c# combobox value from another combobox?

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C#

SQL

combobox

I would like to make like 4 comboboxes where each list within the combobox derives from the previous one. I thought this would be as easy as setting each box to a variable and then using that variable in the query for the combobox after it. I have attempted to use a variable in the query editor in the fillby, and also attempted to use code like this:

            using (SqlConnection cs = new SqlConnection(
@"Data Source=100-olkj-900-ptalog=Report;Persist Security Info=True;User ID=reports;Password=password
            {
                cs.Open();

 
                using DataAdapter da = new SqlDataAdapter(query
                {
                    // 3
                    
                    DataTable t = new DataTable();
                    da.Fill(t);

                    // 4
                    // Render data onto the screen
                    comboBox1.DataSource = t; // <-- From your designer
                    comboBox1.DisplayMember = "station.stationipaddress";
                    comboBox1.ValueMember = "station.stationipaddress";

                }

I am not really sure, what to try...
Any ideas anyone >?