using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

public class District //区
    {
        public string DistrictName { get; set; }    //区名字
        public string DistrictPostcode { get; set; } //区的邮编
    }

    public class City
    {
        public ObservableCollection<District> DistrictObservableCollection { get; set; } //区的集合

        public string CityName { get; set; } //城市的名字
    }

    public class Province
    {
        public ObservableCollection<City> CityObservableCollection { get; set; } //城市的集合

        public string ProvinceName { get; set; } //省份的名字
    }

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

public static class Data
    {
        public static ObservableCollection<Province> Provinces { get; private set; }

        static Data()
        {
            Provinces = new ObservableCollection<Province>()
            {
                new Province()
                {
                    ProvinceName  = "福建省",
                    CityObservableCollection = new ObservableCollection<City>()
                    {
                        new City()
                        {
                            CityName = "福州市",
                            DistrictObservableCollection = new ObservableCollection<District>()
                            {
                                new District()
                                {
                                    DistrictName = "鼓楼区",
                                    DistrictPostcode = "350001"
                                },
                                new District()
                                {
                                    DistrictName = "晋安区",
                                    DistrictPostcode = "350002"
                                },
                                new District()
                                {
                                    DistrictName = "台江区",
                                    DistrictPostcode = "350003"
                                },
                                new District()
                                {
                                    DistrictName = "仓山区",
                                    DistrictPostcode = "350004"
                                },
                            }
                        },
                        new City()
                        {
                            CityName = "厦门市",
                            DistrictObservableCollection = new ObservableCollection<District>()
                            {
                                new District()
                                {
                                    DistrictName = "思明区",
                                    DistrictPostcode = "361001"
                                },
                                new District()
                                {
                                    DistrictName = "湖里区",
                                    DistrictPostcode = "361002"
                                },
                                new District()
                                {
                                    DistrictName = "集美区",
                                    DistrictPostcode = "361003"
                                },
                                new District()
                                {
                                    DistrictName = "海沧区",
                                    DistrictPostcode = "361004"
                                },
                                new District()
                                {
                                    DistrictName = "翔安区",
                                    DistrictPostcode = "361005"
                                },
                            }
                        },
                        new City()
                        {
                            CityName = "泉州市",
                            DistrictObservableCollection = new ObservableCollection<District>()
                            {
                                new District()
                                {
                                    DistrictName = "丰泽区",
                                    DistrictPostcode = "362001"
                                },
                                new District()
                                {
                                    DistrictName = "鲤城区",
                                    DistrictPostcode = "362002"
                                },
                                new District()
                                {
                                    DistrictName = "洛江区",
                                    DistrictPostcode = "362003"
                                },
                                new District()
                                {
                                    DistrictName = "泉港区",
                                    DistrictPostcode = "362004"
                                },
                            }
                        },
                    }
                },
                new Province()
                {
                    ProvinceName  = "浙江省",
                    CityObservableCollection = new ObservableCollection<City>()
                    {
                        new City()
                        {
                            CityName = "杭州市",
                            DistrictObservableCollection = new ObservableCollection<District>()
                            {
                                new District()
                                {
                                    DistrictName = "西湖区",
                                    DistrictPostcode = "310001"
                                },
                                new District()
                                {
                                    DistrictName = "余杭区",
                                    DistrictPostcode = "310002"
                                },
                                new District()
                                {
                                    DistrictName = "萧山区",
                                    DistrictPostcode = "310003"
                                },
                                new District()
                                {
                                    DistrictName = "下城区",
                                    DistrictPostcode = "310004"
                                },
                            }
                        },
                        new City()
                        {
                            CityName = "宁波市",
                            DistrictObservableCollection = new ObservableCollection<District>()
                            {
                                new District()
                                {
                                    DistrictName = "海曙区",
                                    DistrictPostcode = "315001"
                                },
                                new District()
                                {
                                    DistrictName = "江东区",
                                    DistrictPostcode = "315002"
                                },
                                new District()
                                {
                                    DistrictName = "江北区",
                                    DistrictPostcode = "315003"
                                },
                                new District()
                                {
                                    DistrictName = "镇海区",
                                    DistrictPostcode = "315004"
                                },
                                new District()
                                {
                                    DistrictName = "北仑区",
                                    DistrictPostcode = "315005"
                                },
                            }
                        },
                        new City()
                        {
                            CityName = "温州市",
                            DistrictObservableCollection = new ObservableCollection<District>()
                            {
                                new District()
                                {
                                    DistrictName = "鹿城区",
                                    DistrictPostcode = "325001"
                                },
                                new District()
                                {
                                    DistrictName = "龙湾区",
                                    DistrictPostcode = "325002"
                                },
                                new District()
                                {
                                    DistrictName = "瓯海区",
                                    DistrictPostcode = "325003"
                                },
                            }
                        },
                    }
                },
                new Province()
                {
                    ProvinceName  = "安徽省",
                    CityObservableCollection = new ObservableCollection<City>()
                    {
                        new City()
                        {
                            CityName = "合肥市",
                            DistrictObservableCollection = new ObservableCollection<District>()
                            {
                                new District()
                                {
                                    DistrictName = "蜀山区",
                                    DistrictPostcode = "230001"
                                },
                                new District()
                                {
                                    DistrictName = "庐阳区",
                                    DistrictPostcode = "230002"
                                },
                                new District()
                                {
                                    DistrictName = "瑶海区",
                                    DistrictPostcode = "230003"
                                },
                                new District()
                                {
                                    DistrictName = "包河区",
                                    DistrictPostcode = "230004"
                                },
                            }
                        },
                        new City()
                        {
                            CityName = "淮北市",
                            DistrictObservableCollection = new ObservableCollection<District>()
                            {
                                new District()
                                {
                                    DistrictName = "相山区",
                                    DistrictPostcode = "235001"
                                },
                                new District()
                                {
                                    DistrictName = "杜集区",
                                    DistrictPostcode = "235002"
                                },
                                new District()
                                {
                                    DistrictName = "烈山区",
                                    DistrictPostcode = "235003"
                                },
                            }
                        },
                        new City()
                        {
                            CityName = "宿州市",
                            DistrictObservableCollection = new ObservableCollection<District>()
                            {
                                new District()
                                {
                                    DistrictName = "埇桥区",
                                    DistrictPostcode = "234001"
                                },
                            }
                        },
                    }
                },
                new Province()
                {
                    ProvinceName  = "江西省",
                    CityObservableCollection = new ObservableCollection<City>()
                    {
                        new City()
                        {
                            CityName = "南昌市",
                            DistrictObservableCollection = new ObservableCollection<District>()
                            {
                                new District()
                                {
                                    DistrictName = "东湖区",
                                    DistrictPostcode = "330001"
                                },
                                new District()
                                {
                                    DistrictName = "西湖区",
                                    DistrictPostcode = "330002"
                                },
                                new District()
                                {
                                    DistrictName = "青云谱区",
                                    DistrictPostcode = "330003"
                                },
                                new District()
                                {
                                    DistrictName = "湾里区",
                                    DistrictPostcode = "330004"
                                },
                            }
                        },
                        new City()
                        {
                            CityName = "景德镇市",
                            DistrictObservableCollection = new ObservableCollection<District>()
                            {
                                new District()
                                {
                                    DistrictName = "珠山区",
                                    DistrictPostcode = "333001"
                                },
                                new District()
                                {
                                    DistrictName = "昌江区",
                                    DistrictPostcode = "333002"
                                },
                            }
                        },
                        new City()
                        {
                            CityName = "九江市",
                            DistrictObservableCollection = new ObservableCollection<District>()
                            {
                                new District()
                                {
                                    DistrictName = "浔阳区",
                                    DistrictPostcode = "332001"
                                },
                                new District()
                                {
                                    DistrictName = "庐山区",
                                    DistrictPostcode = "332002"
                                },
                            }
                        },
                    }
                }
            };


        }
    }

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="DemoTest_3_28.PickerDemo.PickerDemoPage">
    <StackLayout Orientation="Vertical"
                 VerticalOptions="Center"
                 HorizontalOptions="Center">
        <Label Text="邮编:" 
               HorizontalOptions="Center"
               FontAttributes="Bold"
               FontSize="20"
               x:Name="label_Postcode" />
        <StackLayout Orientation="Horizontal">
            <Picker x:Name="provincePicker" Title="省份" SelectedIndexChanged="ProvincePicker_OnSelectedIndexChanged"/>
            <Picker x:Name="CityPicker" Title="城市" SelectedIndexChanged="CityPicker_OnSelectedIndexChanged"/>
            <Picker x:Name="DistrictPicker" Title="区" SelectedIndexChanged="DistrictPicker_OnSelectedIndexChanged"/>
        </StackLayout>
    </StackLayout>
</ContentPage>

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using DemoTest_3_28.Models;
using Xamarin.Forms;
using Xamarin.Forms.Xaml;

namespace DemoTest_3_28.PickerDemo
{
    [XamlCompilation(XamlCompilationOptions.Compile)]
    public partial class PickerDemoPage : ContentPage
    {
        private ObservableCollection<Province> source;
        private string postcode = null;

        public PickerDemoPage()
        {
            InitializeComponent();
            source = Data.Provinces;
            foreach (var province in source)
            {
                provincePicker.Items.Add(province.ProvinceName);               
            }

            provincePicker.SelectedIndex = 0;
        }

        private void ProvincePicker_OnSelectedIndexChanged(object sender, EventArgs e)
        {
            if (provincePicker.SelectedIndex!=-1)
            {
                var result_1 = source[provincePicker.SelectedIndex].CityObservableCollection;
                CityPicker.Items.Clear();//重要, 而且触发CityPicker_OnSelectedIndexChanged,CityPicker.SelectedIndex=-1
                foreach (var city in result_1)
                {
                    CityPicker.Items.Add(city.CityName);
                }
                CityPicker.SelectedIndex = 0;
            }
        }

        private void CityPicker_OnSelectedIndexChanged(object sender, EventArgs e)
        {
            if (CityPicker.SelectedIndex!=-1)
            {
                var result_2 = source[provincePicker.SelectedIndex].CityObservableCollection[CityPicker.SelectedIndex].DistrictObservableCollection;
                DistrictPicker.Items.Clear(); //重要
                foreach (var district in result_2)
                {
                    DistrictPicker.Items.Add(district.DistrictName);
                }
                DistrictPicker.SelectedIndex = 0; //重要
            }
        }

        private void DistrictPicker_OnSelectedIndexChanged(object sender, EventArgs e)
        {
            if (DistrictPicker.SelectedIndex!=-1)
            {
                postcode = source[provincePicker.SelectedIndex].CityObservableCollection[CityPicker.SelectedIndex].DistrictObservableCollection[DistrictPicker.SelectedIndex].DistrictPostcode;

                label_Postcode.Text = "邮编:" + postcode;
            }
        }
    }
}