This sample provides the demo on how to get the count for all checked nodes WinForms TreeViewAdv.
You can get the count for all checked nodes in WinForms TreeViewAdv control by using CheckedNodes property. Refer the below code for your reference.
C#
int checkedCount = treeView1.CheckedNodes.Count;VB.Net
Dim checkedCount As Integer = treeView1.CheckedNodes.Count
Take a moment to peruse the WinForms TreeViewAdv - CheckBox documentation, to learn more about checkbox with examples.
Visual Studio 2015 and above versions