<ContentPresenter       
      Content="{Binding ElementName=Number, Path=(Validation.Errors).CurrentItem}"
/>

ContentPresenter contentPresenter = new ContentPresenter()
{
    Content = new Binding() { ElementName = "Number", Path = ??? }
};

<!-- CUSTOMER TYPE -->
    <Label 
      Grid.Row="0" Grid.Column="0" 
      Content="Customer _type:" 
      HorizontalAlignment="Right"
      Target="{Binding ElementName=customerTypeCmb}" 
      />
    <ComboBox 
      x:Name="customerTypeCmb"
      Grid.Row="0" Grid.Column="2" 
      ItemsSource="{Binding Path=CustomerTypeOptions, Mode=OneTime}"
      SelectedItem="{Binding Path=CustomerType, ValidatesOnDataErrors=True}"
      Validation.ErrorTemplate="{x:Null}" 
      />
    <ContentPresenter 
      Grid.Row="1" Grid.Column="2"
      Content="{Binding ElementName=customerTypeCmb, Path=(Validation.Errors).CurrentItem}"
      />

<Grid.Resources>
      <DataTemplate DataType="{x:Type ValidationError}">
        <TextBlock 
          FontStyle="Italic"
          Foreground="Red"
          HorizontalAlignment="Right"
          Margin="0,1"
          Text="{Binding Path=ErrorContent}" 
          />
      </DataTemplate>
</Grid.Resources>
    <!-- CUSTOMER TYPE -->
    <Label 
      Grid.Row="0" Grid.Column="0" 
      Content="Customer _type:" 
      HorizontalAlignment="Right"
      Target="{Binding ElementName=customerTypeCmb}" 
      />
    <ComboBox 
      x:Name="customerTypeCmb"
      Grid.Row="0" Grid.Column="2" 
      ItemsSource="{Binding Path=CustomerTypeOptions, Mode=OneTime}"
      SelectedItem="{Binding Path=CustomerType, ValidatesOnDataErrors=True}"
      Validation.ErrorTemplate="{x:Null}" 
      />
    <ContentPresenter 
      Grid.Row="1" Grid.Column="2"
      Content="{Binding ElementName=customerTypeCmb, Path=(Validation.Errors).CurrentItem}"
      />

            Grid grd = new Grid() { Name = "grid", VerticalAlignment = VerticalAlignment.Top, Margin = new Thickness(0, 10, 0, 0) };
            this.RegisterName("grid", grd);
            ColumnDefinition colDef = new ColumnDefinition() { Width = new GridLength(115) };            
            grd.ColumnDefinitions.Add(colDef);
            colDef = new ColumnDefinition() { Width = new GridLength(250) };            
            grd.ColumnDefinitions.Add(colDef);
            for (byte i = 0; i < 9; i++)
            {
                RowDefinition rowDef = new RowDefinition() { Height = GridLength.Auto };
                grd.RowDefinitions.Add(rowDef);
            }

            FrameworkElementFactory fTxt = new FrameworkElementFactory(typeof(TextBlock));            
            fTxt.SetValue(TextBlock.ForegroundProperty, Brushes.Red);
            fTxt.SetValue(TextBlock.FontSizeProperty, 13.0);
            fTxt.SetValue(TextBlock.MarginProperty, new Thickness(0, 3, 0, 3));            
            fTxt.SetBinding(TextBlock.TextProperty, new Binding("ErrorContent"));           
            grd.Resources.Add("error", new DataTemplate(typeof(ValidationError)) { VisualTree = fTxt });     
       
            //f
            TextBlock txtBlock = new TextBlock() { Text = "фамилия", FontSize = 13 };
            grd.Children.Add(txtBlock);
            Grid.SetColumn(txtBlock, 0);
            Grid.SetRow(txtBlock, 0);
            txtBox = new TextBox()
            {
                DataContext = _val,
                MaxLength = 25,
                FontSize = 13,
                Style = new StyleErrorTemplateNull()
            };
            this.RegisterName("F", txtBox);
            txtBox.SetBinding(TextBox.TextProperty, new Binding("F") { Mode = BindingMode.TwoWay, ValidatesOnExceptions = false, ValidatesOnDataErrors = true, UpdateSourceTrigger = UpdateSourceTrigger.LostFocus });                       
            grd.Children.Add(txtBox);
            Grid.SetColumn(txtBox, 1);
            Grid.SetRow(txtBox, 0);

            ContentPresenter cpr = new ContentPresenter() ;//{ ContentTemplate = new ErrorTemplate() };
            cpr.SetBinding(ContentPresenter.ContentProperty, new Binding("(Validation.Errors).CurrentItem") { Source = txtBox });
            grd.Children.Add(cpr);
            Grid.SetColumn(cpr, 1);
            Grid.SetRow(cpr, 1);