title: set Select2 dropdown as readonly in some situation
body: <p>I used the jQuery select2 dropdown in my project.<br>
  I want to set readonly in the select2 dropdown in some situations. How can I set the select2 as readonly from an aspx.cs page (server side)?</p>
  
  <p>And I want to insert the "Select All" checkbox for selecting all the options. How can I do this?</p>