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.

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

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'););

    }

Please help me