Visual Basic 2008 - NullReferenceException ERROR?

Question

Hey all,

I'm using Visual Basic 2008 here, and I'm debugging my code that connects to my SQL Database and writes something in it. It was working fine and all until I came to an error like this. NullReferenceException was unhandled. What's going on? Here is the code I'm working with:

Dim conn As MySqlConnection
                    conn = New MySqlConnection
                    conn.ConnectionString = "server=...; user id=...; password=...; database=..."
                    Try
                        conn.Open()
                    Catch myerror As MySqlException
                        MsgBox("Error connecting to database")
                    End Try
                    Dim myAdapter As New MySqlDataAdapter
                    Dim sqlquery = "SELECT * FROM user WHERE username = '" + TextBox2.Text + "'"
                    Dim myCommand As New MySqlCommand()
                    myCommand.Connection = conn
                    myCommand.CommandText = sqlquery
                    myAdapter.SelectCommand = myCommand
                    Dim myData As MySqlDataReader
                    myData = myCommand.ExecuteReader()

It highlights it right around conn.open(), and gives me that error. It was working fine earlier until I moved my sql database to my mac. (windows -> mac) Is there a difference? I backed up my stuff from my windows vista computer and restored it on my mac. I don't think there is a difference, but I'm just putting that out there. Why does this error come up?

Thanks,

Kevin

Answer 1

I can't really say what the problem is off the top of my head.

But the way to find out would be to put a breakpoint on the conn=New MySqlConnection and look at the variables you have as you step through.

If the problem is happening on the conn.Open line then it would probably be because conn is null (nothing). But if that is the case then conn.ConnectionString should have thrown an exception.

The other step is to look at the stack trace of the exception and see where, exactly, it is being thrown from. It could, for example, be an error somewhere inside the mysql connector.

Sorry I can't really give you a definate answer, but I thought some general comments might help you to the solution or find some more detail.

Answer 2

Whivh version of MySql are you using? Have you looked at the details of this bug...

http://bugs.mysql.com/bug.php?id=26393

... which is related to idle time.

If you had the full stack trace associated with the exception then it might shed some light on the problem.

Answer 3

I would suggest reviewing your connection string. I used your code and connected to our local MYSQL db no errors.