How to populate tree nodes from server side? I am trying to convert nodes ToJson but method seems not working.

TreeNodeCollection nodes = new TreeNodeCollection();
            
TreeNode root = new TreeNode("-1", "Root");
nodes.Add(root);

TreeNode node1 = new TreeNode("1001", "Name1");
root.ChildNodes.Add(node1);

string n = nodes.ToJson();  --> ToJson method not found
ResMgrCustomer.AddScript("TreePnlLevel.initChildren(" + n + ")");