DropDown list not retaining the values(the dropDown Inside gridview control)

I Have the gridview control with 2 dropdown list and 2 text boxes,
When  the textbox chnaged event happen 1 row will create and the dropdown back to initial values( its not retaining the selected values),
I am binding the control  after text changed method,
Is there any properties needs to be set or any other way we can retain the values in postbacks?
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