theres is a option in online center named "Setting the primary draft", but how to set it in code behind?

IContentVersionRepository has a method SetCommonDraft.

Thanks Per!

this is what i had to do:

producLink is my contentreference

var versionRepo = ServiceLocator.Current.GetInstance<IContentVersionRepository>();

var latestVersion = versionRepo.List(productLink).Last().ContentLink;

// get the last item in array

versionRepo.SetCommonDraft(latestVersion);

//set the last item as default

don't forget to use SaveAction.ForceCurrentVersion when saving