I have not clear why the garbage data returning.

Yes it was base64 encoded in a c# app with UTF-8 encoding.
string base64Encoded = Convert.ToBase64String(System.Text.Encoding.UTF8.GetBytes(originalString));

I was trying different combination and found that
System.Text.Encoding.Default works fine for sql decoding.

so this is what I am using now,

string base64Encoded = Convert.ToBase64String(System.Text.Encoding.Default.GetBytes(originalString));

Not sure why my utf8 is returning a different value for certain cases.

Hopefully I will not have any more issues

Apperciate your help.
Thanks Again

Run the follow SQL to see, a properly encoded string
matches its original string after decoding.

-- declare vars.
DECLARE @originalData varchar(max)
DECLARE @encodedData varchar(max)
DECLARE @decodedData varchar(max) 
DECLARE @xmlData XML

SET @originalData = 'h9l+€*c'

-- this is my base 64 encoded string with UTF-8
SET @encodedData = 'aDlsK4AqgWM=' 

-- construct an xml var.
SET @xmlData = CAST('<data>' + @encodedData + '</data>' as xml)

-- base64 decode the @encodedData.
SELECT @decodedData = CONVERT(varchar(MAX),  @xmlData.value('(data)[1]', 'varbinary(max)'))

-- generate the outputs
SELECT @decodedData as DecodedAs

IF @decodedData = @originalData
        SELECT 'Good, @decodedData = @originalData'
ELSE 
        SELECT 'Err, not the same.'

-----Output-------------------------------

DecodedAs
h9l+€*c

"Good, @decodedData = @originalData"
-----Output-------------------------------

DECLARE @xml xml
DECLARE @binary varbinary(max)
SET @xml = CAST(” AS xml)
SET @binary = (SELECT @xml.value(‘(/row/@mystring)[1]‘,’varbinary(max)’))
SELECT CONVERT(varchar(max), @binary)

returns “h9l+€*c” which is not the same as the original string.

Any idea.

Thanks in advance.

The result I got is

(replace “_” with “”) which is as same as ’Добро’

Try this,

DECLARE @data NVARCHAR(MAX), @XmlData XML
SET @data = N'JiMxMDQ0OyYjMTA4NjsmIzEwNzM7JiMxMDg4OyYjMTA4Njs=' 
SET @XmlData = CAST('<data>' + @data + '</data>' AS XML)
SELECT CONVERT(VARCHAR(MAX), @XmlData.VALUE('(data)[1]', 'varbinary(max)'))

SET @data = N’Добро’

SET @XmlData = CAST(” + @data + ” as xml)

SELECT CONVERT(nvarchar(max), @XmlData.value(‘(data)[1]‘, ‘varbinary(max)’))

returns NULL.
If i encode this with http://makcoder.sourceforge.net/demo/base64.php
then text is: JiMxMDQ0OyYjMTA4NjsmIzEwNzM7JiMxMDg4OyYjMTA4Njs=