/// <summary>
/// Check whether the destination file exists. If not, create a file with the same
/// size as the file to be downloaded.
/// </summary>
void CheckFileOrCreateFile()
{
DownloaderHelper.CheckFileOrCreateFile(this, locker);
}
/// <summary>
/// Check the Uri to find its size, and whether it supports "Pause".
/// </summary>
public void CheckUrl(out string fileName)
{
fileName = DownloaderHelper.CheckUrl(this);
}