Question

I have two combobox in my form, using a mysql connection to a remote server. The first combobox populated nicely. However, I need the indexid since that is a foreign key to populate a second combobox. Based on the selection, it will change the data in the second combo (for the xample, i the first combo is for car makes, then the models of each make gets filled, so if I choose Nissan, the models will then have Altima, Maxima, Sentra, … but if I chose Toyota, the combo will then show Corolla, Camry, Prius, …)

My foreign key is always -1 for some reason. I am using the selectindex change method, but it keeps crashing/ bc the value is always -1.

I am very new to MySQL in C# and eager to learn. Any help is appreciated. The code is below.

    private void cboMake_SelectedIndexChanged(object sender, EventArgs e)
    {
        if (cboMake.SelectedIndex >= 0)
            cboModel.Enabled = true;
        else
            cboModel.Enabled = false;

        // get the foreign key value of the model id to get the make for each brand to populate here            
        // MessageBox.Show(cboModel.ValueMember);
        // right now selectindex always shows -1.  why?
        // if it changes, you need to then enable the right cbo, else, disable.
        // but also, you need the mid, fk, so you can then do a new sql statement with the where clause to populate it.
        int fk = cboModel.SelectedIndex;            
        string connStr = "server=123.456.7.8;user=root;database=car;port=3306; password=nowayjose";
        MySqlConnection conn = new MySqlConnection(connStr);
        try
        {
            string sql = "SELECT * FROM cs_model WHERE mid='fk'";
            MySqlCommand cmd = new MySqlCommand(sql, conn);
            DataTable dt = new DataTable();
            MySqlDataAdapter da = new MySqlDataAdapter(cmd);
            da.Fill(dt);
            cboMake.DataSource = dt;
            cboMake.DisplayMember = "model";
            cboMake.ValueMember = "mmid";
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.ToString(), "MySQL Connection Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
        }
        conn.Close();
    }

    private void frmMain_Load(object sender, EventArgs e)
    {
        string connStr = "server=123.456.7.8;user=root;database=car;port=3306; password=nowayjose";
        MySqlConnection conn = new MySqlConnection(connStr);
        try
        {
            string sql = "SELECT * FROM cs_make";
            MySqlCommand cmd = new MySqlCommand(sql, conn);
            DataTable dt = new DataTable();
            MySqlDataAdapter da = new MySqlDataAdapter(cmd);
            da.Fill(dt);
            cboMake.DataSource = dt;
            cboMake.DisplayMember = "make";
            cboMake.ValueMember = "mid";               
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.ToString(), "MySQL Connection Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
        }
        conn.Close();
    }

Answer 1

    private void cboMake_SelectedIndexChanged(object sender, EventArgs e)
    {


        int foreignKey = (int)cboMake.SelectedIndex +1;   // i forgot to cast the int from string since the database was tinyint, it still returns string in c#
        string fk = foreignKey.ToString();   // since i need the string for the query, back it goes, but i needed to do arithmetic 
        string connStr = "server=10.18.30.1;user=notroot;database=car;port=3306;password=password";
        MySqlConnection conn = new MySqlConnection(connStr);
        try
        {
            string sql = "SELECT * FROM cs_model WHERE mid=" + fk;   // forgot to escape out of the string (used to php $var style within double quotes)
            MySqlCommand cmd = new MySqlCommand(sql, conn);                    
            DataTable dt = new DataTable();
            MySqlDataAdapter da = new MySqlDataAdapter(cmd);
            da.Fill(dt);
            cboModel.DataSource = dt;           // i had cboModel by a silly mistake
            cboModel.DisplayMember = "model";
            cboModel.ValueMember = "mmid";

        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.ToString(), "MySQL Connection Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
        }
        conn.Close();            
    }