How can a background thread hang the UI thread?

Question

I am using a background thread to initialize an instrument over USB. The UI hangs when I try to open the device. I would expect the background thread to pause when calling Open on the device, but not the UI thread. I am testing this with no UI interaction from the background thread. I don't know how to debug the problem, and it's too broad a question, but perhaps someone has seen something like this before. There is nothing wrong with the ActiveX interop as far as I know, the device works correctly. This is the general approach:

using System;
using FancyVoltmeterLibrary;

namespace SOQuestion
{
    public class MeterClass
    {
        private FancyVoltmeter meter;
        private Thread meterThread;

        public MeterClass()
        {
            // Create instance of ActiveX/COM object.
            meter = new FancyVoltmeter();

            meterThread = new Thread(UpdateMeter);
            meterThread.Name = "Meter Thread";
            meterThread.Priority = ThreadPriority.Normal;
            meterThread.IsBackground = true;
            meterThread.Start();
        }

        private void UpdateMeter()
        {
            while(true)
            {
                Thread.Sleep(1000);
                if(!meter.IsOpen())
                {
                    // Meter may be powered off here.
                    // The call to Open takes about 1 second. 
                    // UI hangs during the call???
                    meter.Open();
                }
                // code to read meter goes here.
            }
        }
    }
}

Edit: Perhaps unclear what I meant. By 'hang' I should say 'freezes momentarily'.

Answer 1

Does meter require running in an STA? Is the call to Open() actually being marshalled back to the UI thread for this reason?

You can verify this is true by looking at the callstack of the hung UI thread in the debugger.