NUnit for Database retrieval

private string s_connection = @"Provider=Microsoft.Jet.OLEDB.4.0; Data Source=" + FormHelperClass.DataBaseName + ";Jet OLEDB:System Database = " + FormHelperClass.SystemDataBasePath + ";User Id=userid;Password=password"; 
public List<User> GetAllUsers()
{
	List<User> users = new List<User>();
	try
	{
		//query the database 
		string query = "SELECT UserID, UserName FROM User order by UserId;";
		// setup the connection
		OleDbConnection connection = new OleDbConnection(s_connection);
		connection.Open();
		OleDbCommand command = new OleDbCommand(query, connection);
		OleDbDataReader reader = command.ExecuteReader();
		// loop through reader & read configs 
		while (reader.Read())
		{
			User user = new User();
			user.UserId = Convert.ToInt16(reader["UserID"]);
			user.UserName = (reader["UserName"]).ToString();                    
			users.Add(user);
		}
		reader.Close();
		connection.Close();
	}
	catch (Exception ex)
	{                
		log.Debug("Error Message " + ex.Message);
	}
	return users;
}
using NUnit.Framework;
    [TestFixture]
    public class ServiceProvidersTest
    {
        [Test]
        public void TestGetUsers()
        {
            FormHelperClass.DataBaseName = "C:\\Users\\Security.mdb";
            UserManager u_manager = new UserManager();
            List<User> list_manager = u_manager.GetAllUsers();
            
            Assert.IsNotNull(list_manager);
            Assert.Greater(list_manager.Count, 0); 
            foreach(User user in list_manager)
            {
                Assert.IsNull(user);
               
                
            }
        }
    }