postbackurl not working - just posts back to the same page

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C#

ASP.NET

HI all

i am trying to create a login page...
after checking in database about the login user(if the user is valid), i want to navigate to other page,..so i used the following code:

protected void imgb_login_Click(object sender, ImageClickEventArgs e)
    {
        try
        {
            lb_error.Text = "";
            int flag = 0;
            str = "SELECT * FROM [user]";
            c.cmd = new System.Data.SqlClient.SqlCommand(str, c.connect());
            //c.dr = c.cmd.ExecuteReader();
            c.da = new System.Data.SqlClient.SqlDataAdapter(str, c.connect());
            c.da.Fill(c.ds);
            c.dt = c.ds.Tables[0];
            maxrecord = c.dt.Rows.Count;
            string email = "", pwd = "";
            string tbemailvalue = tb_email.Text;
            string tbpwdvalue = tb_pwd.Text;

            for(int i = 0; i < c.dt.Rows.Count; i++)
            {
                email = c.dt.Rows[i].ItemArray[4].ToString();
                pwd = c.dt.Rows[i].ItemArray[5].ToString();
                if ((tb_email.Text == email) && (tb_pwd.Text == pwd))
                {
                    flag = 1;

                    Session["Ufname"] = c.dt.Rows[i].ItemArray[2].ToString();
                    break;
                }
            }


            tb_email.Text = "";
            tb_pwd.Text = "";
            if (flag == 1)
            {
                //Session["Ufname"] = txbemail.Text;
               // Response.Redirect("Default.aspx");
                // Response.Write("authorised user.");
                imgb_login.PostBackUrl = "~/Default.aspx";
            }
            else
            {
                //Label6.Text = "Invalid User,<br/>Check Your Email Id and Password.";
                lb_error.Text ="Not an authorised user.";
            }
        }
        catch (Exception ex)
        {
            Response.Write(ex.ToString());
        }
    }

and now my problem is post backurl is not working in single click...
its working only in double click..

can any one plz help mee


thanks in advance....