I want to add coincident to Component in Assembly.So that I can get the result as the second pictrue I replace some parts with different size,be not the third picture.Thanks
How would the API know how to add the mates? That is kind of a human level problem.
Sorry,I didn't describe it clearly.There are three coincident faces between every two parts.And I just want to add coincident.Thank you for you replay
If the whole assembly is designed about the origin, mate is not required.
Fixed is faster and more stable.
Yes,fixed is faster and more stable.But sometime I need replace some parts with different size .I hope the position to change as well.I need get the result as the second picture 
It is possible with API, but it is not a trivial task I am afraid, and will be outside of the macro scope as it would be better to do with SOLIDWORKS add-in. The task can be simplified if only simple mates to be applied (i.e. concentric and coincident) and if components will be in the exact position for mating.
Yes，you are right.I just want to add coincident.There are three coincident faces between every two parts,as the picture.Someone tell me,Traverse the faces of each part,then measure the distance,if the distance is zero,add coincident.But we don't how to get it by api.Can you help me.Thanks
HI Chen,
Kindly elaborate your problem.
-Abilash