No working append byte array to existing file using visual Basic - vb.net

Question

I have two strings, each string is a PDF etiquette, I must to write this two etiquette into a PDF file. To do this I convert a each string to a byte array (I don't know if this is the best way) and each I Write into a PDF file. When I write one etiquette into PDF file all is good, I see the etiquette, but then I try to appending the second, the result is the same, into the file is only the first etiquette. For example this code write first etiquette and all working good:
Dim fs As FileStream = New FileStream(fullFileName, FileMode.CreateNew)
fs.Close()
fs = New FileStream(fullFileName, FileMode.Append)
Dim str As String = GetPDFString(27)
Dim binaryData As Byte() = ConvertStringToByte(str)
fs.Write(binaryData, 0, binaryData.Length)
fs.Close()
but if I want to append the second etiquette in the same PDF file using this code ... this not appending.
Dim fs As FileStream = New FileStream(fullFileName, FileMode.CreateNew)
fs.Close()
fs = New FileStream(fullFileName, FileMode.Append)
Dim str As String = GetPDFString(25)
Dim str1 As String = GetPDFString(27)
Dim binaryData As Byte() = ConvertStringToByte(str)
Dim binaryData1 As Byte() = ConvertStringToByte(str1)
fs.Write(binaryData, 0, binaryData.Length)
fs.Write(binaryData1, 0, binaryData1.Length)
fs.Close()
both have the same result, and I don't understand why the second etiquette isn't appending? Thank you a lot.

Answer 1

Your question title suggests that you are asking about how to append a byte to a FileStream, not about PDF, and not about Base64 string conversion (which you are using in your code).
Before asking a question on StackOverflow, you need to ensure you are conveying only one problem at a time. Remove everything that is not relevant, and prepare a code sample we can use in a brand new VS project, in order to reproduce your problem and help you solve it.
Now, if your question is really about appending a byte (or a byte array) to a file, it's as simple as one line of code (or two, if you keep FileStream approach). See below link:
C# Append byte array to existing file
Also copy-pasted for your convenience here (and converted from C# to VB.NET):
Dim appendMe As Byte() = New Byte(999) {}
File.AppendAllBytes("C:\test.exe", appendMe)
Or, to avoid memory overflow, if your byte array is expected to be large enough:
Public Shared Sub AppendAllBytes(path As String, bytes As Byte())
'argument-checking here.
Using stream = New FileStream(path, FileMode.Append)
stream.Write(bytes, 0, bytes.Length)
End Using
End Sub

Answer 2

With this line:
fs.Write(binaryData1, binaryData.Length + 1, binaryData1.Length)
Specifically the second argument (binaryData.Length + 1), you're telling it to start appending from the wrong position of binaryData1. If it's 3 bytes long, for example, and so is binaryData, it won't append anything. It should be similar to the first .Write line:
fs.Write(binaryData1, 0, binaryData1.Length)
So it appends all of binaryData1. It will still append it after binaryData - you don't need to specify the length of the preceeding binaryData in this line.
Alternatively, bypassing the above entirely, concatenate your two strings before encoding/writing them to the file:
Dim fs As FileStream = New FileStream(fullFileName, FileMode.CreateNew)
fs.Close()
fs = New FileStream(fullFileName, FileMode.Append)
Dim str As String = GetPDFString(id, token, depot, 25)
Dim str1 As String = GetPDFString(id, token, depot, 27)
Dim binaryData As Byte() = Convert.FromBase64String(str & str1) 'concatenate strings
fs.Write(binaryData, 0, binaryData.Length)
fs.Close()