XmlDocument errors = new XmlDocument();
    errors.Load("../../ErrorFile.xml");
    XmlElement root = errors.DocumentElement;
    XmlNode categoryNode = root.FirstChild;
    XmlNodeList nodes = categoryNode.ChildNodes;
    for (int i = 0; i < nodes.Count; i++)
         A.Items.Add(nodes[i].Name);
    string category = A.SelectedItem.ToString();

<root>
  <a name="1">
    <b>b1</b>
    <b>b2</b>
    <b>b3</b>
  </a>
  <a name="2">
    <b>c1</b>
    <b>c2</b>
    <b>c3</b>
  </a>
</root>

XElement x;

    public Form1()
    {
        InitializeComponent();

        x = XElement.Load("In.xml");
        comboBox1.Items.AddRange(
             x.Elements("a")
              .Select(a => a.Attribute("name").Value)
              .ToArray());
        comboBox1.SelectedIndexChanged += new EventHandler((s, e) =>
        {
            comboBox2.Items.Clear();
            if (comboBox1.SelectedIndex > -1)
            {
                comboBox2.Items.AddRange(
                    x.Elements("a")
                     .First(a => a.Attribute("name")
                                  .Value
                                  .Equals(comboBox1.SelectedItem))
                     .Elements()
                     .Select(b => b.Value)
                     .ToArray());
            }
        });
    }