private void btnDisclose_Click(object sender, RoutedEventArgs e)
        {
            if (cbxNormalList.IsDropDownOpen == false)
            {
                cbxNormalList.IsDropDownOpen = true;
                tblBtnDisclose.Content = "Свернуть список";
                return;
            }
            if (cbxNormalList.IsDropDownOpen == true)
            {
                cbxNormalList.IsDropDownOpen = false;
                tblBtnDisclose.Content = "Развернуть список";
                return;
            }
}

<Window x:Class="Menu_Lists_Trees.ComboBoxWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="ComboBoxWindow" Height="600" Width="1200">
    <Grid>
        <ComboBox x:Name="cbxNormalList" HorizontalAlignment="Left" VerticalAlignment="Top" Width="221" Margin="5,217,0,0" />
        <Button x:Name="tblBtnDisclose" HorizontalAlignment="Left" VerticalAlignment="Top" Width="163" Margin="10,12,0,0" Height="34" Click="btnDisclose_Click">
            <TextBlock TextWrapping="Wrap" Height="30" FontSize="11" FontWeight="Bold" Text="Раскрыть список">                
            </TextBlock>            
        </Button>
   </Grid>
</Window>