Managed ODP: InvalidCastException when returning large number

Question

I get an InvalidCastException "Specified cast is not valid" when trying to get a value bigger than Decimal.MaxValue as a scalar result from a select.
From the stack trace and inspection of the Oracle.ManagedDataAccess.dll code I would expect OracleDataReader to cast to OracleDecimal (or System.Numerics.BigInteger) instead of System.Decimal for large numbers.
Is this a bug?
Stack trace:
   at Oracle.ManagedDataAccess.Client.OracleDataReader.GetDecimal(Int32 i)
   at Oracle.ManagedDataAccess.Client.OracleDataReader.GetValue(Int32 i)
   at Oracle.ManagedDataAccess.Client.OracleCommand.ExecuteScalar()
   at Cora.Data.DBProvider.Tests.CoraParameterTest.BigIntegerOracleTest() in D:\svnroot\tech\lib\Cora.Data.DBProvider\Cora.Data.DBProvider.Tests\CoraParameterTest.cs:line 98
How to reproduce:
- create a table NUNITTEST_PARAM_SIMPLE with a column BIG of type NUMBER(38)
- run sample code
            using ( OracleConnection conn = new OracleConnection(ORACONN) )
                conn.Open();
                using ( OracleCommand cmd = conn.CreateCommand() )
                    cmd.CommandText = "insert into NUNITTEST_PARAM_SIMPLE (BIG) values (1234567890123456789012345678901234567)";
                    cmd.ExecuteNonQuery();
                using ( OracleCommand cmd = conn.CreateCommand() )
                    cmd.CommandText = "select BIG from NUNITTEST_PARAM_SIMPLE";
                    object result = cmd.ExecuteScalar();          // Exception
                conn.Close();

Answer 1

There is no safe type mapping for ExecuteScalar. It's an uncommon use case.
Even if there was, making the correct conversion implicitly would lead to more work for the application developer. Let's assume ODP.NET performs the "right" conversion. Now, the app needs to manipulate the data. What data type does it treat the returned value as: string or decimal? It could be either. That means twice as much code. One set deals with the data if a decimal was returned. The second deals with the data if a string is returned.
It would be much easier to write code for one scenario, rather than two. That is why ODP.NET safe type mapping is an explicit conversion the developer makes.