title: Disable Keyboard Entry to ASP Textbox
body: <p>I have a text box with an ajaxToolkit:calendarextender attached to it.<br>
  I also have an <code>OnTextChanged="do something"</code> with an <code>AutoPostBack=true</code>.</p>
  
  <p><strong>I only want my user to be able to enter data into a textbox by using the calendar extender.</strong></p>
  
  <p><em>I've tried setting the <code>readonly</code> property on the textbox to true. This works only if there is existing text in the textbox (if I <code>textbox.text = "such and such"</code> then entry via the calendar extender works).</em></p>
  
  <p>If I try selecting a date by clicking the textbox and selecting a date in the calendar extender, anything that is entered is immediately cleared because of the <code>autopostback</code> and the <code>OnTextChanged</code> event. I get this exception:  </p>
  
  <pre><code>Exception was thrown at line 3169, column 13 in http://localhost:44994/ScriptResource.axd?d=Es_vqsdtlYssgAF4A3XSWnRVXoRhHI0Nw5kbnX4TQidZ-ER24J-iO0awsG5JH1rtQZR_iEqhZs7sdOBQ9bRTWyylEaGjH7fv-s8DBZ9g2dBrQRgBCGkgUkMcfnp1eLLpQ46PuRCK6kw9bzcz5Laedpe2sZMPd6VRGBt-E7Jq3Hi6WSFYwQmlvlft0Bho70p20&amp;t=7b689585
  0x800a138f - JavaScript runtime error: Unable to get property 'AbbreviatedMonthNames' of undefined or null reference
  </code></pre>
  
  <p>How can I disable entry into the textbox via keyboard and only allow entry by the <code>calendarextender</code>?</p>