Question

In my usercontrol.xaml.cs. I have this dependency proprerty as bleow.

public static readonly DependencyProperty MessageKeyProperty =
                 DependencyProperty.Register("MessageKey", typeof(String),
                 typeof(UC_MessageEntry),
                     new FrameworkPropertyMetadata(null,
                         new PropertyChangedCallback(MessageKeyPropertyChangedCallback)));


        private static void MessageKeyPropertyChangedCallback(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            if (d is UC_MessageEntry)
            {
                UC_MessageEntryucMessageEntryAccessDenied = (UC_MessageEntry)d;
                if (e.NewValue != null)
                {
                    ResourceBundle resourceBundle = App.ResourceBundle;
                    if (e.NewValue.ToString().Equals("enable"))
                    {
                        ucMessageEntryAccessDenied.txtAceessDeniedMsg.Text = "";
                        return;
                    }

                    String actualMessage = resourceBundle.GetString("Resources", e.NewValue.ToString());
                    if (actualMessage == null)
                    {
                        ucMessageEntryAccessDenied.txtAceessDeniedMsg.Text = resourceBundle.GetString("Resources", "ContractSetup.ExchangeAccessDeniedMessage.OTHERS");
                    }
                    else
                    {
                        ucMessageEntryAccessDenied.txtAceessDeniedMsg.Text = actualMessage;
                    }

                }
                else
                {
                    ucMessageEntryAccessDenied.txtAceessDeniedMsg = null;
                }
            }
        }


        public String MessageKey
        {
            get
            {
                return (String)this.GetValue(MessageKeyProperty);  
            }
            set
            {
                this.SetValue(MessageKeyProperty,value);
            }
        }

In mainwindow.xaml , i bind this MessageKey as below.

  <view_MessageEntry:UC_MessageEntry   
                                x:Uid="local:UC_MessageEntry_1" x:Name="UC_OrderEntry"  MessageKey="{Binding MsgAccessDenied}"
                                Style="{DynamicResource contentControlStyle}" SnapsToDevicePixels="True" Margin="0" />

And Behind MessageViewModle.cs,

private static readonly PropertyChangedEventArgs MsgAccessDeniedPropertyChangedEventArgs
    = new PropertyChangedEventArgs("MsgAccessDenied");

    private string _msgAccessDenied;
    public string MsgAccessDenied
    { 
       get
       {
           if (_selectedExchange != null)
           {
               return _msgAccessDenied;
           }
           else
           {
               return "enable";
           }
       }
        set
        {
            _msgAccessDenied = value;
            this.RaisePropertyChanged(this, MsgAccessDeniedPropertyChangedEventArgs);
        }

    }


public void FireMsg()
{
  this.MsgAccessDenied="value";
}

When the combo box selection is changed, I called FireMsg() and it will update the MessageKeyPropertyChangedCallback function in usercontrol.xaml.cs. It’s working fine. But If I call this FireMsg() from Other ViewModels, value of _msgAccessDenied is updated. But MessageKeyPropertyChangedCallback function is not firing. Any solution for this issue? Thanks.

Answer 1

Your code looks generally sound. But if your dependency property change callback is not being called, it is almost certainly because the value of the dependency property isn’t changing. And if the dependency property is bound to a source, then it is probably because the source isn’t changing.

From my quick review of the code, the only way that I can see that happening is:

• the value of MsgAccessDenied is set
• this causes the field _msgAccessDenied to be set
• the RaisePropertyChanged method is called to trigger PropertyChanged
• the dependency subsystem does its thing which forces a get of MsgAccessDenied
• the MsgAccessDenied getter is called
• the getter checks _selectedExchange and it is null
• the getter returns the value "enable" instead of the new value of _msgAccessDenied
• the previous value was also "enable"
• the dependency subsystem says, OK, no change
• the property change callback is not called

In summary, _selectedExchange can hide value changes in _msgAccessDenied, thereby preventing the upstream change callback from firing.

This is just a theory.