how to use textChanged event when from barcode reader

I am using a Barcode reader to get bar code from products . i get the code in a textfield . and i put textchanged event on that textbox . but the problem is that when bar code puts it value ( for example if bar code is 5 digit ) then textchanged event is fired five times . how to get ride of this thing ???

You should be able to program your Barcode reader to output a prefix character and a suffix character (one output before the scanned value and one afterwards). Let's say that you set it up to output an asterisk (*) before the scanned data value and output a carriage return (CR) afterwards. Attach a handler to the TextBox.PreviewTextInput Event and listen out for the asterisk character:

private void PreviewTextInput(object sender, TextCompositionEventArgs e)
{
    if (e.Text == "*") 
    {
        e.Handled = true;
        // Data input has started
    }
}

You can use this to pop up a message saying 'Scanning...', or anything else that you require. Next, attach a handler to the TextBox.KeyUp Event and listen out for the Enter key:

private void KeyUp(object sender, KeyEventArgs e)
{
    if (e.Key == Key.Return)
    {
        string scannedValue = ScanTextBox.Text.Replace("*", string.Empty);
        // Do something with scannedValue 
    }
}

Now the scannedValue variable should contain the scanned barcode value.