Gets or sets the key of the default image shown when a System.Windows.Forms.TreeNode is in a selected state.
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If a System.Windows.Forms.TreeNode does not have a selected image set, the image specified by the TreeView.SelectedImageKey property will be shown when the node is not selected.
The TreeView.SelectedImageKey and TreeView.SelectedImageIndex properties are mutually exclusive; if one is set, the other is ignored. If you set the TreeView.SelectedImageKey property, the TreeView.SelectedImageIndex property is automatically set to -1. Alternatively, if you set the TreeView.SelectedImageIndex property, the TreeView.SelectedImageKey is automatically set to an empty string ("").