This property only reports a useful value if the ListBox.HorizontalScrollbar property is set to true. If the width of the System.Windows.Forms.ListBox is smaller than the value of this property, the horizontal scroll bar horizontally scrolls items in the System.Windows.Forms.ListBox. If the width of the System.Windows.Forms.ListBox is equal to or greater than this value, the horizontal scroll bar is hidden. The value of this property is not dynamically updated by the System.Windows.Forms.ListBox. This property is useful when the items of the System.Windows.Forms.ListBox are owner-drawn. For example, if the owner drawn items of the System.Windows.Forms.ListBox are 200 pixels wide, but the System.Windows.Forms.ListBox is 60 pixels wide, the ListBox.HorizontalExtent property would need to be set to 200 in order to scroll the right edge of the items into the visible region of the control.