Android : calling asmx webservice with ksoap2 failed

i'm struggling to connect my android app with asmx web service using ksoap..

i'm getting http request failed status 400.

my web service is a simple helloworld:

namespace dbservice
{
    /// <summary>
    /// Summary description for dbservice
    /// </summary>
    [WebService(Namespace = "http://tempuri.org/")]
    [WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
    [System.ComponentModel.ToolboxItem(false)]
    // To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line. 
    // [System.Web.Script.Services.ScriptService]
    public class dbservice : System.Web.Services.WebService
    {

        [WebMethod]
        public string HelloWorld()
        {
            return "Hello World";
        }
    }
}

my mainAcitivity:

public class MainActivity extends ActionBarActivity {

    Button save;
    EditText empname;
    TextView view;
    EditText mail;

    private static final String WSDL_TARGET_NAMESPACE = "http://tempuri.org/";
    private static final String SOAP_ADDRESS = "http://10.0.2.2:18442/dbservice.asmx";
    private static final String OPERATION_NAME ="HelloWorld";
    private static final String SOAP_ACTION ="http://tempuri.org/HelloWorld";
    String message;
    String TAG = "SOAP";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        if (android.os.Build.VERSION.SDK_INT > 9) {

        }

        save = (Button) findViewById(R.id.searchbutton);
        empname = (EditText) findViewById(R.id.empname);
        view = (TextView) findViewById(R.id.textView1);
        mail = (EditText) findViewById(R.id.mail);

        save.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {
                myasynck task = new myasynck();
                task.execute();
        }
    });
    }

    private final SoapSerializationEnvelope connectSoap(SoapObject request) {
        SoapSerializationEnvelope envelope = new SoapSerializationEnvelope(SoapEnvelope.VER11);
        envelope.dotNet = true;
        envelope.setOutputSoapObject(request);

        return envelope;
    }

    public void getData()
    {
        SoapObject request = new SoapObject(WSDL_TARGET_NAMESPACE, OPERATION_NAME);

        SoapSerializationEnvelope envelope = connectSoap(request);

        HttpTransportSE  transport = new HttpTransportSE(SOAP_ADDRESS);
        try
        {           
            transport.call(SOAP_ACTION, envelope);
            SoapObject result = (SoapObject) envelope.getResponse();
            message = result.toString(); 

        }catch(Exception ex)
        {
        Log.e("error", ex.getMessage());
        }
}
    public class myasynck extends AsyncTask<String, Void, Void>
    {

        @Override
        protected Void doInBackground(String... params) {

            Log.i(TAG,"backgound");
            getData();
            return null;
        }
         @Override
            protected void onPostExecute(Void result) {
                Log.i(TAG, "onPostExecute");
                mail.setText(message);
            }

            @Override
            protected void onPreExecute() {
                Log.i(TAG, "onPreExecute");
            }
    }

LogCat:

08-29 05:14:10.476: E/error(3253): HTTP request failed, HTTP status: 400

i have internet permission, i'm using android 4.2

hope somebody here will help..