Xamarin Forms with Prism - Problem with using WebService

Question

Can anyone help me? I created simply project Xamarin Forms with Prism i VS2017 on Android (screen). I used Prism Template Pack. I would like connect project with my WebService. here is a link to screen of all project I have two projects PrismCoursApp and PrismCoursApp.Droid. First project contains SecondPageViewModel.cs where I try use connected WebService (wsMES) but I can't add namespace with PrismCoursApp.Droid. The namespace of project PrismCourseApp.Android is PrismCourseApp.Droid and PrismCourseApp.Android depends on PrismCourseApp.
I could add reference to Web service only in PrismCoursApp.Android project but I would like to use it in SecondPageViewModel.cs in PrismCourseApp.

Can someone tell me what I'm doing wrong? Thanks

SecondPageViewModel.cs
using Prism.Commands;
using Prism.Mvvm;
using Prism.Navigation;
using System;
using System.Collections.Generic;
using System.Linq;

using PrismCourseApp.Models;
using System.Collections.ObjectModel;

namespace PrismCourseApp.ViewModels
{
    public class SecondPageViewModel : BindableBase, INavigationAware
    {
        //zmienna do WebService 
        //wsMES.WSwitoMES ws = new wsMES.WSwitoMES();

        private string _title;
        public string Title
        {
            get { return _title; }
            set { SetProperty(ref _title, value); }
        }

        private string _UserCode;
        public string UserCode
        {
            get { return _UserCode; }
            set { SetProperty(ref _UserCode, value); }
        }

        private string _LokalizCode;
        public string LokalizCode
        {
            get { return _LokalizCode; }
            set { SetProperty(ref _LokalizCode, value); }
        }

        public SecondPageViewModel()
        {
            UserCode = AppStateTest.User;
            LokalizCode = AppStateTest.CurrentCode;

            Title = "Użytkownik/Lokalizacja";


        }


        public void OnNavigatedFrom(INavigationParameters parameters)
        {

        }

        public void OnNavigatedTo(INavigationParameters parameters)
        {
            if (parameters.ContainsKey("par1"))
            {
                string par1 = (string)parameters["par1"];
                string par2 = (string)parameters["par2"];
            }

        }

        public void OnNavigatingTo(INavigationParameters parameters)
        {

        }
    }
}

SecondPage.axml
<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:prism="clr-namespace:Prism.Mvvm;assembly=Prism.Forms"
             prism:ViewModelLocator.AutowireViewModel="True"
             x:Class="PrismCourseApp.Views.SecondPage"
             BackgroundColor="White"
             Title="{Binding Title}"
             xmlns:b="clr-namespace:Prism.Behaviors;assembly=Prism.Forms"  
             xmlns:c="clr-namespace:PrismCourseApp.Converters;assembly=PrismCourseApp">
    <ContentPage.Resources>
        <ResourceDictionary>
            <!--<c:ItemTappedEventArgsConverter x:Key="itemTappedEventArgsConverter" />-->
        </ResourceDictionary>
    </ContentPage.Resources>

    <StackLayout
        Spacing="20">

        <Label
            Text="Zalogowany użytkownik:"
            TextColor="Gray"/>
        <Label 
            Text="{Binding UserCode}"
            FontSize="Large" 
            HorizontalOptions="Center" 
            VerticalOptions="CenterAndExpand" />

        <Label
            Text="Lokalizacja:"
            TextColor="Gray"/>
        <Label 
            Text="{Binding LokalizCode}"
            FontSize="Large" 
            HorizontalOptions="Center" 
            VerticalOptions="CenterAndExpand" />

        <ListView 
            x:Name="lstView">
            <!--ItemsSource="{Binding MyDatas}">-->
            <!--<ListView.Behaviors>
                <b:EventToCommandBehavior EventName="ItemTapped" 
                                          Command="{Binding ItemTappedCommand}"
                                          EventArgsConverter="{StaticResource itemTappedEventArgsConverter}" />
            </ListView.Behaviors>-->
            <ListView.ItemTemplate>
                <DataTemplate>
                    <TextCell Text="{Binding name}" Detail="{Binding comment}" />
                </DataTemplate>
            </ListView.ItemTemplate>
        </ListView>


    </StackLayout>
</ContentPage>

SecondPage.axml.cs
using Xamarin.Forms;
using PrismCourseApp.Models;
using System.Collections.ObjectModel;

namespace PrismCourseApp.Views
{
    public partial class SecondPage : ContentPage
    {    

        //Elementy do ListView (klasa MyDate w PrismCourseApp)
        private ObservableCollection<MyDate> MyDatas { get; set; }

        public SecondPage()
        {
            InitializeComponent();
            MyDatas = new ObservableCollection<MyDate>();
            lstView.ItemsSource = MyDatas;

            for (int i = 0; i < 30; i++)
            {
                MyDatas.Add(new MyDate
                {
                    name = "Pozycja " + (i+1).ToString(),
                    comment = "Miejsce na szczegóły " + (i+1).ToString()
                });
            }
        }    
    }
}

MainActivity.cs in Android Project
using Android.App;
using Android.Content.PM;
using Android.OS;
using Prism;
using Prism.Ioc;

namespace PrismCourseApp.Droid
{
    [Activity(Label = "PrismCourseApp", Icon = "@drawable/ic_launcher", Theme = "@style/MainTheme", MainLauncher = true, ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.Orientation)]
    public class MainActivity : global::Xamarin.Forms.Platform.Android.FormsAppCompatActivity
    {
        protected override void OnCreate(Bundle bundle)
        {
            TabLayoutResource = Resource.Layout.Tabbar;
            ToolbarResource = Resource.Layout.Toolbar;

            base.OnCreate(bundle);

            global::Xamarin.Forms.Forms.Init(this, bundle);
            LoadApplication(new App(new AndroidInitializer()));
        }
    }

    public class AndroidInitializer : IPlatformInitializer
    {
        public void RegisterTypes(IContainerRegistry container)
        {
            // Register any platform specific implementations

        }
    }
}

Answer 1

Summarising the discussion in the comments above:

One should not make a web service dependent upon something specific to one of many client platforms. It's preferable to put the service's interface between the server and the part of the client that's shared between the different client implementations.

Say you have a mobile app for Android and IOS. Then you'll have two projects MyApp.Droid and MyApp.IOS for the respective client-specific implementations. Also, there's a project that both of them reference, and that (hopefully) contains most of your app's client-side logic: MyApp.Logic.

Now for the server: you have the MyApp.Server project that implements the service. If you need to define interfaces to communicate between the app and the service (WCF comes to mind), you define a project referenced by both the client-side logic (MyApp.Logic) and the server implementation (MyApp.Server): MyApp.Interface.

MyApp.Droid & MyApp.IOS -ref-> MyApp.Logic -ref-> MyApp.Interface <-ref- MyApp.Server