XAML:

<ComboBox Height="27" Margin="124,0,30,116" Name="cbProductDefaultVatRate" VerticalAlignment="Bottom" ItemsSource="{Binding}">
    <ComboBox.ItemTemplate>
        <DataTemplate>
            <Label Height="26" Content="{Binding Path=Value}"/>
        </DataTemplate>
    </ComboBox.ItemTemplate>
</ComboBox>

Set Data for cbProductDefaultVatRate.Items (Type - VatRate):

private void ShowAllVatRates()
{
    cbProductDefaultVatRate.Items.Clear();
    cbProductDefaultVatRate.ItemsSource = new VatRateRepository().GetAll();
}

Similarly, I defined property:

private Product SelectedProduct
{
    get; set;
}

The Product contains VatRate:

SelectedProduct.DefaultVatRate

How to set SelectedItem equal SelectedProduct.DefaultVatRate? I tried:

cbProductDefaultVatRate.SelectedItem = SelectedProduct.DefaultVatRate;

But it does not work.

Thank you for answers!