SelectedIndexChanged event isn't fired after onchange
<p>I am trying to validate a Dropdown list on client side for its selected index/value.
I want a function that alerts when the selected index is 0 or it should execute the <code>SelectedIndexChandged</code> method in the code behind..</p>

<p>I have written the following code..</p>

<pre><code>function validateDropDown() {
        var indexService = document.getElementById('&lt;%= ddlService.ClientID&gt;').selectedIndex;
        var indexTower = document.getElementById('&lt;%= ddlManager.ClientID&gt;').selectedIndex;
        if (indexTower == 0) {
            document.getElementById('&lt;%= ddlService.ClientID%&gt;').disabled = true;
            document.getElementById('&lt;%= ddlDate.ClientID%&gt;').disabled = true;
            alert("Please select a Tower");
            return false;
        }
        else{
           document.getElementById('&lt;%= ddlService.ClientID%&gt;').disabled = false;
           document.getElementById('&lt;%= ddlDate.ClientID%&gt;').disabled = false;
           _doPostBack('&lt;%= ddlManager.ClientID%&gt;');
        }
}
</code></pre>

<p>This line is throwing an error on runtime.</p>

<pre><code>_doPostBack('&lt;%= ddlManager.ClientID%&gt;');
</code></pre>

<p>the error is : </p>

<blockquote>
  <p>Javascript Runtime error : Object required.</p>
</blockquote>

<p>Note : I Have a master page in the application and the DDL is in the content page.</p>

<p>Can Anyone help me on that.. Thanks is advance.</p>
