Show drop-down list for custom lookup field in the GridView

Hi Everyone,

I created a custom lookup field that inherit from the SPLookupField. My issue is the GridView in editing mode couldn't display my custom lookup field as a drop-down list.

Below is the SPLookupField, values is displayed in a drop down list

and below is my custom lookup field, it doesn't show as a drop-down list

Does anyone give me solutions to solve this issue.

Many Thanks,

Phu Dang.