My organization has a scenario where we need to have the datatype of a column in a dataset be of type 'System.Data.DataSet'. To give some background, we use the dataset to hold the state of our domain entities. Each entity is backed by a dataset. We organize them hierarchically by linking the datasets together at a column level (Account dataset has a Contacts row that holds a Contacts dataset). If you need more background please contact me. This is a very crucial scenario for our entity framework and currently, even though the dataset designer allows for that datatype to be specified, the other critical properties like NullValue, DefaultValue, etc. can not be set in a way that makes sense for that datatype. When attempted we receive the exception:"For columns not defined as System.String, the only valid value is (Throw exception). "Additionally, if we try to set the default value to 'null' we get an error. But it is possible if you use System.String. In general, if the designer followed the same rules for all reference types that it does for System.String, I think we would be fine.