I want to Open Drop Down Automatically.

 <combobox name="comboBox1">
                    DropDownClosed="comboBox1_DropDownClosed" 
                    SelectionChanged="comboBox1_SelectionChanged" />
And your code behind

    private bool keepOpen = false;
    private void comboBox1_DropDownClosed(object sender, EventArgs e)
    {
        new Thread(() =>
        {
            comboBox1.Dispatcher.Invoke(new Action(() =>
            {
                // do your IF logic here
                if (keepOpen)
                {
                    comboBox1.IsDropDownOpen = true;
                    keepOpen = false;
                }
            }), null);
        }).Start();
    }

    private void comboBox1_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        keepOpen = true;
    }</combobox>