MyDataAccess md = new MyDataAccess();
string sql = "INSERT INTO CUS VALUES(N'{0}',N'{1}',N'{2}',N'{3}',N'{4}',N'{5}')";
sql = string.Format(sql,textBox1.Text,textBox2.Text,textBox3.Text,textBox4.Text,textBox5.Text,textBox6.Text);
 md.connect();
 md.docommand(sql);
 md.disconnect();

********************************************************************************************


 public class MyDataAccess
    {

public string servername;
public string Database = "Mydata";
private SqlConnection con;
private SqlCommand cmd;
private SqlDataAdapter da;
public MyDataAccess()
        {
            con = new SqlConnection();
            cmd = new SqlCommand();
            da = new SqlDataAdapter();
            cmd.Connection = con;
            da.SelectCommand = cmd;

        }
        public void connect()
        {
string cs = "Data Source=.\\SQLEXPRESS;AttachDbFilename=|DataDirectory|\\{0}.mdf;Integrated Security=True;User Instance=True";
            cs = string.Format(cs, this.Database);
            con.ConnectionString = cs;
            con.Open();
        }
        public void disconnect()
        {
            con.Close();

        }
        public DataTable select(string sql)
        {
            cmd.CommandText = sql;
            DataTable dt = new DataTable();
            da.Fill(dt);
            return dt;
        }
        public void docommand(string sql)
        {
            cmd.CommandText = sql;
            cmd.ExecuteNonQuery();
        }