How to redeclare an instance of a class? - vb.net

Question

I'm dusting off vb.net needs to work and I have a question that I hope you can clarify. My application require a connection with a database, now when the form is load I execute this function:
Sub OpenConnection()
Try
Dim connection As New Connection
Dim MysqlConn = connection.establishConnection()
MysqlConn.Open()
Catch myerror As MySqlException
MessageBox.Show("Connection failed: " & myerror.Message)
Finally
MysqlConn.Dispose()
End Try
End Sub
The problem's that connection and MysqlConn variable is used also in another function so I've declared this two variable as global. How you can see this variable are re-declared in the OpenConnection() 'cause I've a form where the user can change the settings connection string. Now if the software can't connect to the database for a wrong credentials the user change this into the setting form but if I remove connection and MysqlConn the application use the past instance of a class so even if the credentials are correct, not declaring a new instance of the connection the application can't connect. For the moment I've solved with a re-declaration inside the OpenConnection() but is this a good move? Is there a way to have the two global variables and invoke a new instance without redefine the function?
Connection class - EstablishConnection function
Public Function establishConnection()
Dim MysqlConn As MySqlConnection = New MySqlConnection()
Dim server_name = My.Settings.server_name
Dim username = My.Settings.username
Dim password = My.Settings.password
Dim database = My.Settings.database
MysqlConn.ConnectionString = "server=" & server_name & ";" _
& "user id=" & username & ";" _
& "password=" & password & ";" _
& "database=" & database & ";"
Return MysqlConn
End Function

Answer 1

How to redeclare an instance of a class?
Variables are declared.
Objects are instanced (instantiated).
They are never redeclared; another new one is just created. Take this line (similar to yours):
Dim connection As MySQLConnection
It declares a variable, giving it a name and type.
Dim connection As MySQLConnection = New MySQLConnection
The second part creates an instance - the New operator is the key: it creates a new instance. The short hand form you used does both at once and perhaps makes it unclear that 2 different operations - declaring a variable and creating an instance - are going on.
Apparently, you have a class named Connection to "manage" connections. But the top method, OpenConnection is just using it to set/reset some global object var. dbConnections should generally be created, used and disposed of.
A connection maker-helper like you have can prevent having connection strings
sprinkled all over the code. Eg:
Function GetMySQLConnection(Optional usr As String = "root",
Optional pw As String = "123456") As MySqlConnection
Dim conStr As String
conStr = String.Format("Server=localhost;Port=3306;Database={0};Uid={1}; Pwd={2};",
MySQLDBase, usr, pw)
Return New MySqlConnection(constr)
End Function
This might be part of a small class to hold the DBname and other log in info. But using it doesnt require a global var:
Sub GetSomeData(...)
Using dbCon As MySqlConnection = GetMySQLConnection()
...
dbCon.Open()
myDA.Fill(myDT)
...
End Using
End Sub
At the start, a new connection object is created for this set of operations, which exists only locally (in that method). The Using block assures that the connection is closed (and returned to the pool) and disposed of when you are done with it.