The type 'L:StringValidation' was not found error.

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C#

.NET

Visual-Studio

WPF

C#4.0

, +

HI, sorry for my english.
I have a problem with WPF, and so have the application, where it wants to implement validation controls. I created a class StringValidation that inherits from the ValidationRule. There, I implemented the method validation. in XAML while I did something like this:

<TextBox Height="23" Margin="53,39,0,0" Name="txtNazwa" VerticalAlignment="Top" TabIndex="3" MaxLines="1" GotFocus="txtNazwa_GotFocus" HorizontalAlignment="Left" Width="218"
                             ToolTip="Nazwa wybranej roli" Style="{StaticResource textBoxError}" Validation.ErrorTemplate="{StaticResource errorTemplate}">
            <TextBox.Text>
                    <Binding Path="Nazwa" UpdateSourceTrigger="PropertyChanged" >
                        <Binding.ValidationRules>
                            <s:StringValidation ValidatesOnTargetUpdated="True"
                                                                    ValidationStep="RawProposedValue"
                                                                    ErrorMessage="Error"
                                                                    TylkoNiePuste="True"
                                                                    />
                        </Binding.ValidationRules>
                    </Binding>
            </TextBox.Text>
</TextBox>

And everything started working, so I think that is ok, whereas in the designer crashes error:

Error 1 The type 's:StringValidation' was not found. Verify that you are not missing an assembly reference and that all referenced assemblies have been built.

where I added:

xmlns:s="clr-namespace:MOJA.Kontrolka.Admin"

where the namespace is consistent with the StringValidation class. I do not know what it is, a very difficult job to me. Any suggestions where is the problem and how to fix / avoid?