how to check if user is in database already ?

Question

I have a form which requires new user to register.  If they are already in the database, then they will be re-direct to the login page.  If not, they can proceed with the user registration.

 

 

However, I am not sure what is wrong with my code cos even if the username is not found, it still direct them to the login page.

 

protected void Submit_Button_Click(object sender, EventArgs e)
        {
            using (Team6_PhoneOnlineShopEntities2 cntx = new Team6_PhoneOnlineShopEntities2())
            {
                //try
                {
                    // var checkuser = from u in cntx.Users where u.userName == txtUserName.Text select u;
                    // var checkuser = from u in cntx.Users where txtUserName.Text == u.userName select u;
                     var checkuser = cntx.Users.FirstOrDefault(u => txtUserName.Text == u.userName);
                    //int a = Convert.ToInt32(txtUserId.Text);
                    // var User = cntx.Users.FirstOrDefault(m => m.userID == a);


                   // customer.userName = txtUserName.Text;

                    // String checkUser = "select Count(*) from User where userName = '" + customer.userName + "'";


                    if (checkuser == null)

                    {
                        User customer = new User();
                        customer.userName = txtUserName.Text;
                        customer.lastName = txtLastName.Text;
                        customer.userPassword = txtUserPassword.Text;
                        customer.userPassword = txtConfirmedPassword.Text;
                        customer.moblieNumber = txtMobileNo.Text;
                        customer.userAddress = txtAddress.Text;
                        customer.userEmail = txtEmail.Text;
                        customer.roleID = 1;
                        customer.paymentMethod = DropDownList1.SelectedValue;
                        cntx.Users.Add(customer);
                        cntx.SaveChanges();
                        lblMessageSuccess.Text = "successful registration";
                    }
                    else
                    {
                        lblMessageUser.Text = "You have registered before. Pls proceed to login";
                        Response.Redirect("/Login.aspx");
                    }

                }

                // Response.Redirect("SuperPhoneOnLineShop_Team6/ShoppingPage");// enter the right url here

            }
        }
    }
}

 I hope someone can point out what is the way to do my code so that it will turn out the way I want.  Many thanks.