VB.NET MySQL Connection open for all forms

Question

In the first form I will load Tabels in the secound I want to open the DB Connection.

I wrote a Class which enables me to open and close the connection

Imports System
Imports System.Data
Imports MySql.Data.MySqlClient

Public Class DBConn


Dim conn As New MySqlConnection
Dim connString As String

Dim DataSchnitstelel As MySqlDataAdapter


Public Function verbindungString(ByVal Server As String, ByVal UID As String, ByVal PWD As String, _
                                 ByVal Datenbank As String)
    connString = "server=" & Server & ";uid=" & UID & ";pwd=" & PWD & ";database=" & Datenbank & ";"



    connString = CStr(connString)
End Function



Public Sub Open()
    conn.ConnectionString = connString
    Try
        conn.Open()
        MsgBox(connString)
    Catch ex As Exception
        MessageBox.Show(ex.Message)
    End Try
End Sub



Public Function Close()
    If conn.State = ConnectionState.Open Then
        Try
            conn.Close()
            MessageBox.Show("Closed!")
        Catch ex As Exception
            MessageBox.Show("Something Wrong" & ex.Message)
        End Try
    Else
        MessageBox.Show("Verbindung bereitsgeschlossen")
    End If
End Function


Public Function UpdateStatus()
    Dim Klank As Boolean
    If conn.State = 1 Then
        Klank = True
    End If
    If conn.State = 0 Then
        Klank = False
    End If
    Return Klank
End Function

Public Function SQLSelect()

End Function

End Class

In the Second Form i Connect to the DB. The connection opens successfully but it's not open in the first form :(

Whats Wrong?

Answer 1

If you declare the connection class in each of your forms as Dim someName as new DBConn , your problem is obvious: they know not of each others existence. If you have a module besides your forms, in which you declare the DBConn instance as Friend, it would be known to all forms

Answer 2

I guess the problem is that you use in both forms somewhere

Dim bla as New DBConn

Instead that, you can make use of the Singleton Pattern. Add the following code inside your DBConn class:

'Make Constructor Private to disallow creating an Instance from somewhere else
Private Sub New()
End Sub

'Variable to share the only created instance
Private Shared _Instance As DBConn

'Function to get access to the only instance
Public Shared ReadOnly Property Instance() As DBConn
  Get
    If _Instance Is Nothing Then
      _Instance = New DBConn
    End If
    Return _Instance
  End Get
End Property

After those changes, you need to 'correct' one line in each form to the following:

Dim bla as DBConn = DBConn.Instance

This should assure that both forms are asking for an instance, and only the very first time an instance is created. All further askings receive the same instance.

One more thing: If you have 'WithEvents' or any EventHandling connected to that Singleton, then make sure to set your DBConn variable to nothing when you close/dispose form1. But according to your code this does not matter.