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The code you sent worked fine, after that I changed "TakeMessagesOnly" to "TransferToExtension" and added this line below the "TakeMessagesOnly" line, then received the same error as before (x's are the queue's id). This time I hard coded the queue's id as well, just to make sure it's not breaking when passing

parameters.transfer.extension.id = "xxxxxxx";

Unhandled exception. System.NullReferenceException: Object reference not set to an instance of an object.
   at Program.<Main>$(String[] args) in C:\Users\xxx\source\repos\TestConsoleApp\TestConsoleApp\Program.cs:line 44
   at Program.<Main>(String[] args)

var parameters = new CreateAnsweringRuleRequest();
parameters.enabled = true;
parameters.type = "Custom";
parameters.name = "My weekly meetings";
var schedule = new ScheduleInfo();
var weeklyRanges = new WeeklyScheduleInfo();
TimeInterval day = new TimeInterval();
day.from = "09:00";
day.to = "10:00";
weeklyRanges.monday = new TimeInterval[] { day };

day = new TimeInterval();
day.from = "10:00";
day.to = "15:00";
weeklyRanges.friday = new TimeInterval[] { day };

schedule.weeklyRanges = weeklyRanges;
parameters.schedule = schedule;
parameters.callHandlingAction = "TransferToExtension";
parameters.transfer.extension.id = "xxxxxxxxx";

var response = await rc.Restapi().Account().Extension().AnsweringRule().Post(parameters);

Great. That helps us narrow down the suspecting area. And here is the working code

parameters.callHandlingAction = "TransferToExtension"; 
var transferObj = new TransferredExtensionInfo();
var callQueueInfo = new CallHandlingExtensionInfo();
callQueueInfo.id = "xxxxxxxxx";

transferObj.extension = t;
parameters.transfer = transferObj;

As always, you're a beast.

For future and so I understand better, how were you able to narrow it down to it wanting the CallHandlingExtensionInfo type inside the TransferredExtensionInfo variable?

Here is the trick. When creating a data object, place the cursor to a member and see the data member type.

Hey Phong,

Same issue when leaving those blank, it seems like it is not even getting to the API call and is stopping on the below line.

parameters.transfer.extension = fwdQueueExt;

Also tried adding the following this morning to make sure I have everything it could possibly want, but still receive an error on that same as above (just with the new variable). I'm sure I'm just doing something wrong lol, just not sure what

var transferInfo = new CallHandlingExtensionInfo ();
transferInfo.extensionNumber = callQueues.records[0].extensionNumber.ToString();
transferInfo.id = callQueues.records[0].id.ToString();
transferInfo.uri = callQueues.records[0].uri;

parameters.callHandlingAction = "TransferToExtension";
parameters.transfer.extension = transferInfo;

I think it's because of the extension object you passed to the transfer parameter. The documentation is vague and there are redundant params. Can you try to assign the call queue extension id to the extension.id, instead of using the extension number?

Same error when passing the call queue's ID to the extension.id (snippet below just to make sure we're on the same page)

transferInfo.id = callQueues.records[0].id.ToString();

parameters.callHandlingAction = "TransferToExtension";
parameters.transfer.extension.id = transferInfo.id;

I'm pretty stumped, the documentation says 'If "TransferToExtension" is specified for callHandlingAction, the transfer parameter object is required.'

https://developers.ringcentral.com/guide/voice/call-routing/manual/user-answering-rules

The only thing in the transfer parameter is the id?