MyPage.cs

using System;

using Xamarin.Forms;

namespace contact_picker
{
    public class MyPage : ContentPage
    {
        Button button;
        public MyPage ()
        {
            button = new Button {
                Text = "choose contact"
            };

            button.Clicked += async (object sender, EventArgs e) => {
                
                    if (Device.OS == TargetPlatform.iOS) {
                        await Navigation.PushModalAsync (new ChooseContactPage ());
                        }
                    else if (Device.OS == TargetPlatform.Android)
                    {
                        MessagingCenter.Send (this, "android_choose_contact", "number1");
                    }

            };

            Content = new StackLayout { 
                Children = {
                    new Label { Text = "Hello ContentPage" },
                    button
                }
            };
        }

        protected override void OnSizeAllocated (double width, double height)
        {
            base.OnSizeAllocated (width, height);

            MessagingCenter.Subscribe<MyPage, string> (this, "num_select", (sender, arg) => {
                DisplayAlert ("contact", arg, "OK");
            });
                
        }
    }
}