Random Access Files in VB'NET

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Random Access Files in VB.NET

• ITE285
• R. Johnson

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Text Files Recap

• Read/written with StreamReader and Writer.
  Object-oriented syntax.
• In-memory data is converted to ASCII characters
  upon output.
• Integer 150 becomes digits 150 on disk
• Line oriented
• Variable length, delimited by newline char
  (Java/C \n)
• Text files are Sequential
• Must start reading at beginning.
• Cannot easily edit data in middle of the file.

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Random Access Files

• Record based.
• Fixed length, so no delimiting character.
• In-memory record (a structure variable) is mapped
  to/from disk. Must fix lengths of string fields!
• Data stored in raw binary form.
• In-memory bit pattern for numbers copied to disk
  as-is rather than digits translated to ASCII
  chars.
• Record Number and fixed length allows direct
  access to arbitrary position.
• Byte offset of desired record in file
  (Record -1) x length of record in bytes

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Comparison
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Tasks and Syntax

• FileOpen associates file number with file and
  sets I/O modes needs record length!
• FIlePut (filenum, recordvar, recnum)
• FileGet (filenum, recordvar, recnum)
• FileClose
• records in existing file? recs length of
  file (in bytes) / rec length
• Add new record at position one past last record.
• Can freely overwrite/edit existing record w/o
  disturbing others (fixed length).

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Record Structure

• Define a VB Structure data type for in-memory
  record.
• Strings MUST be made fixed length so overall
  record size is fixed ltVB FixedString(20)gt
  Public name As String
• Put definition at form-level, or in module for
  global use in bigger app.

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More

• This is the old, low-level (but easy!) way to do
  it.
• .NET uses BinaryReader and Writer to do what is
  called serialization of objects, saving their
  state to a file (or XML to be sent over internet)
• You will most likely use ADO.NET, tying VB to
  external databases, for real world data storage.