CIHsieh

class Background
{
    public static void Repeat(Action action, Func keepRunning)
    {
        new Background(action, keepRunning);
    }
    public static void Execute(Action action)
    {
        new Background(action, null);
    }

    private readonly Action _action;
    private readonly Func _keepRunning;
    private Background(Action action, Func keepRunning)
    {
        _action = action;
        _keepRunning = keepRunning;
        _action.BeginInvoke(EndInvoke, this);
    }
    private void EndInvoke(IAsyncResult ar)
    {
        _action.EndInvoke(ar);
        if (_keepRunning != null && _keepRunning())
            _action.BeginInvoke(EndInvoke, this);
    }
}

class Program
{
    static void Main(string[] args)
    {
        var keepRunning = true;
        //Background.Execute(() => Test("Q"));
        //Background.Execute(() => Test("A"));
        //Background.Execute(() => Test("Z"));
        Background.Repeat(() => Test("Q"), () => keepRunning);
        Background.Repeat(() => Test("A"), () => keepRunning);
        Background.Repeat(() => Test("Z"), () => keepRunning);
        Console.ReadKey(true);
        keepRunning = false;
        Console.ReadKey(true);
    }
    static void Test(string id)
    {
        for (int i = 0; i < 5; i++)
        {
            Console.WriteLine("{2} ({0}/5) {1}", i + 1, DateTime.Now, id);
            Thread.Sleep(1000);
        }
    }

}