Project project = new Project("New Project.mpp");
CsvOptions options = new CsvOptions();
options.TasksFilter = new OutlineCodeFilter();
project.Save("CsvOptionsWithFilteredTasks.csv", options);

// ...

class OutlineCodeFilter : ICondition<Task>
{
    public bool Check(Task el)
    {
        var code1 = el.OutlineCodes.GetByFieldId((int)ExtendedAttributeTask.OutlineCode1);
        var code2 = el.OutlineCodes.GetByFieldId((int)ExtendedAttributeTask.OutlineCode2);
        return code1 != null && code2 != null;
    }
}