Wpf datagrid validationrule for unique field

Question

I've got a suclassed ValidationRule named UniqueNameSolidWoodRule to validate for duplicate entries in a datagrid.

Here's the code:

public class UniqueNameSolidWoodRule : ValidationRule
{
    public CollectionViewSource CurrentCollection { get; set; }

    public override ValidationResult Validate(object value, CultureInfo cultureInfo)
    {
        if (value != null)
        {
            ObservableCollection<SolidWood_VM> castedCollection = (ObservableCollection<SolidWood_VM>)CurrentCollection.Source;

            foreach (SolidWood_VM swVM in castedCollection)
            {
                if (swVM.Designation == value.ToString())
                {
                    return new ValidationResult(false, ResourcesManager.Instance.GetString("DuplicatedRecord"));
                }
            }
        }

        return new ValidationResult(true, null);
    }
}

And here's the XAML snippet:

<DataGrid.Resources>
        <CollectionViewSource x:Key="CurrentSolidWoodCollection" Source="{Binding Path=SolidWoods}" />
</DataGrid.Resources>

 <DataGridTextColumn x:Name="Column2" Header="{x:Static p:Resources.Designation}" Width="auto">
        <DataGridTextColumn.Binding>
            <Binding Path="Designation" ValidatesOnDataErrors="True" UpdateSourceTrigger="LostFocus">
                 <Binding.ValidationRules>
                        <my:UniqueNameSolidWoodRule  CurrentCollection="{StaticResource CurrentSolidWoodCollection}"/>
                  </Binding.ValidationRules>
             </Binding>
         </DataGridTextColumn.Binding>
  </DataGridTextColumn>

My problem is that sometimes this rule compares the value with its own row. How could I prevent this? It seems to me that I need a row.index property associated with the value to compare, but unfortunnally I can't figure a way out to achieve this.

Answer 1

I finally got things working!

Here's the solution:

In the XAML I added the following ValidationStep:

<my:UniqueNameSolidWoodRule CurrentCollection="{StaticResource CurrentSolidWoodCollection}" ValidationStep="CommittedValue"/>

This way I'm getting a BindingExpression object instead of a string as the first parameter of the overridden Validate method, which gives me much more information about the record to validate like the HashCode I can use to check if I'm comparing the same object.

Here's the updated Validate method:

public class UniqueNameSolidWoodRule : ValidationRule
{
    public CollectionViewSource CurrentCollection { get; set; }

    public override ValidationResult Validate(object value, CultureInfo cultureInfo)
    {
        if (value != null)
        {
            ObservableCollection<SolidWood_VM> castedCollection = (ObservableCollection<SolidWood_VM>)CurrentCollection.Source;

            SolidWood_VM curValue = (SolidWood_VM)((BindingExpression)value).DataItem;

            foreach (SolidWood_VM swVM in castedCollection)
            {
                if (curValue.GetHashCode() != swVM.GetHashCode() && swVM.Designation == curValue.Designation.ToString())
                {
                    return new ValidationResult(false, ResourcesManager.Instance.GetString("DuplicatedRecord"));
                }
            }
        }

        return new ValidationResult(true, null);
    }
}