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If the ButtonBase.ImageList property value is changed to null, the ButtonBase.ImageIndex property returns its default value, -1. However, the assigned ButtonBase.ImageIndex value is retained internally and used when another System.Windows.Forms.ImageList is assigned to the ButtonBase.ImageList property. If the new System.Windows.Forms.ImageList assigned to the ButtonBase.ImageList property has an System.Windows.Forms.ImageList.ImageCollection.Count property value that is less than or equal to the value assigned to the ButtonBase.ImageIndex property minus one (because the collection is a zero-based index), the ButtonBase.ImageIndex property value is adjusted to one less than the System.Windows.Forms.ImageList.ImageCollection.Count property value. For example, consider a button control whose System.Windows.Forms.ImageList has three images and whose ButtonBase.ImageIndex property is set to 2. If a new System.Windows.Forms.ImageList that has only two images is assigned to the button, the ButtonBase.ImageIndex value changes to 1.