I can't insert data from text boxes into postgre sql table using C#

Question

and here is my code

public partial class Form2 : Form
   {
        NpgsqlConnection conn;
        NpgsqlCommand cmd;
        string connstring = ConfigurationManager.ConnectionStrings["EmployeeData"].ConnectionString;
        private DataSet ds = new DataSet();
        private DataTable dt = new DataTable();

        public Form2()
        {
            InitializeComponent();
            lblErrorMessage.Visible = false;
        }


        string employee = "insert into  VSP_Employee values ('" + txtEmpid.Text + "','" + txtEmpname.Text + "','" + txtEmailid.Text + "','" + txtphnum.Text + "','" + txtDOB.Text + "','" + txtDOJ.Text + "','" + txtNOD.Text + "','" + txtSalary.Text + "','" + txtLeavestaken.Text + "','" + txtLeavesBalance.Text + "')";
        NpgsqlDataAdapter da = new NpgsqlDataAdapter(employee, conn);
        cmd = new NpgsqlCommand(employee, conn);
        da = new NpgsqlDataAdapter(cmd);
        ds = new DataSet();
        da.Fill(ds);