how to save data from gridview on a footer button click

Question

 THIS IS MY GRIDVIEW

Slno	date	type	ACTION
1	2019 /01/10	1	(button) EDIT
2	2019/01/10	2	(button) EDI

(button)SAVE

 

i'm already datakeysname is set in the gridview.and 

i want save data from gridview using footer buttonand that datakeyname value paasing to sql.. 

on that btnclick event is generated. 

 

i didn't get each row datakeynames value. 

 string id=gdvlist.Datakeys[gr.rowindex].value.tostring(); i pass this line but i got this type of error

Index was out of range. Must be non-negative and less than the size of the collection. 

 

 how to access datakeyname and how to save data fron grid on footer button click.

plz help me. thnkx in adv