Get data in a textbox placed in a gridview from a Checkboxlist

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Gridview

Hi,
I have a gridview which is connected to a SQL datasource.In the gridview i have a item template field (Textbox) and a image button. when i click on the image button, i get a popup (using ModalpopupExtender).In the popup there is a checkboxlist with a series of items which are got from the database and a UPDATE button and CANCEL button.The thing is that whenever i check one or more items and click on the UPDATE button, the checked items should be displayed in the textbox for that particular rowIf more than one item with a separator ";". Could any1 pls help me out with this issue.