Use other type than string for ObjectID about algoliasearch-client-csharp

[morganleroi]

Apologies for the late response and thanks for posting this issue @Rickos99.

Your solution is the best one for the 6.x version of the client.

We launched a 7.x new client (Currently in beta) and you can now send the ObjectID with a Guid.

    var saved = await _client.SaveObjectAsync(DefaultIndex,
      new { ObjectID = Guid.NewGuid(), value = "test", otherValue = "otherValue" });