IEnumerable to string

var singleString = string.Join(",", _values.ToArray() );

Write an extension method such as

public static String AppendAll(this IEnumerable<String> collection, String seperator)
{
    using (var enumerator = collection.GetEnumerator())
    {
        if (!enumerator.MoveNext())
        {
            return String.Empty;
        }

        var builder = new StringBuilder().Append(enumerator.Current);

        while (enumerator.MoveNext())
        {
            builder.Append(seperator).Append(enumerator.Current);
        }

        return builder.ToString();
    }
}

and assuming the result of your previous expression is IEnumerable<String>, call:

var _values = _tbl.AsEnumerable().Select(x => x).AppendAll(String.Empty);    

 String.Join(
      ",",
      _tbl.AsEnumerable()
          .Select(r => r.Field<int>("ID").ToString())
          .ToArray())