private void btnAdd_Click(object sender, RoutedEventArgs e)
        {
            List <string> errors = new List <string>();

            if (string.IsNullOrEmpty(txtFirstName.Text))
            {
                errors.Add("FirstName is required.");
            }
            ;

            if (string.IsNullOrEmpty(txtLastName.Text))
            {
                errors.Add("LastName is required.");
            }
            ;

            if (string.IsNullOrEmpty(txtAddress.Text))
            {
                errors.Add("Address is required.");
            }
            ;

            if (string.IsNullOrEmpty(txtSex.Text))
            {
                errors.Add("Sex is required.");
            }
            ;

            if (string.IsNullOrEmpty(txtContactNumber.Text))
            {
                errors.Add("Contact-Number is required.");
            }
            ;

            if (string.IsNullOrEmpty(txtEmail.Text))
            {
                errors.Add("Email is required.");
            }
            ;

            if (string.IsNullOrEmpty(txtPassword.Text))
            {
                errors.Add("Password is required.");
            }
            ;

            if (errors.Count > 0)
            {
                foreach (var error in errors)
                {
                    txtError.Text = txtError.Text + error + "\n";
                }

                return;
            }


            var op = Userbll.Add(new User()
            {
                UserID        = Guid.NewGuid(),
                FirstName     = txtFirstName.Text,
                LastName      = txtLastName.Text,
                Address       = txtAddress.Text,
                Sex           = txtSex.Text,
                ContactNumber = txtContactNumber.Text,
                Email         = txtEmail.Text,
                Password      = txtPassword.Text
            });

            if (op.Code != "200")
            {
                MessageBox.Show("Error : " + op.Message);
            }

            else
            {
                MessageBox.Show(" is successfully added to table");
            }

            this.Close();
        }