button1.Attributes.Add("onclick", "javascript:window.open('page1.aspx?CheckedItem=" + checkedItem.ToString() + "'');"); is not working properly
<p>my client side code button1.Attributes.Add("onclick", "javascript:window.open('page1.aspx?CheckedItem=" + checkedItem.ToString() + "','mywindow',');"); is not executing on the first click.But it is working fine from second click onwards.I wrote this code inside the button click event.</p>

<p>I had tried to put this inside the page_load ,but the problem is i need to return a value from this page.
here is the code</p>

<pre><code>protected void btnPreview_Click(object sender, EventArgs e)
{

    //StringBuilder checkedItem = new StringBuilder();
    checkedItem.Length = 0;
    foreach (ListItem i in chkValidation.Items)
    {
        if (i.Selected)
        {
            if (string.IsNullOrEmpty(Convert.ToString(checkedItem)))
            {
                checkedItem.AppendFormat(i.Text);

            }
            else
            {
                checkedItem.AppendFormat(",");
                checkedItem.AppendFormat(i.Text);
            }
        }
        btn_Preview.Attributes.Add("onclick",
                                   "javascript:window.open('TimePhaseAttributePreview.aspx?CheckedItem=" + 
                                   checkedItem.ToString() + "','mywindow','menubar=0,resizable=0,width=350,height=250');");
    }
</code></pre>

<p>Please help me</p>
