Retreiving all the datas that have been saved in a custom file extension that contains some wizards data and some treenodes using C#

Here are the two methods that save and load the files:

Saving the tree:
saveTree(TreeView tree, string filename)
{
ArrayList al = new ArrayList();
foreach (TreeNode tn in tree.Nodes)
{
//Save each RootNode in TreeView ...
al.Add(tn);
}

// create file
Stream file = File.Open(filename, FileMode.Create);
BinaryFormatter bf = new BinaryFormatter();
try
{
// Serializing Arrays
bf.Serialize(file, al);
}
catch (System.Runtime.Serialization.SerializationException e)
{
MessageBox.Show("Serialization failed : {0}", e.Message);
return -1; // ERROR
}

// Close the file
file.Close();

return 0;
}


Loading the tree:
loadTree(TreeView tree, string filename)
{
if (File.Exists(filename))
{
// open file
Stream file = File.Open(filename, FileMode.Open);
BinaryFormatter bf = new BinaryFormatter();
// Object var. init.
object obj = null;
try
{
//Disable data from the file
obj = bf.Deserialize(file);
}
catch (System.Runtime.Serialization.SerializationException e)
{
MessageBox.Show("De-Serialization failed : {0}", e.Message);
return -1;
}
// Close the file
file.Close();
Create new array
ArrayList nodeList = obj as ArrayList;
// load Root-Nodes
foreach (TreeNode node in nodeList)
{
tree.Nodes.Add(node);
}
return 0;

}
else return -2; // File does not exist

}

how do i serialize the treenode directly?