ODP.NET

I have some Oracle Tables with sequences for primary key and stored procs in packages to wrap up the insert commands. The sequences field are all declared as NUMBER.

I also have Datasets based on the tables and a DataAdapter for each package. The Datasets see the primary keys as System.Decimal. The DataAdapter sees the output primary key parameter to the stored procs as OracleDecimal.

tmp.Parameters.Add(new OracleParameter("P_ID", Oracle.DataAccess.Client.OracleDbType.Decimal, ParameterDirection.Output));
tmp.Parameters["P_ID"].SourceColumn = "ID";


When I call the Update on the DataAdapter the update happens on the DB and then I get the following error

System.ArgumentException : Unable to cast object of type 'Oracle.DataAccess.Types.OracleDecimal' to type 'System.IConvertible'.Couldn't store <231> in ID Column. Expected type is Decimal.
----> System.InvalidCastException : Unable to cast object of type 'Oracle.DataAccess.Types.OracleDecimal' to type 'System.IConvertible'.

If I change the Oracle parameter to Oracle.DataAccess.Client.OracleDbType.Int32 or Oracle.DataAccess.Client.OracleDbType.Int64 it works fine - any ideas why that would be ? I would expect System.Decimal to map to Oracle.DataAccess.Types.OracleDecimal.