How to remove all ListBox items?

Write the following code in the .cs file:

ListBox.Items.Clear();

I think it would be better to actually bind your listBoxes to a datasource, since it looks like you are adding the same elements to each listbox. A simple example would be something like this:

    private List<String> _weight = new List<string>() { "kilogram", "pound" };
    private List<String> _height = new List<string>() { "foot", "inch", "meter" };

    public Window1()
    {            
        InitializeComponent();
    }        

    private void Weight_Click(object sender, RoutedEventArgs e)
    {
        listBox1.ItemsSource = _weight;
        listBox2.ItemsSource = _weight;
    }

    private void Height_Click(object sender, RoutedEventArgs e)
    {
        listBox1.ItemsSource = _height;
        listBox2.ItemsSource = _height;
    }

isn't the same as the Winform and Webform way?

listBox1.Items.Clear();