How to make BackgroundWorker return an object

In your DoWork event handler for the BackgroundWorker (which is where the background work takes place) there is an argument DoWorkEventArgs. This object has a public property object Result. When your worker has generated its result (in your case, a List<FileInfo>), set e.Result to that, and return.

Now that your BackgroundWorker has completed its task, it triggers the RunWorkerCompleted event, which has a RunWorkerCompletedEventArgs object as an argument. RunWorkerCompletedEventArgs.Result will contain the result from your BackgroundWorker.

example:

private void bgw_DoWork(object sender, DoWorkEventArgs e)
{
    int result = 2+2;
    e.Result = result;
}

private void bgw_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
    int result = (int)e.Result;
    MessageBox.Show("Result received: " + result.ToString());
}