I could solve it with the solution posted here.
FileVideoSource fileVideo = new FileVideoSource(dialog.FileName);
AsyncVideoSource asyncVideoSource = new AsyncVideoSource(fileVideo);
asyncVideoSource.NewFrame += asyncVideoSource_NewFrame;
asyncVideoSource.Start();
private void asyncVideoSource_NewFrame(object sender, NewFrameEventArgs eventArgs)
{
Image temp = PictureBoxvideo.Image;
Bitmap bitmap = eventArgs.Frame;
PictureBoxvideo.Image = new Bitmap(bitmap);
if (temp!= null) temp.Dispose();
bitmap.Dispose();
}