Vb.net - Cross-threading exception by closing a form

Question

I'm working on an application to read something from a serial port (COMM-port). In short, it works like this: when you work in a bar or restaurant, before you can enter something in the register, you have to scan a sort of card. If this card returns a good number, you can enter something.

So, there has to be a form that listens to the serial port and checks whether someone scans a card and if it's a card with good rights.

If the person has the good rights, the form can be closed and another form is called.

Now, in code:

Here, the MenuForm is loaded (the form that has to be accesible after the correct code was read). I call the frmWaiterKey to show up.

Private Sub frmMenu_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
    Dim oForm As frmWaiterKey = New frmWaiterKey()
    oForm.ShowDialog()
End Sub

The code of the class frmWaiterKey:

Private Sub frmWaiterKey_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
    nameArray = SerialPort.GetPortNames
    OpenComPort()
    AddHandler myComPort.DataReceived, SerialDataReceivedEventHandler1
End Sub

Sub OpenComPort()

    Try
        ' Get the selected COM port's name 
        ' from the combo box.
        If Not myComPort.IsOpen Then
            myComPort.PortName = _
            nameArray(0).ToString()
            ' Get the selected bit rate from the combo box.
            myComPort.BaudRate = CInt(9600)
            ' Set other port parameters.
            myComPort.Parity = Parity.None
            myComPort.DataBits = 8
            myComPort.StopBits = StopBits.One
            myComPort.Handshake = Handshake.None

            'myComPort.ReadTimeout = 3000
            'myComPort.WriteTimeout = 5000

            ' Open the port.
            myComPort.Open()

        End If

    Catch ex As InvalidOperationException
        MessageBox.Show(ex.Message)
    Catch ex As UnauthorizedAccessException
        MessageBox.Show(ex.Message)
    Catch ex As System.IO.IOException
        MessageBox.Show(ex.Message)
    End Try

End Sub
Sub CloseComPort()

    Using myComPort
        If (Not (myComPort Is Nothing)) Then
            ' The COM port exists.
            If myComPort.IsOpen Then
                ' Wait for the transmit buffer to empty.
                Do While (myComPort.BytesToWrite > 0)
                Loop
            End If
        End If
    End Using

End Sub

Private SerialDataReceivedEventHandler1 As New SerialDataReceivedEventHandler(AddressOf DataReceived)

' Specify the routine that runs when 
' a DataReceived event occurs at myComPort.

' This routine runs when data arrives at myComPort.

Friend Sub DataReceived(ByVal sender As Object, ByVal e As SerialDataReceivedEventArgs)
    Dim newReceivedData As String
    ' Get data from the COM port.
    newReceivedData = myComPort.ReadExisting
    newReceivedData = newReceivedData.Trim()
    MsgBox(newReceivedData)
    If newReceivedData.Equals("00150324294764") Then
        CloseComPort()
        Me.Close()
    End If

End Sub

I get an error in the last line: Me.Close() I get the point: I call the form frmWaiterKey from the frmMenu and can't close it here... But I have no idea how to solve this problem.

I hope someone can help me or tell me what I'm doing wrong.

Answer 1

First, you need to make a method like this:

Private Sub CloseMe()
    If Me.InvokeRequired Then
        Me.Invoke(New MethodInvoker(AddressOf CloseMe))
        Exit Sub
    End If
    Me.Close()
End Sub

Then, close your form by calling that method, like this:

If newReceivedData.Equals("00150324294764") Then
    CloseComPort()
    CloseMe()
End If

The reason this is necessary is because all UI activity in WinForms must be performed from the same thread. Since the DataReceived method is being called from another thread, it must get back onto the UI thread before it can close the form. The InvokeRequired property returns true if you are on any thread other than the UI thread, and the Invoke method invokes the given method from the UI thread.