SQL string to varbinary through XML different than nvarchar

Question

We currently have a function in SQL which I simply do not understand.

Currently we convert a nvarchar to XML, and then select the XML value, and convert that to a varbinary.

When I try to simplify this to convert the nvarchar directly to varbinary, the output is different... Why?

--- Current situation:
Declare @inputString nvarchar(max) = '4d95605d1b8f3bca5ea3e0d2af26027004d17218152e726da0622d669a71f85c'

--1: input to XML
declare @inputXML XML = convert(varchar(max), @inputString)

--2: input XML to binary
declare @inputBinray varbinary(max) = @inputXML.value('(/)[1]', 'varbinary(max)')

select @inputString -- 4d95605d1b8f3bca5ea3e0d2af26027004d17218152e726da0622d669a71f85c
select @inputXML -- 4d95605d1b8f3bca5ea3e0d2af26027004d17218152e726da0622d669a71f85c
select @inputBinray -- 0xE1DF79EB4E5DD5BF1FDDB71AE5E6B77B477669FDBAD36EF4D38775EF6D7CD79D9EEF6E9D6B4EB6D9DEBAF5AEF57FCE5C

--- New situation
--1: Input to binary
declare @inputString2 varbinary(max) = CAST(@inputString as varbinary(max));

select @inputString2 -- 0x3400640039003500360030003500640031006200380066003300620063006100350065006100330065003000640032006100660032003600300032003700300030003400640031003700320031003800310035003200650037003200360064006100300036003200320064003600360039006100370031006600380035006300

Answer 1

Using the value() function to get a XML value specified as varbinary(max) will read the data as if it was Base64 encoded. Casting a string to varbinary(max) does not, it treats it as just any string.

If you use the input string QQA= which is the letter A in UTF-16 LE encoded to Base64 you will see more clearly what is happening.

XML gives you 0x4100 , the varbinary of the letter A , and direct cast on the string gives you 0x5100510041003D00 where you have two 5100 = "Q" and of course one 4100 = "A" followed by a 3D00 = "="

Answer 2

Might be I get something wrong, but - if I understand you correctly - I think you simply want to get a real binary from a HEX-string, which just looks like a binary. Correct?

Above I wrote "simply" , but this was not simple at all a while ago.

I'm not sure at the moment, but I think it was version v2012, which enhanced CONVERT() (read about binary values and how the third parameter works) and try this:

DECLARE @hexString VARCHAR(max)='4d95605d1b8f3bca5ea3e0d2af26027004d17218152e726da0622d669a71f85c';

SELECT CONVERT(varbinary(max),@hexString,2);

The result is a real binary

0x4D95605D1B8F3BCA5EA3E0D2AF26027004D17218152E726DA0622D669A71F85C

What might be the reason for your issue:

Very long ago, I think it was until v2005, the default encoding of varbinaries in XML was a HEX string. Later this was changed to base64. Might be, that you code was used in a very old environment and was upgraded to a higher version?

Today we use XML in a smiliar way to create and to read base64, which is not supported otherwise. Maybe your code did something similar with HEX strings...?

One more hint for this: The many 00 in your New Situation example show clearly, that this is a two-byte encoded NVARCHAR string. Contrary, your Current Situation shows a simple HEX string.

Your final result is just the binary pattern of your input as string: