FileOpen Function 

Opens a file for input or output.

The My feature gives you greater productivity and performance in file I/O operations than FileOpen. For more information, see My.Computer.FileSystem Object.

Public Sub FileOpen( _
   ByVal FileNumber As Integer, _
   ByVal FileName As String, _
   ByVal Mode As OpenMode, _
   Optional ByVal Access As OpenAccess = OpenAccess.Default, _
   Optional ByVal Share As OpenShare = OpenShare.Default, _
   Optional ByVal RecordLength As Integer = -1 _
)

Parameters

• FileNumber
  Required. Any valid file number. Use the FreeFile function to obtain the next available file number.

• FileName
  Required. String expression that specifies a file name—may include directory or folder, and drive.

• Mode
  Required. Enumeration specifying the file mode: Append, Binary, Input, Output, or Random. (For more information, see OpenMode Enumeration.)

• Access
  Optional. Enumeration specifying the operations permitted on the open file: Read, Write, or ReadWrite. Defaults to ReadWrite. (For more information, see OpenAccess Enumeration.)

• Share
  Optional. Enumeration specifying the operations not permitted on the open file by other processes: Shared, Lock Read, Lock Write, and Lock Read Write. Defaults to Lock Read Write. (For more information, see OpenShare Enumeration.)

• RecordLength
  Optional. Number less than or equal to 32,767 (bytes). For files opened for random access, this value is the record length. For sequential files, this value is the number of characters buffered.

Exceptions

Exception type	Error number	Condition
ArgumentException	5	Invalid Access, Share, or Mode.
ArgumentException	5	WriteOnly file is opened for Input.
ArgumentException	5	ReadOnly file is opened for Output.
ArgumentException	5	ReadOnly file is opened for Append.
ArgumentException	5	Record length is negative (and not equal to -1).
IOException	52	FileNumber is invalid (<-1 or >255), or FileNumber is already in use.
IOException	55	FileName is already open, or FileName is invalid.

See the "Error number" column if you are upgrading Visual Basic 6.0 applications that use unstructured error handling. (You can compare the error number against the Number Property (Err Object).) However, when possible, you should consider replacing such error control with Structured Exception Handling Overview for Visual Basic.

Remarks

The FileOpen function is provided for backward compatibility and may affect performance. For non-legacy applications, the My.Computer.FileSystem object provides better performance. For more information, see File Access with Visual Basic.

You must open a file before any I/O operation can be performed on it. FileOpen allocates a buffer for I/O to the file and determines the mode of access to use with the buffer.

Security Note

When writing to a file, an application may need to create a file, if the file to which it is trying to write does not exist. To do so, it needs permission for the directory in which the file is to be created. However, if the file specified by FileName does exist, the application needs Write permission only to the file itself. Wherever possible, to help improve security, create the file during deployment and grant Write permission to that file only, rather than to the entire directory. To help improve security, write data to user directories rather than to the root directory or the Program Files directory.

The channel to open can be found using the FreeFile() function.

Security Note
The FileOpen function requires Read access from the FileIOPermissionAccess enumeration, which may affect its execution in partial trust situations. For more information, see FileIOPermissionAccess enumeration and Requesting Permissions.

Example

This example illustrates various uses of the FileOpen function to enable input and output to a file.

The following code opens the file TestFile in Input mode.

FileOpen(1, "TESTFILE", OpenMode.Input)
' Close before reopening in another mode.
FileClose(1)

This example opens the file in Binary mode for writing operations only.

FileOpen(1, "TESTFILE", OpenMode.Binary,OpenAccess.Write)
' Close before reopening in another mode.
FileClose(1)

The following example opens the file in Random mode. The file contains records of the structure Person.

Structure Person
    <VBFixedString(30)> Dim Name As String
    Dim ID As Integer
End Structure
Public Sub ExampleMethod()
    ' Count 30 for the string, plus 4 for the integer.
    FileOpen(1, "TESTFILE", OpenMode.Random, , , 34)
    ' Close before reopening in another mode.
    FileClose(1)
End Sub

This code example opens the file in Output mode; any process can read or write to file.

FileOpen(1, "TESTFILE", OpenMode.Output, OpenAccess.Default, OpenShare.Shared)
' Close before reopening in another mode.
FileClose(1)

This code example opens the file in Binary mode for reading; other processes cannot read file.

FileOpen(1, "TESTFILE", OpenMode.Binary, OpenAccess.Read, _
   OpenShare.LockRead)

Smart Device Developer Notes

This function is not supported.

Requirements

Namespace: Microsoft.VisualBasic

Module: FileSystem

Assembly: Visual Basic Runtime Library (in Microsoft.VisualBasic.dll)

See Also

Reference

FileClose Function
FreeFile Function

Other Resources

Reading from Files in Visual Basic
Writing to Files in Visual Basic