C# extension method to convert a DataTable to a CSV string.

public static string DataTableToCSV(this DataTable datatable, char seperator)
{
    StringBuilder sb = new StringBuilder();

    for (int i = 0; i < datatable.Columns.Count; i++)
    {
        sb.Append(datatable.Columns[i]);
        if (i < datatable.Columns.Count - 1)
        {
            sb.Append(seperator);
        }
    }

    sb.AppendLine();

    foreach (DataRow dr in datatable.Rows)
    {
        for (int i = 0; i < datatable.Columns.Count; i++)
        {
            if (dr[i] is string[])
            {
                var list = dr[i] as string[];
                var val = string.Join(";", list);

                if (val.Contains(","))
                {
                    val = "\"" + val + "\"";
                }

                sb.Append(val);
            }
            else
            {
                var value = dr[i].ToString();

                if (value.Contains("\""))
                {
                    value = value.Replace("\"", "'");
                }

                if (value.Contains(","))
                {
                    value = "\"" + value + "\"";
                }

                sb.Append(value);
            }

            if (i < datatable.Columns.Count - 1)
            {
                sb.Append(seperator);
            }
        }
        sb.AppendLine();
    }

    return sb.ToString();
}