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MongoDB is a document database that does not have auto-increment or sequence functionality build-in, like e.g. SQL Server. Auto-increment or sequence numbers are often used to create a consecutive sequence of numbers used as order or invoice identifiers.

This is simple to achieve with MongoDB via the atomic operation $inc. If you use the Official MongoDB C# Driver the operation is exposed via the Update class.

To implement auto-increment or sequence functionality with MongoDB a document is required to keep the state of the sequence. The Id is the natural name of the sequence e.g. orderid and the value is the current counter value.

class Counter
  public string Id { get; set; }
  public int Value { get; set; }

Then getting the next ordered is done by executing the below statement:

var client = new MongoClient(connectionString);
MongoServer server = client.GetServer();
MongoDatabase db = server.GetDatabase("myDatabase");
var counterCol = db.GetCollection("counters")

var result = counterCol.FindAndModify(new FindAndModifyArgs()
  Query = Query.EQ(d => d.Id, "orderId"),
  Update = Update.Inc(d => d.Value, 1),
  VersionReturned = FindAndModifyDocumentVersion.Modified,
  Upsert = true, //Create if the document does not exists