Here is my code

1            da.SelectCommand.CommandText = "select * from users where UserName='" + username.Text + "' and Password='" +password.Text+"'";
2    
3            try
4            {
5                da.Fill(ds);
6              
7            }
8            catch
9            {
10               Response.Write("&lt;script language='javascript'>alert('Database maintainance is ongoing please try again later.');</script>");
11           }
12   
13           try
14           {
15             
16                   if (ds.Tables[0].Rows.Count > 0)
17                   {
18                       Session["User"] = username.Text;
19                       Session["Role"] = ds.Tables[0].Rows[0]["Role"].ToString();
20   
21                       Response.Redirect("list.aspx");
22   
23                   }
24                   else
25                   {
26                       Response.Write("&lt;script language='javascript'>alert('Invalid Username or Password');</script>");
27                   }
28            
29           }
30           catch (Exception ex)
31           {
32               Response.Write("&lt;script language='javascript'>alert('" + ex.Message + "');</script>");
33           }

when it comes to Response.Redirect("list.aspx");  it throws exception "base {System.Exception} = {Unable to evaluate expression because the code is optimized or a native frame is on top of the call stack.}"

This is a well known issue, You should add the second parameter of the Response.Redirect method

Response.Redirect("list.aspx",false);

inorder not to stop the execution of the current thread u need to keep it as false.