WPF OneTime databinding gets detached when UI Element data changes

I am trying to set up a data binding between the value property of a NumericUpDown and a property of a cs object. The NumericUpDown is in a Modal dialog box so I only want the databinding to be updated when the user presses the OK button. This NumericUpDown is in a PropertyGrid which works fine in a non Modal dialog box situation, so I don't want to modify the XAML which originally created the databinding. I also don't want to duplicate the XAML just to change the databinding. So, I am trying to copy and modify the databinding in the Loaded event handler for the Modal dialog box.

Here I copy and modify the databinding originally created in XAML.

    void OnLoad(object sender, RoutedEventArgs e)
    {
        GetBindings(DialogPropPanel);
    }  

    private void GetBindings(FrameworkElement root)
   {
      FieldInfo[] infos = root.GetType().GetFields(BindingFlags.Public | BindingFlags.FlattenHierarchy |
         BindingFlags.Instance | BindingFlags.Static);

      foreach(FieldInfo field in infos)
      {
         if(field.FieldType == typeof(DependencyProperty))
         {
            DependencyProperty dp = (DependencyProperty)field.GetValue(null);
            BindingExpression ex = root.GetBindingExpression(dp);
            if(ex != null)
            {
               PropertyElement elem = FindBoundElement(ex.DataItem, GroupContainer.PropertyGroups);
               if(elem != null)
               {
                  Binding bd = ex.ParentBinding;
                  if(bd.Mode == BindingMode.Default || bd.Mode == BindingMode.TwoWay)
                  {
                     // Copy the binding an change mode.
                     Binding newBinding = CreateOneTimeBinding(bd, ex.DataItem);
                     BindingOperations.ClearBinding(root, dp);
                     BindingOperations.SetBinding(root, dp, newBinding);
                     BindingExpression nuExp = root.GetBindingExpression(dp);
                     m_bindings.Add(nuExp);
                  }
               }
            }
         }
      }

      int children = VisualTreeHelper.GetChildrenCount(root);
      for(int i = 0; i < children; i++)
      {
         FrameworkElement child = VisualTreeHelper.GetChild(root, i) as FrameworkElement;

         if(child != null)
            GetBindings(child);
      }
   }

Here I change the mode to OneTime, and the UpdateSourceTrigger to Explicit.

    public static Binding CreateOneTimeBinding(Binding binding, object source)
    {
      var result = new Binding
      {
        Source = source,
        AsyncState = binding.AsyncState,
        BindingGroupName = binding.BindingGroupName,
        BindsDirectlyToSource = binding.BindsDirectlyToSource,
        Converter = binding.Converter,
        ConverterCulture = binding.ConverterCulture,
        ConverterParameter = binding.ConverterCulture,
        //ElementName = binding.ElementName,                              
        FallbackValue = binding.FallbackValue,
        IsAsync = binding.IsAsync,
 开发者_JAVA百科       Mode = BindingMode.OneWay,
        NotifyOnSourceUpdated = binding.NotifyOnSourceUpdated,
        NotifyOnTargetUpdated = binding.NotifyOnTargetUpdated,
        NotifyOnValidationError = binding.NotifyOnValidationError,
        Path = binding.Path,
        //RelativeSource = binding.RelativeSource,                              
        StringFormat = binding.StringFormat,
        TargetNullValue = binding.TargetNullValue,
        UpdateSourceExceptionFilter = binding.UpdateSourceExceptionFilter,
        UpdateSourceTrigger = UpdateSourceTrigger.Explicit,
        ValidatesOnDataErrors = binding.ValidatesOnDataErrors,
        ValidatesOnExceptions = binding.ValidatesOnExceptions,
        XPath = binding.XPath,
      };

      foreach(var validationRule in binding.ValidationRules)      
        result.ValidationRules.Add(validationRule);      

      return result;
    }

When the user changes the target value through the NumericUpDown the DataItem property of the BindingExpression gets set to null. Then when I call UpdateSource() below on that BindingExpression an exception is thrown which says: "Cannot perform this operation when binding is detached."

void ApplyClicked(object sender, RoutedEventArgs e)
{
  foreach(BindingExpression express in m_bindings)
    express.UpdateSource();
}

What am I doing wrong?

I found the problem. The databinding needs to have a mode of TwoWay(or OneWayToSource) in order to update the source. So in the code above the only changed needed is to change OneWay to TwoWay.