Form.show() does not respond - vb.net

Question

I am writing a small postIt programm which should enable the user to send little notes to other computers using a database.
I have a main-Form where I can write messages and see users online, a sheet-form which should display messages and a sql database which has a table for users and messages.
The idea is that I am writing messages into the Messagestable and my programm uses SqlDependency to get the new messages and to show them in sheets. So far so good.
The problem is when I add a new Message to my table the SqlDependency fires an Event and my mainForm creates a new sheet which freezes after sheet.Show() is called. The mainForm keeps on running but my sheets always do not respond.
Here's my code:
DBListener:
Imports System.Data
Imports System.Data.Sql
Imports System.Data.SqlClient
Imports System.Threading
Public Class DBListener
Private changeCount As Integer = 0
Private Const tableName As String = "IMSMessages"
Private Const statusMessage As String = _
"{0} changes have occurred."
Private exitRequested As Boolean = False
Private waitInProgress As Boolean = False
Private recieverHost As String
Private imsMain As IMSMain
Public Sub New(main As IMSMain, recieverHost As String)
Me.imsMain = main
Me.recieverHost = recieverHost
initCommand(recieverHost)
SqlDependency.Start(connectionString)
GetMsg(recieverHost)
End Sub
Private connectionString As String = "Data Source=NB_RANDY\SQLEXPRESS;Initial Catalog=eurom_test;Integrated Security=SSPI;"
Private sqlConn As SqlConnection = New SqlConnection(connectionString)
Private commandMesg As SqlCommand = Nothing
Private commandUsers As SqlCommand = Nothing
Private Sub initCommand(recieverHost As String)
commandMesg = New SqlCommand
commandMesg.CommandText = "SELECT MessageID,SenderHost,RecieverHost,isRead,isRecieved,Stamp from dbo.IMSMessages " &
"WHERE RecieverHost=#RecieverHost" &
" AND isRecieved = 0"
commandMesg.Parameters.Add("#RecieverHost", SqlDbType.VarChar).Value = recieverHost
commandMesg.Connection = sqlConn
commandUsers = New SqlCommand
commandUsers.CommandText = "Select HostName From dbo.IMSUser"
End Sub
Private Sub OnChangeMsg(ByVal sender As System.Object, ByVal e As System.Data.SqlClient.SqlNotificationEventArgs)
Dim dep As SqlDependency = DirectCast(sender, SqlDependency)
RemoveHandler dep.OnChange, AddressOf OnChangeMsg
GetMsg(recieverHost)
End Sub
Private Sub OnChangeUser(ByVal sender As System.Object, ByVal e As System.Data.SqlClient.SqlNotificationEventArgs)
Dim dep As SqlDependency = DirectCast(sender, SqlDependency)
RemoveHandler dep.OnChange, AddressOf OnChangeUser
GetUsers()
End Sub
Public Sub GetMsg(recieverHost As String)
If Not sqlConn.State = ConnectionState.Open Then
sqlConn.Open()
End If
commandMesg.Notification = Nothing
Dim dep As SqlDependency = New SqlDependency(commandMesg)
AddHandler dep.OnChange, New OnChangeEventHandler(AddressOf OnChangeMsg)
Dim table As DataTable = New DataTable
Dim adapter As SqlDataAdapter = New SqlDataAdapter(commandMesg)
adapter.Fill(table)
imsMain.recieveNewMessages(table)
End Sub
Public Sub GetUsers()
If Not sqlConn.State = ConnectionState.Open Then
sqlConn.Open()
End If
commandMesg.Notification = Nothing
Dim dep As SqlDependency = New SqlDependency(commandUsers)
AddHandler dep.OnChange, New OnChangeEventHandler(AddressOf OnChangeUser)
Dim table As DataTable = New DataTable
imsMain.updateOnlineUserList()
End Sub
End Class
IMSMain-Form:
Imports System.Data.SqlClient
Imports System.Threading
Public Class IMSMain
Private user As IMSUser
Private listener As DBListener
Private sqlConn As SqlConnection
Public Sub New()
InitializeComponent()
sqlConn = New SqlConnection("Data Source=NB_RANDY\SQLEXPRESS;Initial Catalog=eurom_test;Integrated Security=SSPI;")
Me.user = New IMSUser(My.Computer.Name)
user.register()
updateOnlineUserList()
listener = New DBListener(Me, user.HostName)
End Sub
Private Sub onSend(sender As Object, e As EventArgs) Handles bt_send.Click
Dim command As SqlCommand = New SqlCommand
Dim msgText = tb_text.Text
Dim reciever As IMSUser = lb_Online.SelectedItem
Dim insert_string As String = "Insert INTO dbo.IMSMessages(Text,RecieverHost,SenderHost,isRead,isRecieved,Stamp) Values(#Text,#RecieverHost,#SenderHost,#isRead,#isRecieved,#Stamp)"
Dim adapter As SqlDataAdapter = New SqlDataAdapter
Dim table As DataTable = New DataTable()
Try
If Not sqlConn.State = ConnectionState.Open Then
sqlConn.Open()
End If
command.Connection = sqlConn
command.CommandText = insert_string
adapter.SelectCommand = command
adapter.Fill(table)
command.CommandText = insert_string
command.Parameters.Add("#Text", SqlDbType.VarChar).Value = msgText
command.Parameters.Add("#RecieverHost", SqlDbType.NChar).Value = reciever.HostName
command.Parameters.Add("#SenderHost", SqlDbType.NChar).Value = user.HostName
command.Parameters.Add("#isRecieved", SqlDbType.Bit).Value = 0
command.Parameters.Add("#isRead", SqlDbType.Bit).Value = 0
command.Parameters.Add("#Stamp", SqlDbType.SmallDateTime).Value = DateTime.Now
command.ExecuteNonQuery()
Catch ex As Exception
Console.WriteLine("onSend: internal database exception" + ex.Message)
End Try
End Sub
Private Sub processMessageId(refMessageID As Integer)
Dim command As SqlCommand = New SqlCommand
Dim msgID_string As String = "SELECT * from dbo.IMSMessages " &
"WHERE MessageID=#MessageID AND isRecieved = 0" &
" ORDER BY Stamp"
Dim isRecievedUpdate_string As String = "Update dbo.IMSMessages " &
"SET isRecieved=1" &
" WHERE MessageID=#MessageID"
Dim adapter As SqlDataAdapter = New SqlDataAdapter
Dim table As DataTable = New DataTable()
Try
If Not sqlConn.State = ConnectionState.Open Then
sqlConn.Open()
End If
command = New SqlCommand
command.CommandText = msgID_string
command.Parameters.Add("#MessageID", SqlDbType.Int).Value = refMessageID
command.Connection = sqlConn
adapter.SelectCommand = command
adapter.Fill(table)
command = New SqlCommand
command.Connection = sqlConn
command.CommandText = isRecievedUpdate_string
command.Parameters.Add("#MessageID", SqlDbType.Int).Value = refMessageID
command.ExecuteNonQuery()
Catch ex As Exception
Console.WriteLine("processMessageID: internal database exception" + ex.Message)
End Try
Try
Dim row As DataRow = table.Rows(0)
Dim senderHost As String = row("SenderHost")
Dim sender As IMSUser = New IMSUser(senderHost)
Dim currentSheet As IMSSheet
Dim stringText As String = row("Text")
Dim stamp As Date = row("Stamp")
currentSheet = New IMSSheet(sender, stringText, stamp)
currentSheet.Show()
Catch ex As Exception
Console.WriteLine("processMessageID: error while showing sheet")
End Try
End Sub
Public Sub recieveNewMessages(newMessageTable As DataTable)
For Each row As DataRow In newMessageTable.Rows
Dim id As Integer = row("MessageID")
processMessageId(id)
Next
End Sub
Public Sub updateOnlineUserList()
lb_Online.Items.Clear()
Dim adapter As SqlDataAdapter = New SqlDataAdapter
Dim table As DataTable = New DataTable()
Dim command As SqlCommand = New SqlCommand
Dim onlineUser_string = "Select * FROM dbo.IMSUser"
command.CommandText = onlineUser_string
Try
If Not sqlConn.State = ConnectionState.Open Then
sqlConn.Open()
End If
command.Connection = sqlConn
adapter.SelectCommand = command
adapter.Fill(table)
Catch ex As Exception
Console.WriteLine("internal database exception" + ex.Message)
End Try
For Each row As DataRow In table.Rows
Dim tmp As IMSUser = New IMSUser(row("HostName"))
If Not user.Equals(tmp) Then
lb_Online.Items.Add(tmp)
End If
Next
End Sub
End Class
IMSSheet-Form:
Public Class IMSSheet
Dim IsDraggingForm As Boolean = False
Private MousePos As New System.Drawing.Point(0, 0)
Public Sub New(session As IMSUser, text As String, stamp As Date)
Ini
tializeComponent()
Me.tb_sender.Text = session.HostName
addText(text, stamp)
End Sub
Private Sub addText(msg As String, stamp As DateTime)
Dim currentText As String = tb_text.Text
currentText = currentText + stamp.ToString + Environment.NewLine + msg + Environment.NewLine
tb_text.Text = currentText
End Sub
Private Sub IMSSheet_MouseDown(ByVal sender As Object, ByVal e As MouseEventArgs) Handles MyBase.MouseDown
If e.Button = MouseButtons.Left Then
IsDraggingForm = True
MousePos = e.Location
End If
End Sub
Private Sub IMSSheet_MouseUp(ByVal sender As Object, ByVal e As MouseEventArgs) Handles MyBase.MouseUp
If e.Button = MouseButtons.Left Then IsDraggingForm = False
End Sub
Private Sub IMSSheet_MouseMove(ByVal sender As Object, ByVal e As MouseEventArgs) Handles MyBase.MouseMove
If IsDraggingForm Then
Dim temp As Point = New Point(Me.Location + (e.Location - MousePos))
Me.Location = temp
temp = Nothing
End If
End Sub
Private Sub onClose(sender As Object, e As EventArgs) Handles bt_Close.Click
Me.Close()
End Sub
Private Sub bt_minimize_Click(sender As Object, e As EventArgs) Handles bt_minimize.Click
Me.WindowState = FormWindowState.Minimized
End Sub
Private Sub bt_Close_MouseHover(sender As Object, e As EventArgs) Handles bt_Close.MouseHover
End Sub
Private Sub bt_minimize_MouseHover(sender As Object, e As EventArgs) Handles bt_minimize.MouseHover
End Sub
End Class
Interesting is that if I have already some new Messages in my Messagestable before I run the programm they are going to be displayed correctly.
If I display the sheets using Form.ShowDialog() it works as well but its not how I want it to work.
As I am out of ideas I hope you can help me.

Answer 1

You are running into issues because the change event of SqlDependency occurs on a different thread that your UI thread. From the SqlDepending documentation:
The OnChange event may be generated on a different thread from the
thread that initiated command execution.
When calling sheet.Show() on a non UI thread, Show() will lockup because there is no message loop to handle the UI events. Use Invoke() on ImsMain to create and show the ImsSheet on your UI thread.