BUG: Seek Returns Unexpected Value When File Opened for Append (190209)

MORE INFORMATION

Opening a file for Append allows you to add new data to an existing file without affecting the information already available in that file. "Byte Position" refers to the byte that will be read or written when the next file operation takes place. Within the file, the first byte is at position 1, the second byte is at position 2, and so on. Therefore, the initial value for the Seek method for a file opened for appending should be the size of that file plus 1.

In the products referenced at the beginning of this article, the Seek method always returns an incorrect value that is less than the actual number of bytes in the file. Earlier versions of Visual Basic, including version 3.0, did not exhibit this behavior.

Although the value reported by the Seek method is incorrect, the "Print" and "Write" methods are not affected by this number and data is written to the file correctly.

Steps to Reproduce Behavior

1. Create a new text file, named "C:\TEST.TXT," using NOTEPAD.EXE. Type the following text into this new file and save the changes:

   Hello World

2. Create a new Visual Basic project. Form1 is created by default.
3. Add a CommandButton, Command1, to Form1.
4. Paste the following code into Form1's code window:

         Private Sub Command1_Click()
            ' Open text file for appending
            Open "c:\test.txt" For Append As #1
   
            ' Display the current read/write position
            MsgBox Seek(1)
   
            ' Close the file
            Close #1
         End Sub
   						

5. Run the sample application and click the button. The value of 1 is displayed indicating that the current byte position is at the beginning of the file. The correct value should be the size of the entire text file plus 1. In the case above, the result should be 12.
6. Stop the project and replace the code in step 4 above with the following to demonstrate a workaround:

         Private Sub Command1_Click()
            Dim SizeOfFile As Long
   
            ' Determine size of file before opening
            SizeOfFile = FileLen("c:\test.txt")
   
            ' Open text file for appending
            Open "c:\test.txt" For Append As #1
   
            ' Display adjusted seek value
            MsgBox Seek(1) + SizeOfFile
   
            ' Close the file
            Close #1
         End Sub
   						

7. Repeat step 5, and note that the correct value is displayed.