public class Program
{
    static public void Main()
    {
        var library = new Library();
        library.Callback = GetMoreData;
        var task = Task.Run( () => library.Foo() );
        Console.WriteLine("Other thread is running.");
        task.Wait();
    }

    static string GetMoreData()
    {
        return "More data";
    }
}



class Library
{
    public Func<string> Callback { get; set; }

    public async Task Foo()
    {
        for (int i=0; i<10; i++)
        {
            var moreData = Callback();
            Console.WriteLine("Library received this data: {0}", moreData);
            await Task.Delay(500);
        }
    }
}