Sometimes, it is not practical to statically predefine the tree structure due to data size or custom content that depends on user input. Because of this, the System.Web.UI.WebControls.TreeView control supports dynamic node population. When the TreeNode.PopulateOnDemand property for a node is set to true, that node gets populated at run time when the node is expanded.
To populate a node dynamically, first set the TreeNode.PopulateOnDemand property for the node to true. Next, define an event-handling method for the TreeView.TreeNodePopulate event that populates the node programmatically. A typical event-handling method retrieves node data from a data source, places the data into a node structure, and then adds the node structure to the TreeNode.ChildNodes collection of the node being populated. A node structure is created by adding System.Web.UI.WebControls.TreeNode objects to the TreeNode.ChildNodes collection of a parent node.
When the TreeNode.PopulateOnDemand property for a node is set to true, the node must be populated dynamically. You cannot declaratively nest another node below it; otherwise, an error will occur on the page.
Supported browsers (Microsoft Internet Explorer 4.0-compatible browsers and later) can also take advantage of client-side node population. When enabled, this allows the System.Web.UI.WebControls.TreeView control to populate a node dynamically on the client when that node is expanded, which prevents the need to post back to the server. For more information on client-side node population, see TreeView.PopulateNodesFromClient.
For more information about handling events, see Consuming Events.