I have in my WPF project file RssInfo.cs in which I have public class

public class DoubleRangeRule : ValidationRule
{
    public double Min { get; set; }

    public double Max { get; set; }

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

and from my XAML code in WPF window class I neet to get to this DoubleRangeRule class..
//reference to my project, all my files are in the WpfCzytanieRSS namespace

           xmlns:valRule="clr-namespace:WpfCzytanieRSS;assembly=WpfCzytanieRSS" 
            <TextBox Validation.ErrorTemplate="{StaticResource TextBoxErrorTemplate}" Name="tbTitle">
                <TextBox.Text>
                    <Binding Path="Nazwa" UpdateSourceTrigger="PropertyChanged">
                        <Binding.ValidationRules>
                            <valRule:DoubleRangeRule Min="0.5" Max="10"/> //error place
                        </Binding.ValidationRules>
                    </Binding>
                </TextBox.Text>
            </TextBox>

And i get two errors:
Error 1
The tag 'DoubleRangeRule' does not exist in XML namespace 'clr-namespace:WpfCzytanieRSS;assembly=WpfCzytanieRSS'.
Error 2
The type 'valRule:DoubleRangeRule' was not found. Verify that you are not missing an assembly reference and that all referenced assemblies have been built.

Please help to get to class DoubleRangeRule !
:)