Adding sub label in Google apps mail account C# application

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C#

Gmail

I am working on C# project in which I need to add the labels in Gapps account. I am able to add the label but not able to create sub label in newly created label. Here is the sample code for creating label.

GoogleMailSettingsService service = new GoogleMailSettingsService(_domain, "Myapp");
service.setUserCredentials(_adminuser, _password);
service.CreateLabel("userName", label);

How to add the sub label in newly created label. Any help on this is greatly appreciated.

Thanks in advance.

Mayuresh