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MyDataType datatype = e.Node.Tag as MyDataType;
if(datatype != null)
{
  // nun kannst du auf die Eigenschaften und Methoden von datatype zugreifen, z.B.
  textbox.Text = datatype.Name;
}					

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 private void xmlDeviceData_Click(object sender, EventArgs e)
        {
            openFileDialog1.InitialDirectory = "C:/Users/Bleri/Documents/Visual Studio 2010/Projects/MieleProjekt";
            openFileDialog1.Title = "Lade die XMLDeviceData-Datei";
            openFileDialog1.FileName = "XMLDeviceData";
            openFileDialog1.Filter = "XML Datei|*.xml";
            if (openFileDialog1.ShowDialog() == DialogResult.OK)
            {
                
                sr = new System.IO.StreamReader(openFileDialog1.FileName);

                xmldoc = new XmlDocument();
                xmldoc.Load(sr);
                treeView.Nodes.Clear();

                treeView.Nodes.Add(new TreeNode(xmldoc.DocumentElement.LocalName));

                TreeNode tNode = new TreeNode();

                tNode = treeView.Nodes[0];

                AddNode(xmldoc.DocumentElement, tNode);

                treeView.ExpandAll();
                treeView.CollapseAll();

            }
        }
private void AddNode(XmlNode inXmlNode, TreeNode inTreeNode)
        {

            XmlNode xNode;
            TreeNode tNode;
            XmlNodeList nodeList;
            int i;
            // Loop through the XML nodes until the leaf is reached.
            // Add the nodes to the TreeView during the looping process.

            if (inXmlNode.HasChildNodes)
            {
                nodeList = inXmlNode.ChildNodes;

                for (i = 0; i <= nodeList.Count - 1; i++)
                {
                    xNode = inXmlNode.ChildNodes[i];
                    inTreeNode.Nodes.Add(new TreeNode(xNode.Name));
                    // inXmlNode
                    tNode = inTreeNode.Nodes[i];
                    AddNode(xNode, tNode);
                }
            }
            else
            {
                // Here you need to pull the data from the XmlNode based on the
                // type of node, whether attribute values are required, and so forth.
                inTreeNode.Text = (inXmlNode.OuterXml).Trim();
            }
        }