R# is Smart YOBA

-- before
foreach (var file in files)
{
    if (!Results.CheckFile(file))
    {
        MessageBox.Show(String.Format("File {0} has unknown format.", file));
        return;
    }
}

-- after
foreach (var file in files.Where(file => !Results.CheckFile(file)))
{
    MessageBox.Show(String.Format("File {0} has unknown format.", file));
    return;
}