Understanding the interaction of WPF data binding and value converters

Question

I am trying to understand what is really going on behind the scenes on the simplified reproducible code below.

I have one Windowwith ListBoxand a TextBlockthat are connected together (i.e. master -> detail). Then I have a ViewModel with two properties - a string and a date. On date, I implemented a value converter ( LongDateConverter).

I have a couple of Debug.WriteLine()calls in code that lead to the following output:

• Launch the application
  • In converter: ConverterProblem.MainWindowViewModel
  • In converter: null
• Click on one of the two items in the list.
  • In converter: ConverterProblem.DataModel

The second and third method calls IValueConverter, I think, I understand. The second one is nullbecause it ListBoxdoes not yet have a selected element. The third is for the item I selected.

I dont understand what:

• MainWindowViewModel?
• ?

:

MainWindow.xaml:

<Window x:Class="ConverterProblem.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:app="clr-namespace:ConverterProblem"
        Title="MainWindow" Height="350" Width="525">
    <Window.Resources>
        <app:LongDateConverter x:Key="longDateConverter"/>
    </Window.Resources>
    <StackPanel Orientation="Horizontal">
        <ListBox SelectedItem="{Binding Data}" ItemsSource="{Binding DataList}"
                 DisplayMemberPath="Name"/>
        <TextBlock Text="{Binding Converter={StaticResource longDateConverter}}" 
                   DataContext="{Binding Data}" />
    </StackPanel>
</Window>

MainWindow.xaml.cs:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Diagnostics;
using System.Globalization;
using System.Windows;
using System.Windows.Data;

namespace ConverterProblem 
{
    public class LongDateConverter : IValueConverter 
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture) 
        {
            if (value == null) {
                Debug.WriteLine("In converter: null");
                return "null";
            }

            Debug.WriteLine("In converter: " + value.GetType().ToString());

            if (value.GetType() == typeof(MainWindowViewModel))
                return "viewmodel";

            return ((DataModel)value).Date.ToLongDateString();
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            return null;
        }
    }

    public class DataModel
    {
        public string Name { get; set; }
        public DateTime Date { get; set; }
    }

    public class MainWindowViewModel : INotifyPropertyChanged
    {
        private DataModel _data;
        private List<DataModel> _dataList;

        public MainWindowViewModel()
        {
            _dataList = new List<DataModel> { 
                new DataModel { Date = DateTime.Now, Name = "John" }, 
                new DataModel { Date = DateTime.Now.AddDays(50), Name = "Sue" }
            };
        }

        public DataModel Data
        {
            get { return _data; }
            set
            {
                if (_data == value) return;

                _data = value;
                RaisePropertyChanged("Data");
            }
        }

        public List<DataModel> DataList
        {
            get { return _dataList; }
        }

        public event PropertyChangedEventHandler PropertyChanged;
        private void RaisePropertyChanged(string propertyName)
        {
            var handler = PropertyChanged;
            if (handler != null) {
                handler(this, new PropertyChangedEventArgs(propertyName));
            }
        }
    }

    public partial class MainWindow : Window
    {
        private MainWindowViewModel _viewModel;

        public MainWindow()
        {
            _viewModel = new MainWindowViewModel();
            DataContext = _viewModel;
            InitializeComponent();
        }
    }
}

Answer 1

, Text DataContext TextBlock.

XAML BAML BAML XAMLLoader, XAML DP .

Text DP , , DataContext TextBlock, , DataContext MainWindowViewModel. , MainWindowViewModel, . DataContext , DP DataContext.

---

XAML , , MainWindowViewModel :

<TextBlock DataContext="{Binding Data}"
           Text="{Binding Converter={StaticResource longDateConverter}}" />

:

In converter: null
In converter: ConverterProblem.DataModel