BindingSample / BindingConsole / Indexer

PersonMap.cs

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

namespace BindingConsole
{
    public class PersonMap : INotifyPropertyChanged
    {
        Dictionary<int, string> People = new Dictionary<int, string>();

        public string this[int id]
        {
            get { return People[id]; }
            set
            {
                if (People.ContainsKey(id) && People[id] == value) return;
                People[id] = value;
                NotifyPropertyChanged(Binding.IndexerName);
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;

        public void NotifyPropertyChanged([CallerMemberName]string propertyName = "") =>
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

Program.cs

using System;
using System.Windows.Controls;
using System.Windows.Data;

namespace BindingConsole
{
    class Program
    {
        [STAThread]
        static void Main(string[] args)
        {
            // Binding Source with indexer.
            var map = new PersonMap { [123] = "Taro" };

            // Binding Target must be FrameworkElement.
            var textBox = new TextBox { Text = "Default" };
            Console.WriteLine(textBox.Text); // Default

            // Binds target to source.
            var binding = new Binding("[123]") { Source = map, UpdateSourceTrigger = UpdateSourceTrigger.PropertyChanged };
            textBox.SetBinding(TextBox.TextProperty, binding);
            Console.WriteLine(textBox.Text); // Taro

            // Changes source value.
            map[123] = "Jiro";
            Console.WriteLine(textBox.Text); // Jiro

            // Changes target value.
            textBox.Text = "Saburo";
            Console.WriteLine(map[123]); // Saburo
        }
    }
}