DP not Working

Kevin Marois

This is a very simple example, and I can't get it to work My UserControl

Code Behind

public partial class MonthViewDayColumnHeader : UserControl
{
#region DP DayName
public static readonly DependencyProperty DayNameCaptionProperty =
DependencyProperty.Register("DayNameCaption",
typeof(string),
typeof(MonthViewDayColumnHeader),
new PropertyMetadata("Today",
new PropertyChangedCallback(OnDayNameCaptionChanged),
new CoerceValueCallback(CoerceDayNameCaption)));

public string DayNameCaption
{
    get { return (string)GetValue(DayNameCaptionProperty); }
    set { SetValue(DayNameCaptionProperty, value); }
}

private static void OnDayNameCaptionChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
    MonthViewDayColumnHeader control = (MonthViewDayColumnHeader)d;
    var dayNameCaption = control.DayNameCaption;  // dayNameCaption is null
}

private static object CoerceDayNameCaption(DependencyObject d, object value)
{
    MonthViewDayColumnHeader control = (MonthViewDayColumnHeader)d;
    var dayNameCaption = control.DayNameCaption;   // dayNameCaption is 'Today', the default value
    return null;
}
#endregion



#region CTOR
public MonthViewDayColumnHeader()
{
    InitializeComponent();

    this.DataContext = this;
}
#endregion

}

Usage:

The text "Monday" doesn't show up. WTF IS WRONG HERE!!!!

If it's not broken, fix it until it is

Lost User

Over-engineering.

Kevin Marois

Thanks for your thoughtful response. That fixed it.

If it's not broken, fix it until it is

PureNsanity

The reason why it's not working is the MonthViewDayColumnHeader class doesn't support INotifyPropertyChanged. It's being initialized, then updated with Monday after. If you take the existing and default it to 'Sunday' you should be able to see the default value. On that note, I know it's an example but you should probably start off with MVVM in your learning process. Have the underlying layers implement the INotifyPropertyChanged and set the context to an appropriate view model.

Kevin Marois

Actually, it wasn't working because I'm a dumba$$...

private static object CoerceDayNameCaption(DependencyObject d, object value)
{
MonthViewDayColumnHeader control = (MonthViewDayColumnHeader)d;
var dayNameCaption = control.DayNameCaption;value
return null; //<=== ALWAYS RETURNS NULL
}

changed it to

private static object CoerceDayNameCaption(DependencyObject d, object value)
{
MonthViewDayColumnHeader control = (MonthViewDayColumnHeader)d;
var dayNameCaption = control.DayNameCaption;value
return dayNameCaption ; //<=== NOW IT WORKS
}

If it's not broken, fix it until it is

PureNsanity

Yes and no. You shouldn't be using a callback in this case. The callback is circumventing the whole point of the binding. Taking my advice would fix it the appropriate way.

Kevin Marois

Well, actually, I put in the callback for testing and ended up breaking my code ;P

If it's not broken, fix it until it is

Richard Deeming

I assume the rogue ;value is just a typo in your message?

var dayNameCaption = control.DayNameCaption;value // <-- This won't compile.

"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer

Richard Deeming

You don't need to implement INotifyPropertyChanged when you're using dependency properties. DependencyProperties or INotifyPropertyChanged ?[^]

"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer

PureNsanity

Yes, you're correct. For some reason I was thinking the underlying source still need to be explicitly hooked, but it does not.

Meshack Musundi

It should be,

You don't need the callback.

"As beings of finite lifespan, our contributions to the sum of human knowledge is one of the greatest endeavors we can undertake and one of the defining characteristics of humanity itself"