Validation rule property: how to bind without using dependency properties - wpf

Question

In my WPF MVVM app I have a TextBox which has bound a validation rule.
In validation rule class I have below property:
public bool CanBeValidated { get; set; } = false;
Then in the view my TextBox has below validation rule bound (I only put the relevant part):
<TextBox.Text>
<Binding Path="myPath"
UpdateSourceTrigger="PropertyChanged"
ValidatesOnDataErrors="True">
<Binding.ValidationRules>
<vRules:RootPathValidationRule
ValidatesOnTargetUpdated="True"
CanBeValidated="{Binding Path=ValidationEnabled}"/>
</Binding.ValidationRules>
</Binding>
</TextBox.Text>
In my view model the property is defined as below:
public bool ValidationEnabled
{
get { return _isValidationEnabled; }
set { this._isValidationEnabled = value; OnPropertyChanged(); }
}
So I receive below compilation error:
A 'Binding' cannot be set on the 'CanBeValidated' property of type
'MyPathValidatorRule'. A 'Binding' can only be set on a
DependencyProperty of a DependencyObject.
For first time when TextBox is loaded I want to avoid validation rule to fire until user edits it and avoid throwing a validation error since TextBox is empty.
Once user edits the TextBox, I would like to enable validation rule by performing a simple this.ValidationEnabled = true from view model.
How can I achieve this without using dependency properties? Is it possible?

Answer 1

You could create a wrapper class that derives from DependencyObject and exposes a dependency property. Then you add a CLR property to the ValidationRule class that returns an instance of this wrapper type:
public class RootPathValidationRule : ValidationRule
{
public Wrapper Wrapper { get; set; }
public override ValidationResult Validate(object value, CultureInfo cultureInfo)
{
bool canBeValidated = Wrapper?.CanBeValidated == true;
...
}
}
public class Wrapper : DependencyObject
{
public static readonly DependencyProperty CanBeValidatedProperty =
DependencyProperty.Register(nameof(CanBeValidated), typeof(bool),
typeof(Wrapper));
public bool CanBeValidated
{
get { return (bool)GetValue(CanBeValidatedProperty); }
set { SetValue(CanBeValidatedProperty, value); }
}
}
Finally, you'll also need a binding proxy object that captures the DataContext where the source property is defined:
public class BindingProxy : System.Windows.Freezable
{
protected override Freezable CreateInstanceCore()
{
return new BindingProxy();
}
public object Data
{
get { return (object)GetValue(DataProperty); }
set { SetValue(DataProperty, value); }
}
public static readonly DependencyProperty DataProperty =
DependencyProperty.Register("Data", typeof(object), typeof(BindingProxy), new PropertyMetadata(null));
}
XAML:
<TextBox>
<TextBox.Resources>
<vRules:BindingProxy x:Key="proxy" Data="{Binding}"/>
</TextBox.Resources>
<TextBox.Text>
<Binding Path="myPath"
UpdateSourceTrigger="PropertyChanged"
ValidatesOnDataErrors="True">
<Binding.ValidationRules>
<vRules:RootPathValidationRule ValidatesOnTargetUpdated="True">
<vRules:RootPathValidationRule.Wrapper>
<vRules:Wrapper CanBeValidated="{Binding Data.ValidationEnabled,
Source={StaticResource proxy}}"/>
</vRules:RootPathValidationRule.Wrapper>
</vRules:RootPathValidationRule>
</Binding.ValidationRules>
</Binding>
</TextBox.Text>
</TextBox>
Please refer to this article for details.