How to have bindable properties of a UserControl which work with OnPropertyChanged

Question

I have a simple usercontrol (WinForms) with some public properties. When I use this control, I want to databind to those properties with the DataSourceUpdateMode set to

Answer 1

Implementing the INotifyPropertyChanged interface is very simple. Here is a sample that shows an object with a single public field...

public class Demo : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    private void NotifyPropertyChanged(String info)
    {
        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs(info));
    }

    private string _demoField;

    public string DemoField
    {
        get {return demoField; }

        set
        {
            if (value != demoField)
            {
                demoField = value;
                NotifyPropertyChanged("DemoField");
            }
        }
    }
}

Then you would create a Binding instance to bind a control property to a property (DemoField) on your source instance (instance of Demo).

Answer 2

I think I've got this figured out. I didn't understand how change notifications were sent from control to bound datasource.

Yes, calling OnValidating() is the wrong way.

From what I've pieced together, there are two ways a control can notify the datasource that a property has changed.

One way is for the control to implement INotifyPropertyChanged. I had never done this from the control side before, and I thought only the datasource side of the binding had to implement it.

When I implemented INotifyPropertyChanged on my user control, and raised the PropertyChanged event at the appropriate time, it worked.

The second way is for the control to raise a specific change event for each property. The event must follow the naming convention: <propertyname>Changed

e.g. for my example it would be

public event EventHandler ControlPropertyChanged

If my property was called Foo, it would be FooChanged.

I failed to notice the relavent part of the MSDN documentation, where it says:

For change notification to occur in a binding between a bound client and a data source, your bound type should either:

Implement the INotifyPropertyChanged interface (preferred).

Provide a change event for each property of the bound type.

This second way is how all existing WinForms controls work, so this is how I'm doing it now. I use INotifyPropertyChanged on my datasource, but I raise the Changed events on my control. This seems to be the conventional way.