How increment ulong correctly?

public class NotificationDto
{
    [JsonProperty( PropertyName = "id" )]
    [JsonRequired]
    public Guid Id { get; set; }

    [JsonProperty( PropertyName = "message" )]
    [JsonRequired]
    public MessageDto Message { get; set; }
}

private async Task OnMessagesSend(
  IDeliveryHandlerContext context,
  MessagesSendArguments arguments )
{
    foreach (var message in arguments.Items)                                          
    {                                                                                   
         switch (message.Behavior)                                                     
         {
          case BehaviorType.Simple:                                                       
              foreach (var operatorName in message.To)                                  
              {
                  var endpoint = await _operatorEndpointCache.EndpointGet(operatorName);
                  await endpoint.EventEmit(new NotificationsReceivedEvent(               
                      new NotificationDto                                                 
                      {
                          Id = Guid.NewGuid(),
                          Message = message
                      }));
              }
              break;
}

public class NotificationDto
{
    private static ulong _nextId = 1;

    [JsonProperty(PropertyName = "id")]
    [JsonRequired]
    public ulong Id { get; set; }

    [JsonProperty(PropertyName = "message")]
    [JsonRequired]
    public MessageDto Message { get; set; }

    public NotificationDto()
    {
        Id = _nextId++;
    }
}