VBA Writing file in binary mode (overwriting given byte makes unexpected change in the next one, setting it's value to 0)

Question

I want to modify part of .bmp file using VBA, everything works great except that, when I overwrite choosen byte, the next byte is being set to zero. My macro is :

Sub WriteBinaryFile()
Dim i As Integer
Dim nFileNum As Integer
Dim sFilename As String

sFilename = "C:\Users\Piotr\Desktop\test1.bmp"

' Get an available file number from the system
nFileNum = FreeFile

' Open the file in binary mode.  Locks are optional

Open sFilename For Binary Lock Read Write As #nFileNum

     ' Put the data in the file
     ' Below code should write 255 value to byte number 100
     ' but it writes also 0 value to byte number 101
     Put #nFileNum, 100, 255 

Close #nFileNum

End Sub

Why when I'm modify byte number 100, value of byte number 101 is setted to 00 ? How to chage it and why is it occour ?

Edit as Cor_Blimey pointed out using conversion function CByte(255) solves problem, because 255 in VBA is integer number, which is 16-bit number so putting it into file overwrites two bytes

Maybe it's doing it in unicode? What does a value of 1 do? Can you write a value of 256? — Nick.Mc, May 11 '14 at 08:55
writing 255 into byte number 100 place value FF there (as I see in hexeditor), but also value of byte 101 is changed into 00 — Qbik, May 11 '14 at 08:58
I know - you already said that - can you write a 256 or does it error out? — Nick.Mc, May 11 '14 at 08:59
Just a guess (i.e. untested) but `255` is an integer, which is 16-bit i.e. 2-bytes in VBA. Try `Put #nFileNum, 100, CByte(255)` — Cor_Blimey, May 11 '14 at 09:44

score 1 · Answer 1 · answered Aug 05 '15 at 19:07

1

As stated by Cor_Blimey above:

...255 is an integer, which is 16-bit i.e. 2-bytes in VBA. Try Put #nFileNum, 100, CByte(255)

answered Aug 05 '15 at 19:07

DiegoAndresJAY

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VBA Writing file in binary mode (overwriting given byte makes unexpected change in the next one, setting it's value to 0)

1 Answers1