A program can call a TreeView node's EnsureVisible method to expand all of the nodes above it in the tree, but there is no simple method to expand an entire subtree below a node. This example uses the following ExpandNode method to expand a node and its subtree.

// Expand this node and its subtree.
private void ExpandNode(TreeNode node)
{
    node.EnsureVisible();
    foreach (TreeNode child in node.Nodes)
    {
        ExpandNode(child);
    }
}

This code calls the node's EnsureVisible method. It then loops through the node's children and recursively calls ExpandNode for each.

That would be the end of the story except a TreeView control can have more than one top-level node. That means if you want to expand the entire tree, you need to call ExpandNode for each top-level node. The following ExpandTreeView method does just that.

// Expand the TreeView control's top-level nodes.
private void ExpandTreeView(TreeView trv)
{
    foreach (TreeNode node in trv.Nodes)
    {
        ExpandNode(node);
    }
}

This method simply loops through the TreeView's top-level nodes, calling ExpandNode for each.

(Note that these methods might make good extension methods for the TreeNode and TreeView classes.)