调用绑定的ViewModel OnPropertyChanged但值相同时，获取IDataErrorInfo验证以在子UserControl中更新

Question

I have a reusable UserControl defined that will be used multiple times within the parent form, to represent different instances of a configured object. This UserControl has several TextBoxes representing configurable properties. For one of these properties, the value must be unique across all instances of the reusable UserControl.

My parent form utilizes these usercontrols like this:

<namespace:ReusableControl
  Property1="{Binding Path=ViewModelProperty1a, Mode=TwoWay}"
  Property2="{Binding Path=ViewModelProperty2a, Mode=TwoWay}"
  UniqueProperty="{Binding Path=VMUniquePropertya, Mode=TwoWay}"/>

<namespace:ReusableControl
  Property1="{Binding Path=ViewModelProperty1b, Mode=TwoWay}"
  Property2="{Binding Path=ViewModelProperty2b, Mode=TwoWay}"
  UniqueProperty="{Binding Path=VMUniquePropertyb, Mode=TwoWay}"/>

And the UserControl property looks like this:

public string this[string columnName]
{
    get
    {
        string error = string.Empty;
        switch (columnName)
        {
        case nameof(UniqueProperty):
            if (!((MyViewModel)DataContext).UniquePropertiesAreUnique())
            {
                error = "not unique";
            }
            break;
        //Other cases here, omitted from example
        }
        return error;
    }
}

//-------------------------------
//Just to show the codebehind for the property:
public string UniqueProperty
        {
            get { return (string)GetValue(UniquePropertyDP); }
            set { SetValue(UniquePropertyDP, value); }
        }

        public static readonly DependencyProperty UniquePropertyDP=
            DependencyProperty.Register(
                "UniqueProperty",
                typeof(string),
                typeof(ReusableControl),
                new PropertyMetadata(string.Empty));

The codebehind for the UserControl contains IDataErrorInfo validation:

//In the setter for VMUniquePropertyb:
var temp = VMUniquePropertya;
VMUniquePropertya = null;
VMUniquePropertya = temp;

Everything appears to be wired up and bound correctly; the values update when the UI is changed as desired. If I change one of the unique property values such that it is no longer unique, I get the red border around that text box, but this is where the issue comes in - the red border only appears around the text box I just changed, not both of the instances of UniqueProperty. In the ViewModel, when either of the UniqueProperty values are changed, it triggers OnPropertyChanged for the other, but this still isn't causing the validation border to appear. If I replace OnPropertyChange with an explicit call to update the value ie:

 //In the setter for VMUniquePropertyb: var temp = VMUniquePropertya; VMUniquePropertya = null; VMUniquePropertya = temp; 

Then I do get the validation border to appear on both text boxes when that value is changed to match the other, and both borders disappear when either value is changed to be unique again. Of course, this is a hack, and also will cause an infinite loop if used on both properties. How can I accomplish the same result with OnPropertyChanged?

Answer 1

I found a solution that works for me. There may be better ways to do this, but this works fairly well.

By using the CoerceValueCallback on the DependencyProperty, we can execute code whenever the value of the property is re-evaluated, not just when it actually changes. This will fire when the PropertyChange event occurs in the ViewModel because the binding is re-evaluated. This looks like this:

public string UniqueProperty
        {
            get { return (string)GetValue(UniquePropertyDP); }
            set { SetValue(UniquePropertyDP, value); }
        }

public static readonly DependencyProperty UniquePropertyDP=
    DependencyProperty.Register(
        "UniqueProperty",
        typeof(string),
        typeof(ReusableControl),
        new PropertyMetadata(string.Empty, null, UniquePropertyCoerceValueCallback));

private static object UniquePropertyCoerceValueCallback(DependencyObject d, object value)
{
    ((ReusableControl)d).UniquePropertyTextBox.GetBindingExpression(TextBox.TextProperty)
        .UpdateTarget();

    return value;
}

When the value of one of the unique properties changes and the ViewModel fires the PropertyChange event for the other unique property in the ViewModel, the DependencyProperty in the UserControl will be re-coerced, triggering this callback and updating validation.