C# WinForms highlight treenode when treeview doesn’t have focus

What you are looking for is the HideSelection property on the TreeView.

From MSDN:

Gets or sets a value indicating whether the selected tree node remains highlighted even when the tree view has lost the focus.

Link: http://msdn.microsoft.com/en-us/library/system.windows.forms.treeview.hideselection.aspx

Code:

TreeView.HideSelection = false;