How can I listen to this event on a different thread?

Question

I've made a little test program to try and get a USB card reader working using an ActiveX control provided by the manufacturer.

The form works fine as long as it doesn't use a separate thread. I create a new instance of Reader and listen to the Read event and everything works fine. I swipe a card, the event fires and the textbox gets updated.

private Reader _reader;

public Form1()
{
    InitializeComponent();
    CreateScanner();
}

public void CreateScanner()
{
    _reader = new Reader();
    _reader.Read += Read;
}

void Read(object sender, EventArgs e)
{
    CardData.Text = "Card Read";
}

Reader class in case it helps:

public class Reader
{
    private AxUSBHIDInsert _axUsbHid;
    public event EventHandler Read;

    public Reader()
    {
        _axUsbHid = new AxUSBHIDInsert();
        _axUsbHid.CreateControl();
        _axUsbHid.BeginInit();
        _axUsbHid.MSRReadDir = MSRReadDirection.ReadWithdrawal;
        _axUsbHid.EndInit();

        _axUsbHid.PortOpen = true;

        _axUsbHid.CardDataChanged += CardDataChanged;
    }

    void CardDataChanged(object sender, EventArgs e)
    {
        if (Read != null)
            Read(this, new EventArgs());
    }
}

However, I need to run this on a separate thread. So I change the constructor to be

Thread thread = new Thread(CreateScanner);
thread.SetApartmentState(ApartmentState.STA);
thread.Start();

It has to be an STA thread otherwise the ActiveX control will complain that it cannot be instantiated. However doing this, the event doesn't fire anymore. I'm not that familiar with how threading works, so I'm not sure why.

Any ideas? Note that it has to work this way (separate thread, hooked up to the Read event), because the code will eventually reside in a class library that gets deployed with an application that I cannot change.

Answer 1

Your COM object sends messages to the second thread, that means it must be alive all the time application is running.

Try to do like this:

public class Reader
{
  public Reader()
  {
    // leave empty
  }

  public Read() {
    _axUsbHid = new AxUSBHIDInsert();
    ...
  }
}

public Form1()
{
  InitializeComponent();
  _reader = new Reader();
  _reader.Read += Read;
  StartRead(_reader);
}

void StartRead(Reader reader) {
  Thread thread = new Thread(ReadRoutine);
  thread.SetApartmentState(ApartmentState.STA);
  thread.Start(reader);
}

void ReadRoutine(object param) {
  Reader reader = (Reader)param;
  reader.Read();
  while (AppIsAlive) { // add logic here
    // bad code, import GetMessage from user32.dll
    Application.DoEvents();
    Thread.Sleep(100);
  }
}

But the Read event must be processed synchronously:

void Read(object sender, EventArgs e)
{
  if (this.InvokeRequired)
    this.BeginInvoke(new EventHandler(Read), new object[2] { sender, e } );
  else {
    CardData.Text = "Card Read";
  }
}