Output Parameter cast problems

Question

Wondering if anyone has seen this ...
Migrating a data access layer (based on the MS DAAB) using the ODP.NET provider. I wrote my own parameter discovery code to dynamically configure parameters at runtime - just like the SqlCommandBuilder.DeriveParameters() method does for SQL Server sprocs.
(I query the "all_arguments" view to get this data, by the way, passing the package name and procedure name.)
Here's the problem: you cannot discover the Size property for a parameter via this query. Bummer. So for VarChar2 and Char types I tried setting the Size to somethng big, like 2000.
When I retrieve the parameters after an ExecuteNonQuery statement, I am getting Type cast errors:
"Cast from type 'OracleString' to type 'String' is not valid"
Here's the weird part:
- CType(params(5).Value, Integer) 'WORKS
- CType(params(2).Value, String) 'DOESN'T WORK
- params(2).Value.ToString 'WORKS
- CType(params(2).Value, Date) 'DOESN'T WORK
What the ...? Why can't I cast a VarChar2 or Char type back to a .NET String? Why does ToString work but Ctype() not work?
I hope somebody can offer guidance, I've got a lot invested in this DAL :-)
Thanks eh
Kurt Mang
Vancouver BC

Answer 1

There's not really an elegant way to provide these conversions in VB. In C# OracleString uses operator overloading to provide type conversions, but this version of VB doesn't support it.
They might have implemented IConvertible on the oracle types.
Then you could write
Dim osw as new OracleString("Hello")
Dim s as String = CType(osw, IConvertible).ToString
But that's not really better than
Dim osw as new OracleString("Hello")
Dim s as String = osw.Value
The long and short of it is, OracleTypes ( and other ADO.NET provider types), are special high-performance types (usually value types), and they convert using a different idiom. By convention they are converted to the corresponding framework type using a property called .Value, and constructed from framework types using a constructor.
String s = New OracleString("Hello").Value
Converts the Unicode string "Hello" to an OracleString structure, and then back to a unicode string.
David