MSDN
Iterating Through All Nodes of a Windows Forms TreeView Control
// C#
private void PrintRecursive(TreeNode treeNode)
{
// Print the node.
System.Diagnostics.Debug.WriteLine(treeNode.Text);
MessageBox.Show(treeNode.Text);
// Print each node recursively.
foreach (TreeNode tn in treeNode.Nodes)
{
PrintRecursive(tn);
}
}

// Call the procedure using the TreeView.
private void CallRecursive(TreeView treeView)
{
// Print each node recursively.
TreeNodeCollection nodes = treeView.Nodes;
foreach (TreeNode n in nodes)
{
PrintRecursive(n);
}
}