[RESOLVED] How to use Try... Catch in following code?

[marsias]

If no internet connection exist stop, (not execute any other codes), with a message like "No Internet connection , Try again later...".

 Private Sub btnGetGold_Click(sender As Object, e As EventArgs) Handles btnGetGold.Click
     Dim pos1 As Long, pos2 As Long, gold As String
     Dim request As WebRequest = WebRequest.Create("https://www.livepriceofgold.com/greece-gold-price.html")
     Dim response As HttpWebResponse = CType(request.GetResponse(), HttpWebResponse)
         Dim datastream As Stream = response.GetResponseStream
         Dim reader As New StreamReader(datastream)
         Dim strData As String = reader.ReadToEnd
         pos1 = InStr(strData, "22GXAUUSD_EUR_BID") + 18
         pos2 = pos1 + 5
         gold = strData.Substring(pos1, pos2 - pos1)
         TextBox5.Text = Val(gold)
     Using conn As New SQLiteConnection(connection)
         conn.Open()
         Dim query As String = "SELECT SUM(Gold_gr) from users"
         Using command As New SQLiteCommand(query, conn)
             TextBox6.Text = Convert.ToDecimal(command.ExecuteScalar())
         End Using
         TextBox5.Select()

     End Using

 End Sub

[Niya]

Wrap the line with request.GetResponse in the Try....Catch. That is where your code will try to use the internet so that is where the point of failure would be if there is a problem with your internet connection.

[sapator]

greece-gold-price ???

[wqweto]

greece-gold-price ???

Relax, it's only goverment selling sovereign gold for peanuts. . . Just what EU told them to do :-))

cheers,
</wqw>

[sapator]

WHAT?! HOW DARE YOU?!
That is....
Accurate.

[Red.x]

If My.Computer.Network.IsAvailable = False Then
    MsgBox("No network connection detected. Cancelling all operations.", MsgBoxStyle.Critical, "Network Error")
    End
End If

[Shaggy Hiker]

It would be best to also wrap the SQL connection steps in an exception handler, and not the same one that you have for the request. Getting an exception from a SQL call is pretty rare, normally, as your database is usually going to be highly reliable, but it IS a place where an exception that can only be dealt with through exception handling can arise.

[Niya]

If My.Computer.Network.IsAvailable = False Then
    MsgBox("No network connection detected. Cancelling all operations.", MsgBoxStyle.Critical, "Network Error")
    End
End If

I dislike this approach because the chances of the internet failing right after that check succeeds is non-zero. I think it's a bad practice to write code in a way that relies on a low probability of exceptional circumstances.

[Red.x]

I dislike this approach because the chances of the internet failing right after that check succeeds is non-zero. I think it's a bad practice to write code in a way that relies on a low probability of exceptional circumstances.

Help him out
Don't talk like a grandma

[Niya]

Help him out
Don't talk like a grandma

I already did. This is the answer:-

Wrap the line with request.GetResponse in the Try....Catch. That is where your code will try to use the internet so that is where the point of failure would be if there is a problem with your internet connection.

[ChrisE]

Help him out
Don't talk like a grandma

send a Ping to check

... just one Ping
https://www.youtube.com/watch?v=jr0JaXfKj68

[Shaggy Hiker]

One ping isn't enough to figure everything out. One ping is good enough for some things, but it's not the case that one ping will rule them all.

[Shaggy Hiker]

I agree with Niya, of course. Exception handling should be used for exceptional circumstances. You can never be utterly sure that a network connection will be available, and will remain available. Checking that there is a network connection available is a good thing to do, as it avoids some nuisance failures, but it doesn't catch all possible failures in a network operation, so any network operation should be wrapped in exception handling, and so should any SQL operation. In both cases, the key is that your program is reaching outside itself to some other process, and you program can't control what that other process does or how it behaves, so you have to protect yourself, which is what exception handling does.

[marsias]

thanks to all.
I have wraped the whole code in the Button.click event.
if the internet connection is not avalaible any operations, variables,textboxes and SQL operations are pointless.
this include if something wrong is with the SQL Operation, but i will have the same message?

 Private Sub btnGetGold_Click(sender As Object, e As EventArgs) Handles btnGetGold.Click
     Try
         Dim pos1 As Long, pos2 As Long, gold As String
         Dim request As WebRequest = WebRequest.Create("https://www.livepriceofgold.com/greece-gold-price.html")
         Dim response As HttpWebResponse = CType(request.GetResponse(), HttpWebResponse)
         Dim datastream As Stream = response.GetResponseStream
         Dim reader As New StreamReader(datastream)
         Dim strData As String = reader.ReadToEnd
         pos1 = InStr(strData, "22GXAUUSD_EUR_BID") + 18
         pos2 = pos1 + 5
         gold = strData.Substring(pos1, pos2 - pos1)
         TextBox5.Text = Val(gold)
         Using conn As New SQLiteConnection(connection)
             conn.Open()
             Dim query As String = "SELECT SUM(Gold_gr) from users"
             Using command As New SQLiteCommand(query, conn)
                 TextBox6.Text = Convert.ToDecimal(command.ExecuteScalar())
             End Using
             TextBox5.Select()
         End Using
     Catch ex As Exception
         MsgBox("No Internet connection.. Try again later...")
     End Try
 End Sub

[jmcilhinney]

That's a VERY bad solution. If there is a problem with the database, you're going to lie to the user and tell them that there's no internet connection. What if that happens and the user checks their internet connection, finds it's fine and then tries again, only to get the same message?

Why ask a question of you're just going to ignore the good advice that people take the time and make effort to provide to you? You should almost NEVER catch just an Exception. You should pretty much always know exactly what types of exceptions can be thrown by your code and catch those specifically, leaving any unexpected exceptions to be handled by a global exception handler.

[marsias]

This is not the final solution.....
The app should work even if i dont have a internet connection!
i try to figure out where to put the database exception.

[jmcilhinney]

i try to figure out where to put the database exception.

Around the database code. Pull all the UI stuff out and do it at the end, assigning the data to variables first. Wrap the WebRequest code in one Try block and put a Return in the Catch for that, then put the ADO.NET code in another Try block and put a return in that Catch block too. If no exceptions are caught, you'll end up at the code that populates and manipulates the UI.

Try
    'Web stuff here
Catch
    '...
    Return
End Try

Try
    'Database stuff here
Catch
    '...
    Return
End Try

'UI stuff here

That's basically what people have already said.

[marsias]

the database exception goes to the form load event.
It works but starts the form with empty fields,
How to make that the app does not load the form and close ?

ok found with add me.close() before return

Public Sub ShowData()
    table.Rows.Clear()
    Try
        Dim command As New SQLiteCommand("SELECT id,Ruler,Gold_Coin,year,Gold_gr,Image FROM users order by year", connection)
        Dim adapter As New SQLiteDataAdapter(command)
        adapter.Fill(table)
    Catch ex As Exception
        MsgBox("No Database Connection..Contact your admin..")
me.close()
        Return
    End Try
    TextBox1.Text = table.Rows(index)(1).ToString
    TextBox2.Text = table.Rows(index)(2).ToString
    TextBox3.Text = table.Rows(index)(3).ToString
    TextBox4.Text = table.Rows(index)(4).ToString
    Dim imgData As Byte() = CType(table.Rows(index)(5), Byte())
    Using ms As New MemoryStream(imgData)
        PictureBox1.Image = Image.FromStream(ms)
    End Using
    DataGridView1.DataSource = table
End Sub

[jmcilhinney]

You're still catching the base Exception instead of the specific type(s) of exception you expect to be thrown.

[Shaggy Hiker]

I'm not so opposed to the base exception for SQL code, especially just starting on a program. For most things I've done, errors in DB code are first because of typos in the SQL. Those are just going to get caught during debugging, and I don't care what exception is thrown for that purpose. Once those trivial issues are cleaned up, any exception in the DB code is usually so utterly fatal that I'm not even going to TRY (or catch) to recover, I'm just going to exit as politely as I can. If it becomes apparent that there is one type of exception that has some meaningful way to recover, then that can be split out. I rarely find that to be the case with SQL.

[wes4dbt]

I don't like doing this,

    Catch ex As Exception
        MsgBox("No Database Connection..Contact your admin..")
me.close()
        Return
    End Try

You get more information with,

        Catch ex As Exception
            MessageBox.Show(ex.Message)
        End Try

or

        Catch ex As Exception
            MessageBox.Show(ex.ToString)
        End Try

The "ex" message isn't always helpful but many times it can be.

[PlausiblyDamp]

I would typically avoid showing those kind of error messages to an end user, they are often overly technical and can even reveal information that could be used to identify security weakness.

Ideally those errors should be logged somewhere, and a more user friendly message displayed to the users.

[wes4dbt]

I would typically avoid showing those kind of error messages to an end user, they are often overly technical and can even reveal information that could be used to identify security weakness.

Ideally those errors should be logged somewhere, and a more user friendly message displayed to the users.

I don't mind logging the error, just as long as it's available in some form to aid in debugging.

I would just display it while I am developing the program then change it to logging at the end.

[jmcilhinney]

I don't mind logging the error, just as long as it's available in some form to aid in debugging.

I would just display it while I am developing the program then change it to logging at the end.

That's why you use Debug.WriteLine. You'll see that in the Output window while debugging but it won't affect a Release build at all without the need to modify the code.

[wes4dbt]

That's why you use Debug.WriteLine. You'll see that in the Output window while debugging but it won't affect a Release build at all without the need to modify the code.

I've used that a lot.

Most of the time I would leave the "ex" messagebox display. It made phone support easier. It wasn't till towards the end of my career that I started getting remote access. Most of my work the last few years was just maintaining old programs.