Remember to add the usual using directives at the start of the codebehind

[codesyntax lang=”csharp”]
using System.Data;
using System.Data.SqlClient;
[/codesyntax]

For a DropDownList on your ASP.NET page with an ID of Category such a

[codesyntax lang=”html4strict”]
<asp:DropDownList ID="Category" runat="server">
</asp:DropDownList>
[/codesyntax]

The following in your code behind will bind data from an example Northwind database on SQL Server to the text and values of the DropDownList

[codesyntax lang=”csharp”]
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
using System.Data.SqlClient;

namespace AppDevStuff
{
  public partial class _Default : System.Web.UI.Page
  {
    protected void Page_Load(object sender, EventArgs e)
    {
      using(SqlConnection cnn = new SqlConnection("Data Source=YOURSERVERNAME; Database=Northwind; UID=YOURUSERNAME; PWd=YOURPASSWORD"))
      {
        SqlCommand cmd;
        SqlDataReader rdr;
        if (!Page.IsPostBack)
        {
          cmd = new SqlCommand("SELECT CategoryID, CategoryName FROM Categories ORDER BY CategoryName;", cnn);
          cnn.Open();
          rdr = cmd.ExecuteReader();

          Category.DataSource = rdr;
          Category.DataTextField = "CategoryName";
          Category.DataValueField = "CategoryID";
          Category.DataBind();
        }
      }
    }
  }
}
[/codesyntax]