Gets or sets the content of column style with MText format string, this property is usable for most table columns,
but for alignment segment table, please use GetContentString() and specified type.
Thrown when the column is obtained from a alignment segment table style or parcel segment table style or structure table style.
Remarks
Invalid property for the SegmentColumnStyle and StructureColumnStyle.
We can use method Autodesk.AutoCAD.DatabaseServices.MText.SetContentsRtf and Autodesk.AutoCAD.DatabaseServices.MText.Content to convert rtf format string to MText format string.