Question

 SqlConnection con = new SqlConnection("Data Source=.\\SQLEXPRESS;AttachDbFilename=|DataDirectory|\\Db.mdf;Integrated Security=True;User Instance=True");

 SqlCommand com2 = new SqlCommand();
 com2.Connection = con;
 com2.CommandText = "insert tb_items (item_name, item_unit) values ('" + this.txtitemname.Text + "','" + this.txtitemunit.Text + "')";
 con.Open();

  com2.ExecuteScalar();

Answer 1

You should be using ExecuteNonQuery instead of ExecuteScalar.

(You should also use using statements, and use parameterized SQL instead of including user input directly in your SQL statement, but that’s another matter. A very important matter, admittedly…)