Binding to different property of bound object in Validation Rule

Question

Given the following View Model example

public class MyViewModel
{
  public ObservableCollection<MyObjType> BoundItems { get; }
}

and MyObjType

public class MyObjType
{
  public string Name { get; set; }
  public int Id { get; set; }
}

I have added a Validation rule to a DataGrid Column, where the DataGrid is bound to the BoundItems collection in my ViewModel, and the Text property in the Template Column is bound to the Name.

<DataGrid ItemsSource="{Binding BoundItems}">
      <DataGrid.Columns>
             <DataGridTemplateColumn>
                <DataGridTemplateColumn.CellTemplate>
                    <DataTemplate>
                        <TexBox>
                          <TextBox.Text>
                            <Binding Path="Name" ValidatesOnDataErrors="True">
                              <Binding.ValidationRules>
                                <xns:MyValidationRule>
                                  <xns:MyValidationRule.SomeDependencyProp>
                                    <xns:SomeDependencyProp SubProp={Binding Id} /> <!-- Not Working -->
                                  </xns:MyValidationRule.SomeDependencyProp>
                                </xns:MyValidationRule>
                              </Binding.ValidationRules>
                            </Binding>
                          </TextBox.Text>
                        </TextBox>
                    </DataTemplate>
                </DataGridTemplateColumn.CellTemplate>
            </DataGridTemplateColumn>
            ...
      </DataGrid.Columns>
</DataGrid>

I want to pass another property Id of my collection type (MyObjType) to the validation rule, how do I access that from the rule. I know about the freezable and getting the context of the view model, but i need another property of my collection type that is bound to the Datagrid.

The ValidationRule and SomeDependencyProp is modeled after the example here: https://social.technet.microsoft.com/wiki/contents/articles/31422.wpf-passing-a-data-bound-value-to-a-validation-rule.aspx

public class SomeDependencyProp : DependencyObject
{
  public static readonly SubPropProperty =
     DependencyProperty.Register("SubProp", typeof(int),
     typeof(SomeDependencyProp), new FrameworkPropertyMetadata(0));

  public int SubProp{
    get { return (int)GetValue(SubPropProperty ); }
    set { SetValue(SubPropProperty, value); }
  }
}

public class MyValidationRule: System.Windows.Controls.ValidationRule
{
  public override ValidationResult Validate(object value, CultureInfo cultureInfo) 
  {
    ...
  }

  public SomeDependencyProp SomeDependencyProp { get; set; }
}

Answer 1

The solution to this situation is to use a BindingProxy.