Credits: fcamuso https://www.youtube.com/playlist?list=PL0qAPtx8YtJe3WjjoRaB28ZGlX9heBqn3

*MVVM*

Credits: fcamuso

1-MainWindow.xaml

<Window x:Class="HelloWpf.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:HelloWpf"
        xmlns:sys="clr-namespace:System;assembly=mscorlib"
        mc:Ignorable="d"
        Title="MainWindow" Height="300" Width="500">

  <Window.Resources>
  
  </Window.Resources>
  
    <StackPanel Margin="10" >
        <StackPanel Orientation="Horizontal">
            <TextBlock Text="Nome:" />
            <TextBox x:Name="tboxName" Width="120" Margin="8,0"/>
            <TextBlock Text="Cognome:" />
            <TextBox x:Name="tboxSurname" Width="120" Margin="8,0"/>
            <Button x:Name="btnSalutami" Click="BtnSalutami_Click" Content="Salutami" />
        </StackPanel>
        <TextBlock/>
            <TextBlock x:Name="tboxResult" x:FieldModifier="private"/>
        </StackPanel>
  
</Window>

1-MainWindow.xaml.cs

namespace HelloWpf
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }
        private void BtnSalutami_Click(object sender, RoutedEventArgs e)
        {
            SalutaMi();
        }
        private void SalutaMi()
        {
            tboxResult.Text = String.Format("Ciao {0} {1}", tboxName.Text, tboxSurname.Text);
        }
    }
}

2-MainWindow.xaml

<Window x:Class="HelloWpf.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:HelloWpf"
        mc:Ignorable="d"
        Title="MainWindow" Height="135" Width="367">

    <StackPanel >
        <StackPanel Orientation="Horizontal" Margin="10,10,10,0">
        <StackPanel Orientation="Horizontal" Background="Aquamarine" Margin="0,0,5,0">

            <Label>Surname</Label>
            <ComboBox x:Name="comboSurname" Width="150" Margin="3,3,10,3" />

        </StackPanel>

            <Button Content="Salutami" Width="105" Click="Button_Click"/>

        </StackPanel>

        <StackPanel Background="Beige" Height="50" Margin="10,10,10,0">
        <TextBox x:Name="tboxResult" Margin=" 10,10,0,0" Width="150" Background="Transparent" HorizontalAlignment="Center" BorderBrush="{x:Null}"/>
        </StackPanel>
        
    </StackPanel>
    
</Window>

2-MainWindow.xaml.cs

public partial class MainWindow : Window
    {
        private Models.PeopleService peopleService = null; 

        public MainWindow()
        {
            InitializeComponent();

            peopleService = new Models.PeopleService();
            comboSurname.ItemsSource = peopleService.People;
            comboSurname.DisplayMemberPath = "Surname";

        }

        private void BtnSalutami_Click(object sender, RoutedEventArgs e)
        {
            SalutaMi();
        }

        private void SalutaMi()
        {
            if (comboSurname.SelectedItem != null)
            {
                Models.Person chosenPerson = comboSurname.SelectedItem as Models.Person;
                tboxResult.Text = String.Format("Ciao {0} {1}", chosenPerson.Name, chosenPerson.Surname);
            }

        }

    }

2-PeopleService.cs

 class PeopleService
    {
        private List<Person> _people = null; //private -> need a getter to make it public

        public PeopleService()
        {
            _people = new List<Person>();
            _people.Add(new Person { Name = "Mark", Surname = "Gallagher" });
            _people.Add(new Person { Name = "John", Surname = "Snow" });

        }
        public IList<Person> People => _people; //return an interface (more generic)

        /* equivalent to:
        public IList<Person> People
        {
            get { return _people; }
        }*/
    }

2-Person.cs

    public class Person
    {
        public string Name { get; set; }
        public string Surname { get; set; }

    }

3-App.xaml.cs

    public partial class App : Application
    {/*
        private void Application_Startup(object sender, StartupEventArgs e)
        {
            MainWindow mwin = new MainWindow(new Models.PeopleService());

            mwin.Show();

        }*/

        protected override void OnStartup(StartupEventArgs e)
        {
            base.OnStartup(e);

            MainWindow mainWindow = new MainWindow(new Models.PeopleService());
            MainWindow.Show();

        }
    }

3-MainWindow.xaml.cs

public partial class MainWindow : Window
    {
        private Models.IPeopleService peopleService = null; 
        //private Models.PeopleService peopleService = null;

        public MainWindow(Models.IPeopleService _peopleService)
        {
            InitializeComponent();
            //peopleService = new Models.PeopleService();
            peopleService = _peopleService;
            comboSurname.ItemsSource = peopleService.People;
            comboSurname.DisplayMemberPath = "Surname";
        }

        private void BtnSalutami_Click(object sender, RoutedEventArgs e)
        {
            SalutaMi();
        }

        private void SalutaMi()
        {
            if (comboSurname.SelectedItem != null)
            {
                Models.Person chosenPerson = comboSurname.SelectedItem as Models.Person;
                tboxResult.Text = String.Format("Ciao {0} {1}", chosenPerson.Name, chosenPerson.Surname);
            }
        }
    }

5-App.xaml.cs

public partial class App : Application
{
    protected override void OnStartup(StartupEventArgs e)
    {
        Models.IPeopleService peopleService = new Models.PeopleService();

        ViewModels.MWViewModel viewModel = new ViewModels.MWViewModel(peopleService);

        MainWindow mw = new MainWindow(viewModel);

        mw.Show();
    }
}

5-MainWindow.cs

public partial class MainWindow : Window
{
    public MainWindow(ViewModels.MWViewModel vM)
    {   
        InitializeComponent();
        this.DataContext = vM;
    }
    private void Button_Click(object sender, RoutedEventArgs e)
    {
        (DataContext as ViewModels.MWViewModel).SalutaMi();
    }
}

5-MainWindow.xaml.cs

<Window x:Class="HelloWpf.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:HelloWpf"
        mc:Ignorable="d"
        Title="MainWindow" Height="140" Width="380">
    <Grid ShowGridLines="False" Margin="3">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="1*"/>
            <ColumnDefinition Width="0.5*"/>
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition/>
            <RowDefinition/>
        </Grid.RowDefinitions>

        <StackPanel Orientation="Horizontal" Background="Bisque" Margin="5">
            <Label Content="Surname" VerticalAlignment="Center"/>
            <ComboBox Width="170" Height="25"
                      x:Name="cbox"
                      ItemsSource="{Binding People}"
                      DisplayMemberPath="Surname"
                      SelectedItem="{Binding Path = ChosenP}"
                      />
        </StackPanel>
        <Button Grid.Column="1" Width="100" Height="25" Content="Salutami" Click="Button_Click"/>
        
        
        <StackPanel Grid.Row="1" Background="PaleGreen" Grid.ColumnSpan="2" Margin="10" Width=" 150">
            <TextBox x:Name="tbox" FontSize="15" TextAlignment="Center" Width="150" BorderBrush="Transparent" Background="Transparent"
              Text="{Binding Path = TextSaluto}"/>
            
        </StackPanel>
        
    </Grid>

    
</Window>

5-MWViewModel.cs

using System.Collections.Generic;
using System.ComponentModel;

namespace HelloWpf.ViewModels
{
    public class MWViewModel : INotifyPropertyChanged
    {
        private Models.IPeopleService _peopleService = null;

        public MWViewModel(Models.IPeopleService peopleService)
        {
            _peopleService = peopleService;
        }

        public IList<Models.Person> People => _peopleService.PeopleList;
        
        
        //properties for binding
        public Models.Person ChosenP { get; set; }
        private string _txtSaluto;

        public event PropertyChangedEventHandler PropertyChanged;

        public string TextSaluto
        {
            get { return _txtSaluto; }
            set
            {
                _txtSaluto = value;
                PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(TextSaluto))); //better to use nameof(Property)

            }
        }
        
        public void SalutaMi()
        {
            if (ChosenP != null)
            {
                TextSaluto = string.Format("Ciao {0} {1}", ChosenP.Name, ChosenP.Surname);
            }

        }
    }
}

5-PeopleService.cs

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

namespace HelloWpf.Models
{
    public interface IPeopleService
    {
        IList<Person> PeopleList { get; }
    }
    public class PeopleService : IPeopleService
    {
       private List<Person> _peopleList = new List<Person>();
       public PeopleService()
        {
            _peopleList.Add(new Person { Name = "Pablo", Surname = "Luis" });
            _peopleList.Add(new Person { Name = "Ana", Surname = "Maria" });
            _peopleList.Add(new Person { Name = "Sara", Surname = "Lucia" });
        }
        public IList<Person> PeopleList => _peopleList;
    }
}

6-MWViewModel.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Runtime.CompilerServices;
using System.Windows.Input;

namespace HelloWpf.ViewModels
{

    public abstract class BaseViewModel : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;
        protected void NotifyPropertyChanged([CallerMemberName] string propertyName = "")
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName)); //better than using the string for refactoring
        }
    }

    public class RelayCommand : ICommand
    {
        public event EventHandler CanExecuteChanged;
        private Action<object> executeMethod;
        private Predicate<object> canExecuteMethod;

        public RelayCommand(Action<object> Execute, Predicate<object> CanExecute)
        {
            canExecuteMethod = CanExecute;
            executeMethod = Execute;
        }

        public bool CanExecute(object parameter)
        {
            return (canExecuteMethod == null) ? true : canExecuteMethod.Invoke(parameter);
        }

        public void Execute(object parameter)
        {
            executeMethod.Invoke(parameter);
        }
        public void RaiseCanExecuteChanged()
        {
            CanExecuteChanged?.Invoke(this, EventArgs.Empty);
        }
    }

    public class MWViewModel : BaseViewModel
    {
        private Models.IPeopleService _peopleService = null;
        public IList<Models.Person> People => _peopleService.PeopleList;
        public ViewModels.RelayCommand SalutaCommand { get; private set; } //private set -> read only

        private Models.Person _chosenP;

        public Models.Person ChosenP
        {
            get
            {
                return _chosenP;
            }
            set
            {
                if (_chosenP == value) return;
                _chosenP = value;
                NotifyPropertyChanged();
                SalutaCommand.RaiseCanExecuteChanged();
            }
        }


        private string _txtSaluto;

        public string TextSaluto
        {
            get { return _txtSaluto; }
            set
            {
                _txtSaluto = value;
                NotifyPropertyChanged();

            }
        }

        public void SalutaMi()
        {
            if (ChosenP != null)
            {
                TextSaluto = string.Format("Ciao {0} {1}", ChosenP.Name, ChosenP.Surname);
            }

        }

        public MWViewModel(Models.IPeopleService peopleService)
        {
            _peopleService = peopleService;
            SalutaCommand = new RelayCommand(salutaMethod, salutaExecute);

        }

        private bool salutaExecute(object obj)
        {
            return ChosenP != null;
        }

        private void salutaMethod(object obj)
        {
            SalutaMi();
        }

    }
}