在Renuiz的答案的基础上，我会这样做:

Building on the answer from Renuiz, I would do it this way:

private object lockObj;

private void backgroundWorkerN_RunWorkerCompleted(
    object sender, 
    RunWorkerCompletedEventArgs e)
{
    lock (lockObj)
    {
        y = true;
        if (cb && cr) // if cb and cr flags are true - 
                      // other backgroundWorkers finished work
        {
            someMethodToDoOtherStuff();
        }
    }
}