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Gets the image list that is used to indicate the state of the C1TreeView and its nodes.
[TypeConverter(typeof(ImageListConverter))]
public ImageList CheckImageList { get; }
When the CheckImageList property is set, each C1TreeNode that is contained in the C1TreeView displays the first and second images from the CheckImageList to indicate an unchecked or checked state, respectively. Additionally the C1TreeView can displays the twelve state of checkbox if the CheckImageList property contains twelve images with keys from CheckBoxState enum.