how to populate comma delimited string to checkboxlist?

Hi, All,
I have a problem to populate a comma delimited string to a checkboxlist inside gridview edittemplate. I want to retrieve the value of the checkboxlist first then use gridview rowdatabound event to split the "," and display the selected value. But how do I retrieve the value of the checkboxlist inside gridview?

IMy code is as below:

protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)    
{         CheckBoxList c = (CheckBoxList)GridView1.FindControl("chkAchieved");

            string commastring = ???? //checkboxlist selectedvalue, it will be a commadelimited string. but How?

            char sep = ',';

            String[] s = commastring. ...