ValidationRule with DependencyProperty

I would like to send a value from a bound  viewmodel to my inherited ValidationRule. I have read several articles but had no success implementing this.

Firstly my xaml.

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:assets="clr-namespace:DiskManagerView.Assets"
	xmlns:telerik="http://schemas.telerik.com/2008/xaml/presentation"><ControlTemplate x:Key="validationTemplate"><DockPanel><Border BorderBrush="Red" BorderThickness="1" Margin="1"><AdornedElementPlaceholder/></Border></DockPanel></ControlTemplate><Style x:Key="textBoxInError" TargetType="{x:Type TextBox}"><Style.Triggers><Trigger Property="Validation.HasError" Value="true"><Setter Property="ToolTip" Value="{Binding RelativeSource={x:Static RelativeSource.Self},
                              Path=(Validation.Errors)/ErrorContent}"/></Trigger></Style.Triggers></Style><DataTemplate x:Key="EditTemplate"><Grid DataContext="{Binding}"
              Name="Grid1"><Grid.RowDefinitions><RowDefinition Height="250*"/><RowDefinition Height="30"/></Grid.RowDefinitions><StackPanel Grid.Row="0" 
                        Orientation="Vertical"><TextBox Validation.ErrorTemplate="{StaticResource validationTemplate}"
                         Style="{StaticResource textBoxInError}"><TextBox.Text><Binding Path="Name" ValidatesOnDataErrors="True" UpdateSourceTrigger="PropertyChanged" Mode="TwoWay"><Binding.ValidationRules><assets:NullStringValidationRule><assets:NullStringValidationRule.Content><assets:ValidationParams Value="{Binding Path=IsValid, Mode=TwoWay}"/></assets:NullStringValidationRule.Content></assets:NullStringValidationRule></Binding.ValidationRules></Binding></TextBox.Text></TextBox></StackPanel>        </Grid></DataTemplate></ResourceDictionary>

And my inherited ValidationRule and the Inherited DependencyObject.

Namespace Assets

    Public Class NullStringValidationRule
        Inherits ValidationRule

        Private m_content As ValidationParams
        Public Property Content() As ValidationParams
            Get
                Return m_content
            End Get
            Set(ByVal value As ValidationParams)
                m_content = value
            End Set
        End Property


        Public Overrides Function Validate(value As Object, cultureInfo As System.Globalization.CultureInfo) As System.Windows.Controls.ValidationResult
            Dim _val As String = TryCast(value, String)
            If String.IsNullOrEmpty(value.ToString) Then
                'Content.Value = False
                Return New ValidationResult(False, "NullStringValidationRule: String cannot be null.")
            Else
                'Content.Value = True
                Return New ValidationResult(True, Nothing)
            End If
        End Function

    End Class

    Public Class ValidationParams
        Inherits DependencyObject

        Public Property Value As Object
            Get
                Return GetValue(ValueProperty)
            End Get

            Set(ByVal value As Object)
                SetValue(ValueProperty, value)
            End Set
        End Property

        Public Shared ReadOnly ValueProperty As DependencyProperty = _
                               DependencyProperty.Register("Value", _
                               GetType(Object), GetType(ValidationParams), _
                               New FrameworkPropertyMetadata(Nothing))


    End Class

End Namespace

and the error message i receive.

System.Windows.Data Error: 2 : Cannot find governing FrameworkElement or FrameworkContentElement for target element. BindingExpression:Path=IsValid; DataItem=null; target element is 'ValidationParams' (HashCode=8674443); target property is 'Value' (type 'Object')

Can somebody set me straight on this, I'm stuck. I think i need to supply a DataContext to my ValidationParams but not sure.

Gary