Upload file to local SFTP but hit error.

public static void UploadToSFTP(List<string> selectedFileList)
        {
                using (var connection = new SQLiteConnection(GeneralFunc.DBFile))
                {
                    List<Model.SFTPSetting> sftpList = new List<Model.SFTPSetting>();
                    string host = "localhost";
                    int port = 22;
                    string username = "gth";
                    string password = "1234";
                    string dir = @"/D/SFTP";
                    using (SftpClient client = new SftpClient(host, port, username, password))
                    {
                        client.Connect();
                        client.ChangeDirectory(dir);
                        foreach (var file in selectedFileList)
                        {
                            using (FileStream fs = new FileStream(testPath + "/" + file, FileMode.Open))
                            {
                                client.BufferSize = 4 * 1024;
                                client.UploadFile(fs, Path.GetFileName(testPath + "/" + file));
                            }
                        }

                    }
                }

01-09 11:47:25.081 E/mono    (12159): 
01-09 11:47:25.081 E/mono    (12159): Unhandled Exception:
01-09 11:47:25.081 E/mono    (12159): System.Net.Sockets.SocketException (0x80004005): Connection refused
01-09 11:47:25.081 E/mono    (12159):   at (wrapper dynamic-method) System.Object.5(intptr,intptr,intptr)
01-09 11:47:25.081 E/mono-rt (12159): [ERROR] FATAL UNHANDLED EXCEPTION: System.Net.Sockets.SocketException (0x80004005): Connection refused
01-09 11:47:25.081 E/mono-rt (12159):   at (wrapper dynamic-method) System.Object.5(intptr,intptr,intptr)
01-09 11:47:25.099 E/CameraBase(12159): SCANNING dlclose for instance 0x437bbb5ee79d6b9f
01-09 11:47:25.100 E/CameraBase(12159): SCANNING AFTER DLCLOSE