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

Answer 1

1. Implement IDataErrorInfo in your model or Viewmodel depending logic of binding property. You may implement in both classes.

2. Implement this too in your base validation class. Here validation will trigger when binding IDataErrorInfo does not work.

   public virtual bool HasError
   {
       get { return _hasError; } 
       set
       {
           // if (value.Equals(_hasError)) return;
           _hasError = value;
           RaisePropertyChanged(() => HasError);
       }
   }
   

3. Next, add global class

   public class ProtocolSettingsLayout
   {
       public static readonly DependencyProperty MVVMHasErrorProperty = DependencyProperty.RegisterAttached("MVVMHasError", typeof(bool), typeof(ProtocolSettingsLayout), new FrameworkPropertyMetadata(false, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault, null, CoerceMVVMHasError));
   
       public static bool GetMVVMHasError(DependencyObject d)
       {
           return (bool)d.GetValue(MVVMHasErrorProperty);
       }
   
       public static void SetMVVMHasError(DependencyObject d, bool value)
       {
           d.SetValue(MVVMHasErrorProperty, value);
       }
   
       private static object CoerceMVVMHasError(DependencyObject d, Object baseValue)
       {
           bool ret = (bool)baseValue;
   
           if (BindingOperations.IsDataBound(d, MVVMHasErrorProperty))
           {
               if (GetHasErrorDescriptor(d) == null)
               {
                   DependencyPropertyDescriptor desc = DependencyPropertyDescriptor.FromProperty(Validation.HasErrorProperty, d.GetType());
                   desc.AddValueChanged(d, OnHasErrorChanged);
                   SetHasErrorDescriptor(d, desc);
                   ret = System.Windows.Controls.Validation.GetHasError(d);
               }
           }
           else
           {
               if (GetHasErrorDescriptor(d) != null)
               {
                   DependencyPropertyDescriptor desc = GetHasErrorDescriptor(d);
                   desc.RemoveValueChanged(d, OnHasErrorChanged);
                   SetHasErrorDescriptor(d, null);
               }
           }
           return ret;
       }
   
       private static readonly DependencyProperty HasErrorDescriptorProperty = DependencyProperty.RegisterAttached("HasErrorDescriptor",
                                                                               typeof(DependencyPropertyDescriptor),
                                                                               typeof(ProtocolSettingsLayout));
   
       private static DependencyPropertyDescriptor GetHasErrorDescriptor(DependencyObject d)
       {
           var ret = d.GetValue(HasErrorDescriptorProperty);
           return ret as DependencyPropertyDescriptor;
       }
   
       private static void OnHasErrorChanged(object sender, EventArgs e)
       {
           DependencyObject d = sender as DependencyObject;
   
           if (d != null)
           {
               d.SetValue(MVVMHasErrorProperty, d.GetValue(Validation.HasErrorProperty));
           }
       }
   
       private static void SetHasErrorDescriptor(DependencyObject d, DependencyPropertyDescriptor value)
       {
           var ret = d.GetValue(HasErrorDescriptorProperty);
           d.SetValue(HasErrorDescriptorProperty, value);
       }
   }
   

4. xaml