public class Downloader : IBindStatusCallback, IDisposable, IAuthenticate

    URLDownloadToFile(IntPtr.Zero, SourcePath , LocalPath ,0,(IBindStatusCallback)this);

    public interface IBindStatusCallback

    public class Downloader : IBindStatusCallback,IDisposable

            [MarshalAs(UnmanagedType.Interface)] IBindStatusCallback lpfnCB);

                    URLDownloadToFile(IntPtr.Zero, SourcePath , LocalPath ,0,(IBindStatusCallback)this);//use 0x10 for new download