ODP.NET OracleCommand optional parameter never getting default value - vb.net

Question

When I'm calling a function from a PL/SQL package, there is a IN optional parameter (Date) with default value of SYSDATE.
When I execute my commmand, I don't have a choice but to add the parameter. Even if there is no value set, the function always receive null as the entered value, so it never affects the default value.
Dim cmd As New OracleCommand(con)
cmd.CommandType = CommandType.StoredProcedure
cmd.CommandText = "PACK.RefreshData"
Dim param1 As New OracleParameter()
param1.Direction = ParameterDirection.Input
param1.ParameterName = "p_date"
param1.DbType = DbType.Date
cmd.Parameters.Add(param1)
param1 = New OracleParameter()
param1.Direction = ParameterDirection.Output
param1.ParameterName = "po_errorCode"
param1.DbType = DbType.String
cmd.Parameters.Add(param1)
param1 = New OracleParameter()
param1.Direction = ParameterDirection.Output
param1.ParameterName = "po_errorDescription"
param1.DbType = DbType.String
cmd.Parameters.Add(param1)
con.Open()
cmd.ExecuteNonQuery()
In another question, somebody said the command property BindByName set to true works only for regular queries, not storedProcs. So what can I do to call the function without passing a value with my IN parameter?
Here is the Stored Proc header
Procedure RefreshData
(
p_date IN DATE := SYSDATE,
po_errorCode OUT varchar2,
po_errorDescription OUT varchar2
);
N.B. : I'm using Oracle.DataAccess.dll version 4.112.4.0
UPDATE : Here is the explanation taken from the oracle thread, from my answer below :
I think what it boils down to is that DervieParameters is deriving parameters for all of the proc params, even though they have default values, and that seems to me to be the correct behavior. Once you have parameters in the collection, if you don't assign a value to them, null will be passed.
What you probably need to do is
1) the best option, is not to use deriveparameters in the first place, and manually build the parameters collection, adding parameters for only the things you don't want to have default values:
DeriveParameters incurs a database round-trip and should only be used during design time. To avoid unnecessary database round-trips in a production environment, the DeriveParameters method itself should be replaced with the explicit parameter settings that were returned by the DeriveParameters method at design time.
2) If you want to continue using DeriveParameters, remove the unwanted parameters from the OracleParameters collection.

Answer 1

If you have default-value parameter not at the tail of the parameters chain you should perform named parameters call to avoid passing this default-value parameter. I think your data provider doesn't support this call type, so you should wrap your call in anonymous pl\sql block like this :
begin
PACK.RefreshData(po_errorCode => :po_errorCode, po_errorDescription=>:po_errorDescription);
end;
and then do what did before in VB code (ofcourse you CommandType will be Text).
Another solution is to move your default-value parameter to the end of parameters chain like this:
Procedure RefreshData
(
po_errorCode OUT varchar2,
po_errorDescription OUT varchar2,
p_date IN DATE := SYSDATE
);

Answer 2

Finally I have been able to find a way to call the procedure with this thread :
https://community.oracle.com/thread/2248928?tstart=0
Here is my corrected code :
con.Open()
cmd = con.CreateCommand()
cmd.CommandType = CommandType.StoredProcedure
cmd.CommandText = "Pack.RefreshData"
cmd.BindByName = True
OracleCommandBuilder.DeriveParameters(cmd)
cmd.Parameters.RemoveAt(0) 'I remove the optional parameter
cmd.ExecuteNonQuery()
Then I retreive my OUT parameters
Dim outPrm1 as String = ""
If cmd.Parameters("po_errorDescription").Value IsNot Nothing Then
outPrm1 = cmd.Parameters("po_errorDescription").Value
End If