Get unique array of letters in C#

GetUniqueStringValue.cs

        public static string GetUniqueStringValue(int size)
        {
            var data = new byte[size];
            using var crypto = RandomNumberGenerator.Create();
            crypto.GetBytes(data);
            for (var i = 0; i < size; i++)
            {
                data[i] = (byte)('a' + data[i] % 26);
            }
            return Encoding.ASCII.GetString(data);
        }