Why do I get the error message " CS1502: The best overloaded method match for 'xxx(string)' has some invalid arguments " when I use Helper function in DataGrid
The function xxx() is expecting a string, but DataBinder.Eval() returns an object. So you must cast the result of DataBinder.Eval() to a string (or, in this case, just use .ToString()).
i.e, in your template do:
Either of these should work.