Scriptmanager异常处理问题

Question

关于应用AspncPostBackError事件，来让浏览器报错。引发一个除数为0的异常，代码有执行，没有语法错误，但是浏览器没有反应，不能提示错误信息。调试的时候编译也会提示有异常，高手帮帮小弟。 protected void Button1_Click(object sender, EventArgs e)
    {
        int a = Int32.Parse(TextBox1.Text);
        int b = Int32.Parse(TextBox2.Text);
        try
           {
           
              if(DropDownList1 .SelectedValue =="+")
             TextBox3 .Text = (a + b).ToString ();
            if (DropDownList1.SelectedValue == "-")
                TextBox3.Text = (a - b).ToString();
            if (DropDownList1.SelectedValue == "*")
                TextBox3.Text = (a * b).ToString();
            else if (DropDownList1.SelectedValue == "/")
                TextBox3.Text = (a / b).ToString();            
            }
    catch (Exception ex)
    {
        if (TextBox1.Text.Length > 0 && TextBox2.Text.Length > 0)
        {
            ex.Data["ExtraInfo"] = " 不能将 " +
                TextBox1.Text + "除以" + TextBox2.Text + ".";
        }
        throw ex;
   }        
}
    protected void ScriptManager1_AsyncPostBackError(object sender, AsyncPostBackErrorEventArgs e)
    {
       
        if (e.Exception.Data["ExtraInfo"] != null)
        {
            
            ScriptManager1.AsyncPostBackErrorMessage =
                e.Exception.Message +
                e.Exception.Data["ExtraInfo"].ToString();
        }
        else
        {
            ScriptManager1.AsyncPostBackErrorMessage =
                "未指定的错误发.";
        }