Passing state of WPF ValidationRule to View Model in MVVM

Question

I am stuck in a seemingly common requirement. I have a WPF Prism (for MVVM) application. My model implements the IDataErrorInfo for validation. The IDataErrorInfo works great for non-numeric properties. However, for numeric properties, if the user enters invalid characters (which are not numeric), then the data doesn't even reach the model because wpf cannot convert it to numeric type.

So, I had to use WPF ValidationRule to provide user some meaningful message for invalid numeric entries. All the buttons in the view are bound to DelegateCommand of prism (in view model) and the enabling/disabling of buttons is done in View Model itself.

Now if a wpf ValidationRule fail for some TextBox, how do I pass this information to View Model so that it can appropriately disable buttons in the view ?

Answer 1

For MVVM I prefer to use Attached Properties for this type of thing because they are reusable and it keeps the view models clean.

In order to bind to the Validation.HasError property to your view model you have to create an attached property which has a CoerceValueCallback that synchronizes the value of your attached property with the Validation.HasError property on the control you are validating user input on.

This article explains how to use this technique to solve the problem of notifying the view model of WPF ValidationRule errors. The code was in VB so I ported it over to C# if you're not a VB person.

The Attached Property

public static class ValidationBehavior
{
    #region Attached Properties

    public static readonly DependencyProperty HasErrorProperty = DependencyProperty.RegisterAttached(
        "HasError",
        typeof(bool),
        typeof(ValidationBehavior),
        new FrameworkPropertyMetadata(false, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault, null, CoerceHasError));

    private static readonly DependencyProperty HasErrorDescriptorProperty = DependencyProperty.RegisterAttached(
        "HasErrorDescriptor",
        typeof(DependencyPropertyDescriptor),
        typeof(ValidationBehavior));

    #endregion

    private static DependencyPropertyDescriptor GetHasErrorDescriptor(DependencyObject d)
    {
        return (DependencyPropertyDescriptor)d.GetValue(HasErrorDescriptorProperty);
    }

    private static void SetHasErrorDescriptor(DependencyObject d, DependencyPropertyDescriptor value)
    {
        d.SetValue(HasErrorDescriptorProperty, value);
    }

    #region Attached Property Getters and setters

    public static bool GetHasError(DependencyObject d)
    {
        return (bool)d.GetValue(HasErrorProperty);
    }

    public static void SetHasError(DependencyObject d, bool value)
    {
        d.SetValue(HasErrorProperty, value);
    }

    #endregion

    #region CallBacks

    private static object CoerceHasError(DependencyObject d, object baseValue)
    {
        var result = (bool)baseValue;
        if (BindingOperations.IsDataBound(d, HasErrorProperty))
        {
            if (GetHasErrorDescriptor(d) == null)
            {
                var desc = DependencyPropertyDescriptor.FromProperty(System.Windows.Controls.Validation.HasErrorProperty, d.GetType());
                desc.AddValueChanged(d, OnHasErrorChanged);
                SetHasErrorDescriptor(d, desc);
                result = System.Windows.Controls.Validation.GetHasError(d);
            }
        }
        else
        {
            if (GetHasErrorDescriptor(d) != null)
            {
                var desc = GetHasErrorDescriptor(d);
                desc.RemoveValueChanged(d, OnHasErrorChanged);
                SetHasErrorDescriptor(d, null);
            }
        }
        return result;
    }
    private static void OnHasErrorChanged(object sender, EventArgs e)
    {
        var d = sender as DependencyObject;
        if (d != null)
        {
            d.SetValue(HasErrorProperty, d.GetValue(System.Windows.Controls.Validation.HasErrorProperty));
        }
    }

    #endregion
}

Using The Attached Property in XAML

<Window
  x:Class="MySolution.MyProject.MainWindow"
  xmlns:v="clr-namespace:MyNamespace;assembly=MyAssembly">  
    <TextBox
      v:ValidationBehavior.HasError="{Binding MyPropertyOnMyViewModel}">
      <TextBox.Text>
        <Binding
          Path="ValidationText"
          UpdateSourceTrigger="PropertyChanged">
          <Binding.ValidationRules>
            <v:SomeValidationRuleInMyNamespace/>
          </Binding.ValidationRules>
        </Binding>
     </TextBox.Text>
  </TextBox>
</ Window >

Now the property on your view model will be synchronized with Validation.HasError on your textbox.

Answer 2

Nirvan

The simplest way to solve this particular issue is to use a numeric textbox, which prevents the user from entering an invalid value (you can either do this via a Third Party Vendor, or find an open source solution, such as a class derived from Textbox that suppresses non numeric input).

The second way to handle this in MVVM without doing the above, is to define another field in you ViewModel which is a string, and bind that field to your textbox. Then, in the setter of your string field you can set the Integer, and assign a value to your numeric field:

Here is a rough example: (NOTE I did not test it, but it should give you the idea)

// original field
private int _age;
int Age 
{
   get { return _age; }
   set { 
     _age = value; 
     RaisePropertyChanged("Age");
   }
}


private string _ageStr;
string AgeStr
{
   get { return _ageStr; }
   set { 
     _ageStr = value; 
     RaisePropertyChanged("AgeStr");
     if (!String.IsNullOrEmpty(AgeStr) && IsNumeric(AgeStr) )
         Age = intVal;
    }
} 

private bool IsNumeric(string numStr)
{
   int intVal;
   return int.TryParse(AgeStr, out intVal);
}

#region IDataErrorInfo Members

    public string this[string columnName]
    {
        get
        {

            if (columnName == "AgeStr" && !IsNumeric(AgeStr)
               return "Age must be numeric";
        }
    }

    #endregion