Once you add the RadDropDownTree1.EmbeddedTree.NodeDataBound += EmbeddedTree_NodeDataBound, you need to have a method with the same name and the correct arguments:
private void EmbeddedTree_NodeDataBound(object sender, Telerik.Web.UI.RadTreeNodeEventArgs e)
if (e.Node.Text == "World Continents")
e.Node.Enabled = false;
Also, you can add use Visual Studio to generate the handler for you by typing RadDropDownTree1.EmbeddedTree.NodeDataBound += and then press the TAB key twice.
The link that you have shared is actually showing the TreeView control's events, but the snippet below it is actually the one for the DropDownTree, not the TreeView:
protected void ddtForums_NodeDataBound(object sender, DropDownTreeNodeDataBoundEventArguments e)
The DropDownTreeNode, unfortunately, does not have all the properties of the TreeView's RadTreeNode. That is why the TreeView's NodeDataBound is needed.
Regarding the Checkable property, it controls the checkbox of the node when the checkboxes are enabled for the DropDownTree. If you want to control if the node can be selected, you need to use the Enabled property of the TreeView node, not the DropDownTree's:
Once the TreeView's node is disabled, it will not be selectable.
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