<form id="form1" runat="server">
    <div>
        <asp:DropDownList runat="server" Enabled="False" ID="ddlTest"/>
        <asp:Button runat="server" ID="btnTest" Text="Test" OnClick="btnTest_OnClick"/>
    </div>
</form>

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
         DataTable dt=new DataTable();
         DataColumn dtc = new DataColumn("key", typeof(string));
         dt.Columns.Add(dtc);

         dtc = new DataColumn("value", typeof(string));
         dt.Columns.Add(dtc);

         //添加数据到DataTable
         DataRow dr = dt.NewRow();
         dr["key"] = "张";
         dr["value"] = "张三";
         dt.Rows.Add(dr);

         dr = dt.NewRow();
         dr["key"] = "李";
         dr["value"] = "李四";
         dt.Rows.Add(dr);

         dr = dt.NewRow();
         dr["key"] = "王";
         dr["value"] = "王五";
         dt.Rows.Add(dr);

         ddlTest.DataSource = dt;
         ddlTest.DataTextField = "value";
         ddlTest.DataValueField = "key";
         ddlTest.SelectedValue = "李";
         ddlTest.DataBind();
    }
}

protected void btnTest_OnClick(object sender, EventArgs e)
{
    var value= ddlTest.SelectedValue;
    Response.Write("<script language='javascript'>alert('当前选中:" + value + "');</script>");
}