while(true){
    messages = clientAPI.getMessages();
    // do stuff
}

public class ClientApi
{

    public EventHandler SomethingHappened;

    public void StartWorkerMethod()
    {
        var i = 0;
        while (i < 10)
        {
            System.Threading.Thread.Sleep(2000);
            if (SomethingHappened != null)
                SomethingHappened(this, EventArgs.Empty);
            i++;
        }
    }
}

var clientApi = new ClientApi();
clientApi.SomethingHappened += (s, e) => Console.WriteLine("Something happened on the server!");
clientApi.StartWorkerMethod();