How to merge features in a multi feature part

I have multi part Cone created programatically using API. after it has been created i want all parts to merge programatically but i can't.

The situaation is when i open part its merge checkbox is clicked already. If i open and close the feature it merges correctly even without changing anything.

Please advise or send me a working code .

Thank you.

Aqeel

imms@skype.com