, what is "ExceptionMessage"?

This is bad. Now I'm quoting myself (much worse than talking to oneself!)

ExceptionMessage is the name of the script block, correct? So if I do more than one, I would give each block a unique name? Can you explain this in the bigger picture of things (like the whole program, not just a procedure) so I understand it and can implement it correctly?

And I've figured out the concatenation question.

[MMock]

I have an exception handler in my RowDataBound event and my RowUpdating event. I am hitting them both. The former works but the latter is saying "Done, but with errors on page" and the details say Expected ')'. I double-checked and ensured it is the same code both places. Here are the two blocks.

This is in RowDataBound. I see this message.

                Catch ex As Exception
                    Dim t As Type = Me.[GetType]()
                    If Not ClientScript.IsClientScriptBlockRegistered(t, "ExceptionMessage") Then
                        ClientScript.RegisterClientScriptBlock( _
                            t, _
                            "ExceptionMessage", _
                            "<script type='text/JavaScript'>alert('Exception in RowDataBound event of grid: " & ex.Message & "')</script>")
                        End If
                    Exit Sub
                End Try

This is in RowUpdating. I don't see the message and I get the warning icon:

            Catch ex As Exception
                Dim t As Type = Me.[GetType]()
                If Not ClientScript.IsClientScriptBlockRegistered(t, "ExceptionMessage2") Then
                    ClientScript.RegisterClientScriptBlock( _
                        t, _
                        "ExceptionMessage2", _
                        "<script type='text/JavaScript'>alert('Exception in RowDataBound event of grid: " & ex.Message & "')</script>")
                    End If

            End Try

Do you see what is wrong?

[mendhak]

Okay, this "simpler" way worked. I don't know what I was doing wrong before. I have a question. I replaced 'hello world' with 'Program Error in RowDataBound event of grid'. Can I concatenate ex.message to this? (Assuming the answer is yes, I guess I should ask how?) Also, what is "ExceptionMessage"?

Now to get *really* picky, the reason I resurrected this thread is because in my other thread More Advanced Gridview Updating I was having an error processing my even-numbered rows. I took me a while to find because I didn't have a proper exception handler. So now that I do, how would I bail out of a row edit in the midst of a RowDataBound event? I'd like to put the entire grid back in view mode because I am getting an exception, there is no current data displayed for the row, yet the user is in edit mode and could really mess things up (not that this bug would ever make its way into production... )

Thanks.

The even rows thing sounds like you're missing out on a check for Row and Alternate Row. Do you have that check in your If statement in the handler event? You can check the rowtype to see if it's a normal row or alternate row, or header and so on.

[mendhak]

This is bad. Now I'm quoting myself (much worse than talking to oneself!)

ExceptionMessage is the name of the script block, correct? So if I do more than one, I would give each block a unique name? Can you explain this in the bigger picture of things (like the whole program, not just a procedure) so I understand it and can implement it correctly?

And I've figured out the concatenation question.

Yes, each script 'block' would get a unique name. This is used for when you want to check in 'some other' postback whether a script block of that name has already been registered or not.

[mendhak]

I have an exception handler in my RowDataBound event and my RowUpdating event. I am hitting them both. The former works but the latter is saying "Done, but with errors on page" and the details say Expected ')'. I double-checked and ensured it is the same code both places. Here are the two blocks.

This is in RowDataBound. I see this message.

                Catch ex As Exception
                    Dim t As Type = Me.[GetType]()
                    If Not ClientScript.IsClientScriptBlockRegistered(t, "ExceptionMessage") Then
                        ClientScript.RegisterClientScriptBlock( _
                            t, _
                            "ExceptionMessage", _
                            "<script type='text/JavaScript'>alert('Exception in RowDataBound event of grid: " & ex.Message & "')</script>")
                        End If
                    Exit Sub
                End Try

This is in RowUpdating. I don't see the message and I get the warning icon:

            Catch ex As Exception
                Dim t As Type = Me.[GetType]()
                If Not ClientScript.IsClientScriptBlockRegistered(t, "ExceptionMessage2") Then
                    ClientScript.RegisterClientScriptBlock( _
                        t, _
                        "ExceptionMessage2", _
                        "<script type='text/JavaScript'>alert('Exception in RowDataBound event of grid: " & ex.Message & "')</script>")
                    End If

            End Try

Do you see what is wrong?

Do a view source on the page, look at the javascript created. You may be able to see the problem there. Paste it here if you can't find it.

[MMock]

The even rows thing sounds like you're missing out on a check for Row and Alternate Row. Do you have that check in your If statement in the handler event? You can check the rowtype to see if it's a normal row or alternate row, or header and so on.

Yes that was it, quit cross-threading! And maybe it's a moot point because really bad bugs like that one are usually caught in test, but my question was once I am correctly catching an exception...how do I handle it? But skip it, it was too picky.

[MMock]

Do a view source on the page, look at the javascript created. You may be able to see the problem there. Paste it here if you can't find it.

Once again, this is taking a back burner (until Monday).Today I want to finish a page so my customer can start testing and tomorrow I have a day off - one daughter is reader of the month and the other is performing african dance at school.

Thanks.

[mendhak]

Yes that was it, quit cross-threading! And maybe it's a moot point because really bad bugs like that one are usually caught in test, but my question was once I am correctly catching an exception...how do I handle it? But skip it, it was too picky.

Depends on your needs. You can throw the exception (don't handle it) and let the user see it. But that confuses users. What I see most often is that the exception is caught, logged, and an error message shown to the user. "An error has occurred. Please contact your administrator. Look at the event logs. Nag nag nag."

Then when they report it to you, have a look at the log to see what the error was. It does inconvenience the user but is a better alternative to that YSOD.

[MMock]

Maybe this week I will actually understand enough to be able to close this thread...

Here is my question now. As you might remember, I coded a lot of my pages with response.write in my exception handler with no intention of leaving them that way when they went into production. So now I am doing my finishing touches on them. I have an exception in a class file I am calling. So what is the best way to code the exception handling? I can't put RegisterClientScriptBlock because there's no such thing as ClientScript (since it's a vb module, not vb code-behind, I am assuming). Do I code my class file function to pass back something to indicate "an exception occured" and also pass back ex.message, or do I ignore exception handling in the class file and let the exeption get bubbled up to its caller? Or none of the above???

Thanks.

[MMock]

I chose none of the above and went this way:

        Try

            conn.Open()
            command.ExecuteNonQuery()
            ' int count = Int32.Parse(cmd.Parameters["@LastNameCount"].Value.ToString());
            iCount = CInt(command.Parameters("@mktarea_cnt").Value)
            GetMarketAreaCnt = iCount

        Catch ex As Exception
            ' Can't do reponse.write() here
            Dim s As String
            s = ex.Message
            Throw New Exception(ex.Message)
        Finally
            conn.Close()
            conn.Dispose()
            conn = Nothing
        End Try

Good?

[MMock]

Yes, each script 'block' would get a unique name. This is used for when you want to check in 'some other' postback whether a script block of that name has already been registered or not.

And if it has already been registered, I have to unregister it to use it again? Is something like this what I need to do application-wide:

ClientScript.RegisterClientScriptBlock(Me.GetType, "ExcPageAFunction1", "<script type='text/JavaScript'>alert('Exception');</script>")
ClientScript.RegisterClientScriptBlock(Me.GetType, "ExcPageAFunction2", "<script type='text/JavaScript'>alert('Exception');</script>")
ClientScript.RegisterClientScriptBlock(Me.GetType, "ExcPageAFunction3", "<script type='text/JavaScript'>alert('Exception');</script>")
ClientScript.RegisterClientScriptBlock(Me.GetType, "ExcPageBFunction1", "<script type='text/JavaScript'>alert('Exception');</script>")

etc., with all unique names?

[mendhak]

The uniqueness is on a page level. You don't need to make it unique site-wide. If you have only one RegisterClientScriptBlock() on your page, then you can name it whatever you want.

[mendhak]

I chose none of the above and went this way:

        Try

            conn.Open()
            command.ExecuteNonQuery()
            ' int count = Int32.Parse(cmd.Parameters["@LastNameCount"].Value.ToString());
            iCount = CInt(command.Parameters("@mktarea_cnt").Value)
            GetMarketAreaCnt = iCount

        Catch ex As Exception
            ' Can't do reponse.write() here
            Dim s As String
            s = ex.Message
            Throw New Exception(ex.Message)
        Finally
            conn.Close()
            conn.Dispose()
            conn = Nothing
        End Try

Good?

You're catching an exception and throwing an exception - that's equivalent to not handling it. What is the purpose of doing it this way?

[mendhak]

Also, you declare a string s, give it ex.Message, but then don't use it anywhere...

[MMock]

Also, you declare a string s, give it ex.Message, but then don't use it anywhere...

Sorry...at the time, that was just to have somewhere to put a bp and look at ex.message. It was never meant to stay, but thanks for pointing it out in case I needed a little jarring to see it for myself. I think it's gone already.

[MMock]

You're catching an exception and throwing an exception - that's equivalent to not handling it. What is the purpose of doing it this way?

Is it? I just tend to cringe if I don't see any exception handling, so I wanted to make it obvious that I *want* to not handle it and bubble it up to my caller, not that I just forgot to handle it. I wasn't sure about that. So you're saying just remove it? Maybe with a comment that I'm letting the caller handle it?

[mendhak]

To make it obvious that you're not handling it, don't handle it.

When you do what you did, the operation becomes expensive because, first, the compiler stops execution immediately and 'catches' the exception. Then when you throw the exception, that's another code break.

Using try_catch is not expensive, it's catching and throwing errors that's expensive.

I'm not sure who the 'caller' is in this case but if you want your caller to handle it, then let it bubble up on its own. It is logical.

[MMock]

Okay, that makes sense.

But what happens to things you would do in a Finally block if you are forsaking your exception handling? E.g...

            Try
                adapter.Fill(ds)
            Catch ex As Exception
                Throw New Exception(ex.Message)
                Exit Function
            Finally
                conn.Close()
            End Try

My conn will stay open?

[mendhak]

This is where things like Using comes in handy.

If you're using (ha) .net 2.0 onwards, then you can use using.

Using conn
'conn.open()
'something
conn.close()
End Using

But you have to keep in mind, because a catch/throw is expensive it doesn't mean you get rid of it and just not use it. The rule of thumb is to use exception handling around code which does have a possibility of failure. So in your case, it's fine to leave it there.

To address the specific example there, here's how I'd do it

Try
                adapter.Fill(ds)
            Catch ex As Exception
                'Write to log file maybe
            Finally
                conn.Close()
            End Try
 
If ds Is Not Null Then
'Rest of Code
 
 
End If
 
End Function