ODP.NET

/* 
 CREATE OR REPLACE PROCEDURE defPROC
(
p1 IN VARCHAR2 DEFAULT 'foo',
p2 IN VARCHAR2 DEFAULT 'bar',
p3 IN VARCHAR2 DEFAULT 'baz',
p4 out varchar2
) AS
begin
 p4 := 'p1='||p1||', p2='||p2||', p3='||p3;
 end;
 /
 */
using System;
using System.Data;
using Oracle.DataAccess.Client;

class testdefvals
{
    static void Main(string[] args)
    {
        using (OracleConnection con = new OracleConnection("data source=orcl;user id=scott;password=tiger"))
        {
            con.Open();
            using (OracleCommand cmd = new OracleCommand("defProc", con))
            {
                cmd.CommandType = CommandType.StoredProcedure;
                cmd.BindByName = true;
                cmd.Parameters.Add("p2", "secondval");
                OracleParameter outval = new OracleParameter("p4", OracleDbType.Varchar2, 4000);
                outval.Direction=ParameterDirection.Output;
                cmd.Parameters.Add(outval);
                cmd.ExecuteNonQuery();
                Console.WriteLine(outval.Value.ToString());

            }
        }
    }
}

/* 

CREATE OR REPLACE PROCEDURE defPROC
 (
 p1 IN VARCHAR2 DEFAULT 'foo',
 p2 IN VARCHAR2 DEFAULT 'bar',
 p3 IN VARCHAR2 DEFAULT 'baz',
 p4 out varchar2
 ) AS
 begin
  p4 := 'p1='||nvl(p1,'p1null')||', p2='||nvl(p2,'p2null')||', p3='||nvl(p3,'p3null');
  end;
 */
using System;
using System.Data;
using Oracle.DataAccess.Client;

class testguid
{
    static void Main(string[] args)
    {
        using (OracleConnection con = new OracleConnection("data source=orcl;user id=scott;password=tiger"))
        {
            con.Open();
            using (OracleCommand cmd = new OracleCommand("defProc", con))
            {
                cmd.CommandType = CommandType.StoredProcedure;
                cmd.BindByName = true;
                OracleCommandBuilder.DeriveParameters(cmd);
                cmd.Parameters[1].Value = "secondval";
                cmd.ExecuteNonQuery();
                Console.WriteLine(cmd.Parameters[3].Value.ToString());

            }
        }

        using (OracleConnection con = new OracleConnection("data source=orcl;user id=scott;password=tiger"))
        {
            con.Open();
            using (OracleCommand cmd = new OracleCommand("defProc", con))
            {
                cmd.CommandType = CommandType.StoredProcedure;
                cmd.BindByName = true;
                OracleCommandBuilder.DeriveParameters(cmd);
                // remove the first param
                cmd.Parameters.RemoveAt(0);
                // the third param is now the second param, remove it too.
                cmd.Parameters.RemoveAt(1);
                // this is the second value of the proc, but the first param of the collection, jsut
                // as it was when we coded it manually
                cmd.Parameters[0].Value = "secondval";
                cmd.ExecuteNonQuery();
                // the 4th proc param is now the second in the collection
                Console.WriteLine(cmd.Parameters[1].Value.ToString());

            }
        }
    }
}