Can't write date to a file if I opened a video

Question

Hi,
I have a problem of DirectX. I can not write data to a file if I opened a video.

StreamWriter sw = new StreamWriter ( "config1.cfg", false);
Sw.WriteLine (imageWidth);
Sw.Close ();
MessageBox.show ( "ABC");

I closed on video before, and I know that this section of code is executed (a messagebox displayed). But there is no file created. It works well if I did not have the video.
Can you help me?