score:3

Something like this?

var query = (from c in dt.AsEnumerable()
             select new { Value=c.Field<int>("RollNo"), 
             Text=c.Field<string>("Name")}).ToList();

And then just bind query to the ListBox.

UPDATE

OP wants just strings; so this should do it:

var query = (from c in dt.AsEnumerable()
         select "RollNo: "+ c.Field<int>("RollNo")+ " , Name: "+ c.Field<string>("Name")).ToList();

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