create or replace function test_function return sys_refcursor
is
  cur sys_refcursor;
begin
  
  open cur for
      select count(*) from test_table;

  return cur;
  
end;

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Data.OracleClient;

namespace test_base
{
  public partial class Form1 : Form
  {
    public Form1()
    {
      InitializeComponent();
    }

    private void button1_Click(object sender, EventArgs e)
    {
      int a = 0;
      
      OracleConnection conn = new OracleConnection("Data Source=my_alias; User Id=my_name; Password=my_password;");
      conn.Open();

      OracleCommand cmd = new OracleCommand("test_function", conn);
      cmd.CommandType = CommandType.StoredProcedure;
      cmd.Parameters.Add("cur", OracleType.Cursor).Direction = ParameterDirection.ReturnValue; 

      OracleDataReader dr = cmd.ExecuteReader(); 

      while (dr.Read())
      {
          a = Convert.ToInt32(dr[0]); 
      }
      
      conn.Close();

      MessageBox.Show("В базе " + a.ToString() + " записей");

    }
  }
}