public bool Login(User user)
        {
            try
            {
                User aUser =
                dataContext.Users.SingleOrDefault(
                    u => u.UserName.Equals(user.UserName) && u.Password.Equals(user.Password));
                if (aUser != null)
                {
                    return true;
                }
                return false;
            }
            catch (Exception exception)
            {

                throw new Exception(exception.Message);
            }
        }
        private void btnOK_Click(object sender, EventArgs e)
        {
            if (txtUserName.Text == "")
            {
                MessageBox.Show("Please enter user name", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
                txtUserName.Focus();
                return;
            }
            if (txtPassword.Text == "")
            {
                MessageBox.Show("Please enter password", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
                txtPassword.Focus();
                return;
            }
            try
            {
                User user = new User()
                {
                    UserName = txtUserName.Text,
                    Password = txtPassword.Text
                };
                if (userGateway.Login(user))
                {
                    DialogResult = DialogResult.OK;
                }

                else
                {
                    MessageBox.Show("Login is Failed...Try again !", "Login Denied", MessageBoxButtons.OK,
                        MessageBoxIcon.Error);

                    txtUserName.Clear();
                    txtPassword.Clear();
                    txtUserName.Focus();
                }
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message, "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
        }