Why does focusing a TextBox speed up another thread's progress? - wpf

Question

Hello Stackoverflowers!
I have a strange situation here. I wrote a small C# tool, to measure the time it takes to send a request via usb and wait until the response arrives. The communication is no problem, nor are the devices.
I put a simple TextBox on the GUI together with a "Start" button.
Pressing "Start" starts a thread with a loop (loops defined in the TextBox) sending requests and receiving the answers. In the GUI thread you can see a progress bar showing the status.
When I pressed "Start" I measured around 15 ms per command and response (in average) which is quite a long time.
When I pressed "Start" and clicked inside the TextBox (only setting the cursor inside), the worker thread ran through with an average time of 1.6 ms per command and response.
This is reproducible in my case. I even put in a new, totally useless TextBox. When clicking inside, the thread speeds up, when I focus another element, like a slider, the thread slows down again.
Has anyone ever heard about something like that and can explain to me, why this happens? And even better, how I can avoid this behavior?
FYI:
- I tried both BackgroundWorker and Thread, same behavior.
- For taking the times, I'm using .NET's stopwatch.
- Executing the measurement method in the GUI thread changes nothing. EDIT: Tried again and everything's as it should be. But now I have the problem, that the GUI freezes, since I'm working in its thread.
- Removing the progress bar changes nothing.
Thanks a lot!
EDIT:
private void tbAmount_Copy_PreviewKeyUp(object sender, KeyEventArgs e)
{
if (e.Key == Key.Enter)
{
if (String.IsNullOrEmpty(tbDevAddrTime.Text))
return;
pbTimeMeasure.Value = 0;
worker = new BackgroundWorker();
worker.DoWork += new DoWorkEventHandler(StartTimeMeasureHandler);
worker.RunWorkerAsync();
}
}
private void StartTimeMeasure()
{
int address = 1;
int loops = 1;
DispatchIfNecessary(() =>
{
address = Convert.ToInt32(tbDevAddrTime.Text);
loops = Convert.ToInt32(tbAmount_Copy.Text);
pbTimeMeasure.Maximum = loops;
});
System.Diagnostics.Stopwatch stopwatchLong = new System.Diagnostics.Stopwatch();
System.Diagnostics.Stopwatch stopwatchShort = new System.Diagnostics.Stopwatch();
var cmd = String.Format(":{0:X3}4010200300000000\r", RequestAddress + address);
for (int i = 0; i < loops; i++)
{
stopwatchLong.Start();
//SND
_canbus.Write(cmd, cmd.Length, ref _bytesWritten);
stopwatchShort.Start();
Thread.Sleep(1);
//RCV
var answer = ReadContent();
stopwatchShort.Stop();
stopwatchLong.Stop();
DispatchIfNecessary(() =>
{
pbTimeMeasure.Value++;
});
}
Double resLong = Convert.ToDouble(stopwatchLong.ElapsedMilliseconds);
Double resLongAvg = resLong / loops;
Double resShort = Convert.ToDouble(stopwatchShort.ElapsedMilliseconds);
Double resShortAvg = resShort / loops;
DispatchIfNecessary(() =>
{
lbTotalResLong.Content = resLong.ToString();
lbTotalResShort.Content = resShort.ToString();
int length = 5;
if (resLongAvg.ToString().Length < 5)
{
length = resLong.ToString().Length;
}
if (resShortAvg.ToString().Length < length)
{
length = resShortAvg.ToString().Length;
}
lbAvgResLong.Content = resLongAvg.ToString().Substring(0,length);
lbAvgResShort.Content = resShortAvg.ToString().Substring(0,length);
});
}