How do I load the list of a ComboBox from the database?

Question

I have a sql statement but i am not sure how to implement it so i can use it to populated a combobox in a winforms app.

This is how i had it in a previous app:

<asp:SqlDataSource ID="Sections" runat="server" ConnectionString="<%$ ConnectionStrings:MyConnection%>" 
            SelectCommand="SELECT e.DisplayName,  e.ID , e.GUID
            FROM ATable e
            INNER JOIN RootTable re
            ON e.ID = re.TablesID
            AND re.InitID = 1">
</asp:SqlDataSource>

How can i alter this to suite a winforms combobox

----UPDATE----

After implementing suggestion below: i am getting an error when i compile stating that the method must be non-generic static class.

Code:

public partial class InfoGet : Form
    {
        public InfoGet()
        {
            InitializeComponent();
        }
        static string conString = "Data Source=.;Initial Catalog=MyConnection;Integrated Security=True";
        public static void FillDropDownList(this ComboBox sections_drp, string conString)
        {
            using (SqlConnection con = new SqlConnection(conString))
            {
                con.Open();
                using(SqlDataAdapter sda = new SqlDataAdapter("SELECT....", conString))
                {
                    DataTable dt = new DataTable();
                    sda.Fill(dt);

                    sections_drp.ValueMember = "ID";
                    sections_drp.DisplayMember = "DisplayName";
                    sections_drp.DataSource = dt;
                }
            }

        }
    }

Answer 1

In a Windows Forms application you're going to need to go get it in code, or at least that's what I'm going to recommend. One possible approach is:

using (SqlConnection cnn = new SqlConnection(connString))
{
    cnn.Open();
    using (SqlDataAdapter sda = new SqlDataAdapater("SELECT e.DisplayName,  e.ID , e.GUID FROM ATable e INNER JOIN RootTable re ON e.ID = re.TablesID AND re.InitID = 1", cnn))
    {
        DataTable dt = new DataTable();
        sda.Fill(dt);

        comboBox.ValueMember = "ID";
        comboBox.DisplayMember = "DisplayName";
        comboBox.DataSource = dt;
    }
}

Now, to make this code reusable, you might build an extension method like this:

namespace System.Windows.Forms
{
    public static class ComboBoxExtensions
    {
        public static void Load(this ComboBox comboBox, string sql, string valueMember, string displayMember)
        {
            using (SqlConnection cnn = new SqlConnection(connString))
            {
                cnn.Open();
                using (SqlDataAdapter sda = new SqlDataAdapater(sql, cnn))
                {
                    DataTable dt = new DataTable();
                    sda.Fill(dt);

                    comboBox.ValueMember = valueMember;
                    comboBox.DisplayMember = displayMember;
                    comboBox.DataSource = dt;
                }
            }
        }
    }
}

With that extension method it would be pretty easy to use:

comboBox.Load("SELECT e.DisplayName,  e.ID , e.GUID FROM ATable e INNER JOIN RootTable re ON e.ID = re.TablesID AND re.InitID = 1",
    "ID",
    "DisplayName");