XmlNodeList elemList = xmlDoc.GetElementsByTagName("company");
foreach (XmlNode node in elemList)
{
    if (node.Attributes[0].Value == company)
    {
        foreach (XmlNode child in node.ChildNodes)
        {
            foreach (XmlNode detail in child.ChildNodes)
            {
                ddlCodes.Items.Add(detail.Value.ToString());
            }
        }
    }
}

<companies>
    <company id="company1">
       <code>12</code>
       <detail>detail of 12 code</detail>
    </company>
    <company id="company2">
       <code>15</code>
       <detail>detail of 15 code</detail>
    </company>
</companies>

[XmlRoot("companies")]
public class Root
{
    [XmlElement("company")]
    public company[] companies;
}

public class company
{
    [XmlAttribute("id")]
    public string id;
    public string code;
    public string detail;
}


XmlSerializer xml = new XmlSerializer(typeof(Root));
Root r = (Root)xml.Deserialize(new StringReader(xmlstr));