❔ C# WebAPI - how to make a field 'optional' ?

[HttpGet("users")]
public IActionResult GetUsers(
    [FromQuery] int? dbId,
    [FromQuery] int? userId,
    [FromQuery] DateTime? fromDate,
    [FromQuery] DateTime? toDate,
    [FromQuery] string username)
{
    var query = _context.Users.AsQueryable();
    if (dbId.HasValue)
    {
        query = query.Where(d => d.dbId == dbId.Value);
    }
    if (userId.HasValue)
    {
        query = query.Where(d => d.userId == userId.Value);
    }
    if (fromDate.HasValue)
    {
        query = query.Where(d => d.registrationDate >= fromDate.Value);
    }
    if (toDate.HasValue)
    {
        query = query.Where(d => d.registrationDate <= toDate.Value);
    }
    if (!string.IsNullOrEmpty(username))
    {
        query = query.Where(d => d.username == username);
    }
    var users = query.ToList();
    return Ok(users);
}