if (!sqlDataReader.IsDBNull(n)) value = r.GetString(n);

static class DataReaderExtensions
{
    public static string GetStringOrNull(this IDataReader reader, int ordinal)
    {
        return reader.IsDBNull(ordinal) ? null : reader.GetString(ordinal);
    }

    public static string GetStringOrNull(this IDataReader reader, string columnName)
    {
        return reader.IsDBNull(columnName) ? null : reader.GetString(columnName);
    }
}

value = reader.GetStringOrNull(n);

value = r[n] as string;