Binding is not displaying the correct value

Question

Basically, I am reading a value from an xml file and displaying that value in a text box. This text box is editable so the user can make changes to it and when the session ends that value gets stored back in the xml file.

I know that the values correctly get stored and loaded to the text box. But I'm baffled as to why I cannot see the bound value that should be displayed in the text box..

This is the text box:

<UserControl x:Class="test.myView"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
         xmlns:ui="clr-namespace:test.myView"  
         xmlns:local="test.myControls;assembly=test.mycontrols">

<UserControl.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="Style.xaml"/>
    </ResourceDictionary>
</UserControl.Resources>

<StackPanel Margin="8">
        <TextBlock Text="Starting URL"/>
        <TextBox Margin="0,5" FontSize="12" Height="30" Width="360" Text="{Binding myValue, UpdateSourceTrigger=PropertyChanged, Mode=TwoWay}" 
                 Style="{StaticResource WaterMarkTextBox}" local:WaterMarkTextHelper.WatermarkText="ENTER Value" />
    </StackPanel>

This is the INotifyPropertyChanged:

public abstract class ViewModelBase : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    protected void RaiseChangeNotification(string propertyName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }

}

This is the property im binding to:

public string myValue
    {
        get { return _settings.myValue; }
        set { _settings.myValue= value;   }
    }

I think it maybe because the value of the text box to empty before the code reaches the part where the start url is set using data binding. Does there exist a validate command in wpf that forces it to view the most up to date value?

Answer 1

Try to add one dummy converter and see if binding is getting fired when you change value from UI textbox (Convert back method needs to be executed). Else take help of snoop (http://snoopwpf.codeplex.com/) This will help you check status of binding.

And add RaiseNotification in setter if you want to update data from source to target.

public string myValue
    {
        get { return _settings.myValue; }
        set { _settings.myValue= value;  RaiseChangeNotification("myValue") }
    }

Answer 2

There is not enough information to know the exact problem, but there are a few things to check when this sort of things happen.

1. Check your Output window in Visual Studio. This will give you any binding errors. This will help solve some of the following problems:

   • Not having your property set to public

   • Typo with your property name

   • Not having the DataContext set

2. Make sure your DataContext implements the INotifyPropertyChanged interface. This is required by the implementation of WPF to update the binding. You would do this like the following:

   public class YourDataContext : INotifyPropertyChanged
   {
       private object _myvalue;
   
       public object myvalue
       {
           get
           {
               return _myvalue;
           }
           set
           {
               if (_myvalue == value)
                   return;
   
               _myvalue = value;
               OnPropertyChanged("myvalue");
           }
       }
   
       public event PropertyChangedEventHandler PropertyChanged;
   
       public void OnPropertyChanged(string propertyName)
       {
           if (PropertyChanged != null)
           {
               PropertyChanged(this, 
                               new PropertyChangedEventArgs(propertyName));
           }
       }
   }
   

   You can also look up several strategies to simplify this, like INotifyPropertyChanged weaving or a BaseViewModel class.

Edit

If myvalue is in your ViewModel that inherits from your ViewModelBase then all you need to do is change the implementation of your property to:

public string myValue
{
    get 
    {
        return _settings.myValue;
    }
    set 
    {
        if (_settings.myValue == value)
            return;

        _settings.myValue = value;
        RaiseChangeNotification("myValue");            
    }
}