private class AsyncLogin extends AsyncTask<String, String, LoginModel>
{
    HttpURLConnection conn;
    BufferedReader reader = null;
    URL url = null;
    @Override
    protected  LoginModel doInBackground(String... params) {
        try {

            // Enter URL address where your php file resides
            url = new URL("http://192.168.1.5:80/api/ventas/Login");

        } catch (MalformedURLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
            //return "exception";
        }
        try {
            // Setup HttpURLConnection class to send and receive data from php and mysql
            conn = (HttpURLConnection)url.openConnection();
            conn.setReadTimeout(READ_TIMEOUT);
            conn.setConnectTimeout(CONNECTION_TIMEOUT);
            conn.setRequestMethod("POST");
            conn.setRequestProperty("Content-Type", "application/json;");
            conn.setRequestProperty( "charset", "utf-8");

            // setDoInput and setDoOutput method depict handling of both send and receive
            conn.setDoInput(true);
            conn.setDoOutput(true);

            // Append parameters to URL
            Uri.Builder builder = new Uri.Builder()
                    .appendQueryParameter("User", params[0])
                    .appendQueryParameter("Pass", params[1]);
            String query = builder.build().getEncodedQuery();

            // Open connection for sending data
            OutputStream os = conn.getOutputStream();
            BufferedWriter writer = new BufferedWriter(
                    new OutputStreamWriter(os, "UTF-8"));
            writer.write(query);
            writer.flush();
            writer.close();
            os.close();
            conn.connect();

        } catch (IOException e1) {
            // TODO Auto-generated catch block
            e1.printStackTrace();
           // return "exception";
        }

        InputStream stream = null;
        try {
            stream = conn.getInputStream();
            reader = new BufferedReader(new InputStreamReader(stream));

            StringBuffer buffer = new StringBuffer();
            String line = "";

            while ((line = reader.readLine()) != null)
            {
                buffer.append(line + "");
            }

            String finalJson = buffer.toString();
            JSONObject object = new JSONObject(finalJson);

            if(object.has("Message"))
            {
                return null;
            }
            else
            {
                LoginModel objModel = new LoginModel();

                objModel.setIdEmpleado(object.getInt("IdEmpleado"));
                objModel.setNombres(object.getString("Nombres"));
                objModel.setUsuario(object.getString("Usuario"));
                objModel.setClave(object.getString("Clave"));

                return objModel;
            }
        } catch (IOException e1) {
            e1.printStackTrace();
        } catch (JSONException e) {
            e.printStackTrace();
        } finally {
            if(conn != null)
            {
                conn.disconnect();
            }

            try {
                if(reader != null){reader.close();}

            } catch (IOException e) {
                e.printStackTrace();
            }
        }

        return  null;
    }

    @Override
    protected void onPostExecute( LoginModel loginModel) {
        super.onPostExecute(loginModel);

        if (loginModel != null)
        {

            datosUsuario.setNombre_usuario(loginModel.getUsuario());
            datosUsuario.setClave(loginModel.getClave());
            datosUsuario.setNombreCompleto(loginModel.getNombres());
            datosUsuario.setIdEmpleado(loginModel.getIdEmpleado());

            SharedPreferences
                preferences
                = getSharedPreferences("temp", getApplicationContext().MODE_PRIVATE);
            SharedPreferences.Editor editor = preferences.edit();
            editor.putString("username", datosUsuario.getNombre_usuario());
            editor.putString("clave", datosUsuario.getClave());
            editor.putString("nombres", datosUsuario.getNombreCompleto());
            editor.putInt("idempleado", datosUsuario.getIdEmpleado());
            editor.commit();
            openPrincipal();
        }
        else
        {
            Toast.makeText(ctx, "Datos incorrectos" ,Toast.LENGTH_SHORT).show();
        }
    }
}

[HttpPost]
public IHttpActionResult Login(String User, String Pass)
{
    try
    {
        SombraEntities contexto = new SombraEntities();

        if (String.IsNullOrEmpty(User) || String.IsNullOrEmpty(Pass))
        {
            var data = new
            {
                Message = "No llegaron los datos."
            };

            return Json(data);
        }

        Usuario objusuario = (from data in contexto.Usuario
                              where data.User == User && data.Password == User
                              select data).SingleOrDefault();

        if (objusuario != null)
        {
            Empleado objempleado = objusuario.Empleado.FirstOrDefault();

            var data = new
            {
                IdEmpleado = objempleado.IdEmpleado,
                Nombres = objempleado.Nombres + " " + objempleado.Apellidos,
                Usuario = objempleado.Usuario.User,
                Clave = objempleado.Usuario.Password
            };

            return Json(data);
        }
        else
        {
            var data = new
            {
                Message = "Datos de Login Incorrectos"
            };

            return Json(data);
        }
    }
    catch (Exception e)
    {
        throw;
    }
}

<uses-permission android:name="android.permission.INTERNET" />

Uri.Builder builder = new Uri.Builder()
                        .appendQueryParameter("User", params[0])
                        .appendQueryParameter("Pass", params[1]);

[HttpPost]
    public IHttpActionResult Login(String User, String Pass)
    {
        try
        {
            SombraEntities contexto = new SombraEntities();

            if (String.IsNullOrEmpty(User) || String.IsNullOrEmpty(Pass))
            {
                var data = new
                {
                    Message = "No llegaron los datos."
                };

                return Json(data);
            }




            Usuario objusuario = (from data in contexto.Usuario
                                  where data.User == User && data.Password == User
                                  select data).SingleOrDefault();


            if (objusuario != null)
            {

                Empleado objempleado = objusuario.Empleado.FirstOrDefault();

                var data = new
                {
                    IdEmpleado = objempleado.IdEmpleado,
                    Nombres = objempleado.Nombres + " " + objempleado.Apellidos,
                    Usuario = objempleado.Usuario.User,
                    Clave = objempleado.Usuario.Password
                };

                return Json(data);

            }
            else
            {
                var data = new
                {
                    Message = "Datos de Login Incorrectos"
                };

                return Json(data);
            }
        }
        catch (Exception e)
        {
            throw;
        }



    }