Question

I have this code and it always returns -1.I have three tables (a picture is more suggestive ):

I want to see if the row is already in the ReservationDetails table, and if it’s not to insert it.

try
        {
            SqlConnection conn = new SqlConnection... 
            SqlCommand slct = new SqlCommand("SELECT * FROM ReservationDetails WHERE rID=@rID AND RNumber=@RNumber", conn);
            slct.Parameters.AddWithValue("@rID", (int)comboBox1.SelectedValue);
            slct.Parameters.AddWithValue("@RNumber", dataGridView1.SelectedRows[0].Cells[0].Value);

            int noRows;//counts if we already have the entry in the table
            conn.Open();
            noRows = slct.ExecuteNonQuery();
            conn.Close();
            MessageBox.Show("The result of select="+noRows);

            if (noRows ==0)    //we can insert the new row

Answer 1

You should

1) Change your TSQL to

 SELECT COUNT(*) FROM ReservationDetails WHERE ...

(better still, use IF EXISTS …)

2) and use ExecuteScalar():

noRows = (int) slct.ExecuteScalar();

Also: you will need to use a transaction (or some other atomic technique), or else someone could insert a row in-between you testing and trying to insert it…

All that said, it would be better to create a stored procedure that given your parameters, atomically tests and inserts into the table, returning 1 if successful, or 0 if the row already existed.