c# binding to fields on a nested object

Question

I can't seem to find a simple, concrete explanation of how to bind controls in a WinForms app to nested objects using data binding. For example:

class MyObject : INotifyPropertyChanged
{
    private string _Name;
    public string Name { get { return _Name; } set { _Name = value; OnPropertyChanged("Name"); } }

    private MyInner _Inner;
    public MyInner Inner { get { return _Inner; } set { _Inner = value; OnPropertyChanged("Inner"); } }

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

class MyInner : INotifyPropertyChanged
{
    private string _SomeValue;
    public string SomeValue { get { return _SomeValue; } set { _SomeValue = value; OnPropertyChanged("SomeValue"); } }

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

Now imagine a form with just two textboxes, the first for Name and the second for Inner.SomeValue. I'm easily able to get binding to work against Name, but Inner.SomeValue is flaky. If I populate the object and then set up the binding, it shows Inner.SomeValue in the textbox but I can't edit it. If I start from a fresh object without initializing Inner, I can't seem to get data to stick in Inner.SomeValue.

Thanks in advance -- I've checked all over MSDN, all over StackOverflow, and dozens of searches with different keywords. Everyone wants to talk about binding to databases or DataGrids, and most examples are written in XAML.

Update: I've tried Marc's full test harness and have partial success. If I hit the "all change!" button, I seem to be able to write back to the inner object. However, starting with MyObject.Inner null, it doesn't know how to create an inner object. I think for now, I can work around it by just making sure my inner references are always set to a valid object. Still, I can't help feeling like I'm missing something :)

--Matt

Answer 1

Hmm - an excellent question; I've done lots of data-binding to objects, and I would have sworn that what you are doing should work; but indeed it is very reluctant to notice the change to the inner object. I've managed to get it working by:

var outer = new BindingSource { DataSource = myObject };
var inner = new BindingSource(outer, "Inner");
txtName.DataBindings.Add("Text", outer, "Name");
txtSomeValue.DataBindings.Add("Text", inner, "SomeValue");

Not ideal, but it works. Btw; you might find the following utility methods useful:

public static class EventUtils {
    public static void SafeInvoke(this EventHandler handler, object sender) {
        if(handler != null) handler(sender, EventArgs.Empty);
    }
    public static void SafeInvoke(this PropertyChangedEventHandler handler,
               object sender, string propertyName) {
        if(handler != null) handler(sender,
               new PropertyChangedEventArgs(propertyName));
    }
}

Then you can have:

class MyObject : INotifyPropertyChanged
{
    private string _Name;
    public string Name { get { return _Name; } set {
        _Name = value; PropertyChanged.SafeInvoke(this,"Name"); } }
    private MyInner _Inner;
    public MyInner Inner { get { return _Inner; } set {
        _Inner = value; PropertyChanged.SafeInvoke(this,"Inner"); } }
    public event PropertyChangedEventHandler PropertyChanged;
}

class MyInner : INotifyPropertyChanged
{
    private string _SomeValue;
    public string SomeValue { get { return _SomeValue; } set {
        _SomeValue = value; PropertyChanged.SafeInvoke(this, "SomeValue"); } }
    public event PropertyChangedEventHandler PropertyChanged;
}

And in the bargain it fixes the (slim) chance of a null-exception (race-condition).

---

Full test rig, to iron out kinks (from comments):

using System;
using System.ComponentModel;
using System.Windows.Forms;
public static class EventUtils {
    public static void SafeInvoke(this PropertyChangedEventHandler handler, object sender, string propertyName) {
        if(handler != null) handler(sender, new PropertyChangedEventArgs(propertyName));
    }
}
class MyObject : INotifyPropertyChanged
{
    private string _Name;
    public string Name { get { return _Name; } set { _Name = value; PropertyChanged.SafeInvoke(this,"Name"); } }
    private MyInner _Inner;
    public MyInner Inner { get { return _Inner; } set { _Inner = value; PropertyChanged.SafeInvoke(this,"Inner"); } }
    public event PropertyChangedEventHandler PropertyChanged;
}

class MyInner : INotifyPropertyChanged
{
    private string _SomeValue;
    public string SomeValue { get { return _SomeValue; } set { _SomeValue = value; PropertyChanged.SafeInvoke(this, "SomeValue"); } }
    public event PropertyChangedEventHandler PropertyChanged;
}
static class Program
{
    [STAThread]
    public static void Main() {
        var myObject = new MyObject();
        myObject.Name = "old name";
        // optionally start with a default
        //myObject.Inner = new MyInner();
        //myObject.Inner.SomeValue = "old inner value";

        Application.EnableVisualStyles();
        using (Form form = new Form())
        using (TextBox txtName = new TextBox())
        using (TextBox txtSomeValue = new TextBox())
        using (Button btnInit = new Button())
        {
            var outer = new BindingSource { DataSource = myObject };
            var inner = new BindingSource(outer, "Inner");
            txtName.DataBindings.Add("Text", outer, "Name");
            txtSomeValue.DataBindings.Add("Text", inner, "SomeValue");
            btnInit.Text = "all change!";
            btnInit.Click += delegate
            {
                myObject.Name = "new name";
                var newInner = new MyInner();
                newInner.SomeValue = "new inner value";
                myObject.Inner = newInner;
            };
            txtName.Dock = txtSomeValue.Dock = btnInit.Dock = DockStyle.Top;
            form.Controls.AddRange(new Control[] { btnInit, txtSomeValue, txtName });
            Application.Run(form);
        }
    }

}