مشکل در هنگام insert کردن

[naghneh]

این کلاسیه که تعریف کردم

class DB

    {

        public static SqlConnection con = new SqlConnection("SERVER=.; DataBase=db; Integrated Security = True");

        public static SqlDataAdapter da = new SqlDataAdapter("", con);

        public static DataTable dt = new DataTable();

        public static SqlCommand cmd = new SqlCommand("", con);



        public static DataTable exec(string s)

        {

            DataTable dt = new DataTable();



            DB.cmd.CommandText = s;



            if (DialogResult.No == MessageBox.Show("آیامطمئنید؟", "هشدار", MessageBoxButtons.YesNo, MessageBoxIcon.Question, MessageBoxDefaultButton.Button2, MessageBoxOptions.RightAlign))

                return dt;



            DB.con.Open();

            DB.cmd.ExecuteNonQuery();

            DB.con.Close();



            DB.da.Fill(dt);



            return dt;

        }

واین هم کامندی که براش میفرستم

  private void button1_Click(object sender, EventArgs e)

        {

            s = "insert into project Values(";

            s += projectsubject.Text + " , ";

            s += comboBox1.SelectedItem + " , ";

            s += comboBox2.SelectedItem + " , ";

            s += textBox7.Text + " , '";

            s += textBox2.Text + "' , '";

            s += textBox5.Text + "' , '";

            s += textBox3.Text + "' , '";

            s += textBox4.Text + "')";

            dt = DB.exec(s);

        }